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acc-phys/9411004 | null | High-Brightness Photocathode Electron Sources | acc-phys physics.acc-ph | Most present and future electron accelerators require bright sources.
Invented less than ten years ago, the photo-injector the principle of which is
briefly recalled, has already demonstrated that it can provide very bright
beams. In this paper, the most advanced photo-injector projects are reviewed,
their specific features are outlined, and their major issues are examined. The
state-of-the-art in photocathode and laser technologies is presented. Beam
dynamics issues are also considered since they are essential in the production
of bright beams. Finally, the question of the maturity of photo-injector
technology is addressed.
| Accelerator Physics | Physics |
adap-org/9904001 | Nino Boccara | Correlated random walks with a finite memory range | adap-org nlin.AO | We study a family of correlated one-dimensional random walks with a finite
memory range M.These walks are extensions of the Taylor's walk as investigated
by Goldstein, which has a memory range equal to one. At each step, with a
probability p, the random walker moves either to the right or to the left with
equal probabilities, or with a probability q=1-p performs a move, which is a
stochastic Boolean function of the M previous steps. We first derive the most
general form of this stochastic Boolean function, and study some typical cases
which ensure that the average value <R_n> of the walker's location after n
steps is zero for all values of n. In each case, using a matrix technique, we
provide a general method for constructing the generating function of the
probability distribution of R_n; we also establish directly an exact analytic
expression for the step-step correlations and the variance <R_n^2> of the walk.
From the expression of <R_n^2>, which is not straightforward to derive from
the probability distribution, we show that, for n going to infinity, the
variance of any of these walks behaves as n, provided p>0. Moreover, in many
cases, for a very small fixed value of p, the variance exhibits a crossover
phenomenon as $n$ increases from a not too large value. The crossover takes
place for values of $n$ around 1/p. This feature may mimic the existence of a
non-trivial Hurst exponent, and induce a misleading analysis of numerical data
issued from mathematical or natural sciences experiments.
| Adaptation and Self-Organizing Systems | Physics |
adap-org/9511002 | Denis V. Juriev | Octonionic binocular mobilevision. An overview | adap-org nlin.AO | This paper is a compact overview of the heuristic approach to the recently
elaborated octonionic binocular mobilevision.
| Adaptation and Self-Organizing Systems | Physics |
adap-org/9409004 | null | A New Look at Broken Ergodicity | adap-org nlin.AO | We study the nature and mechanisms of broken ergodicity (BE) in specific
random walk models corresponding to diffusion on random potential surfaces, in
both one and high dimension. Using both rigorous results and nonrigorous
methods, we confirm several aspects of the standard BE picture and show that
others apply in one dimension, but need to be modified in higher dimensions.
These latter aspects include the notions that at fixed temperature confining
barriers increase logarithmically with time, that ``components'' are
necessarily bounded regions of state space which depend on the observational
timescale, and that the system continually revisits previously traversed
regions of state space. We examine our results in the context of several
experiments, and discuss some implications of our results for the dynamics of
disordered and/or complex systems.
| Adaptation and Self-Organizing Systems | Physics |
adap-org/9909003 | Viva | Analysis of the optimality principles responsible for vascular network
architectonics | adap-org nlin.AO q-bio | The equivalence of two optimality principles leading to Murray's law has been
discussed. The first approach is based on minimization of biological work
needed for maintaining the blood flow through the vessels at required level.
The second one is the principle of minimal drag and lumen volume.
Characteristic features of these principles are considered. An alternative
approach leading to Murray's law has been proposed. For that we model the
microcirculatory bed in terms of delivering vascular network with symmetrical
bifurcation nodes, embedded uniformly into the cellular tissue. It was shown
that Murray's law can be regarded as a direct consequence of the organism
capacity for controlling the blood flow redistribution over the
microcirculatory beds.
| Adaptation and Self-Organizing Systems | Physics |
alg-geom/9608006 | David R. Morrison | The Geometry Underlying Mirror Symmetry | alg-geom hep-th math.AG | The recent result of Strominger, Yau and Zaslow relating mirror symmetry to
the quantum field theory notion of T-duality is reinterpreted as providing a
way of geometrically characterizing which Calabi-Yau manifolds have mirror
partners. The geometric description---that one Calabi-Yau manifold should serve
as a compactified, complexified moduli space for special Lagrangian tori on the
other Calabi-Yau manifold---is rather surprising. We formulate some precise
mathematical conjectures concerning how these moduli spaces are to be
compactified and complexified, as well as a definition of geometric mirror
pairs (in arbitrary dimension) which is independent of those conjectures. We
investigate how this new geometric description ought to be related to the
mathematical statements which have previously been extracted from mirror
symmetry. In particular, we discuss how the moduli spaces of the `mirror'
Calabi-Yau manifolds should be related to one another, and how appropriate
subspaces of the homology groups of those manifolds could be related. We treat
the case of K3 surfaces in some detail.
| High Energy Physics - Theory, Algebraic Geometry | Physics |
alg-geom/9708020 | Gunnar Floystad | A property deducible from the generic initial ideal | alg-geom math.AC math.AG | Let $S_d$ be the vector space of monomials of degree $d$ in the variables
$x_1, ..., x_s$. For a subspace $V \sus S_d$ which is in general coordinates,
consider the subspace $\gin V \sus S_d$ generated by initial monomials of
polynomials in $V$ for the revlex order. We address the question of what
properties of $V$ may be deduced from $\gin V$. % This is an approach for
understanding what algebraic or geometric properties of a homogeneous ideal $I
\sus k[x_1, ..., x_s]$ that may be deduced from its generic initial ideal $\gin
I$.
| Commutative Algebra, Algebraic Geometry | Mathematics |
alg-geom/9702005 | Marco Andreatta | Moishezon Manifolds | alg-geom math.AG | Let X be a compact Moishezon manifold which becomes projective after blowing
up a smooth subvariety $Y \subset X$. We assume also that there exists a proper
map $\rho :X \to X'$ onto a projective variety X' with $\rho(Y)$ a point, such
that $Pic(X/X') = \Z$ and $K_X$ is $\rho$-big. We prove some inequalities
between the dimensions of Y and X and we construct examples which shows the
optimality of the inequalities. Then we discuss some differential geometry
properties of these examples which lead to a conjecture.
| Algebraic Geometry | Mathematics |
alg-geom/9211001 | Daniel Huybrechts | Stable pairs on curves and surfaces | alg-geom math.AG | We describe stability conditions for pairs consisting of a coherent sheaf and
a homomorphism to a fixed coherent sheaf on a projective variety. The
corresponding moduli spaces are constructed for pairs on curves and surfaces.
We consider two examples. The fixed sheaf is the structure sheaf or is a vector
bundle on a divisor, i.e. Higgs pairs or framed bundles, resp. (unencoded
version)
| Algebraic Geometry | Mathematics |
alg-geom/9512015 | Dan Abrmovich | Lang maps and Harris's conjecture - a note in search for content | alg-geom math.AG | The Lang map, namely the universal dominant rational map to a variety of
general type, is constructed and briefly discussed in relation with arithmetic
conjectures of Harris, Lang and Manin. Existence of the Lang map follows from
the additivity of Kodaira dimension, but the fine structure depends on
conjectures on birational classification of algebraic varieties. Serious
applications of the Lang map are still being searched.
| Algebraic Geometry | Mathematics |
alg-geom/9502001 | null | A sextic surface cannot have 66 nodes | alg-geom math.AG | Let S be a surface in complex projective 3-space, having only nodes as
singularities. Suppose that S has degree 6. We show that the maximum number of
nodes which S can have is 65.
An abbreviated history of this is as follows. Basset showed that S can have
at most 66 nodes. Catanese and Ceresa and Stagnaro constructed sextic surfaces
having 64 nodes. Barth has recently exhibited a 65 node sextic surface. We
complete the story by showing that S cannot have 66 nodes.
Let f: S~ --> S be a minimal resolution of singularities. A set N of nodes on
S is even if there exists a divisor Q on S~ such that 2Q ~ f^{-1}(N). We show
that a nonempty even set of nodes on S must have size 24, 32, 40, 56, or 64.
This result is key to showing the nonexistence of the 66 node sextic.
We do not know if a sextic surface can have an even node set of size 56 or
64. The existence or nonexistence of large even node sets is related to the
following vanishing problem. Let S be a normal surface of degree s in CP^3. Let
D be a Weil divisor on S such that D is Q-rationally equivalent to rH, for some
r \in \Q. Under what circumstances do we have H^1(O_S(D)) = 0? For instance,
this holds when r < 0. For s=4 and r=0, H^1 can be nonzero. For s=6 and r=0, if
a 56 or 64 node even set exists, then H^1 can be nonzero. The vanishing of H^1
is also related to linear normality, quadric normality, etc. of set-theoretic
complete intersections in P^3.
| Algebraic Geometry | Mathematics |
alg-geom/9503012 | P. G. J. Nijsse | The irreducibility of the moduli space of stable vector bundles of rank
2 on a quintic in $\pp^3$ | alg-geom math.AG | In this paper I consider a quintic surface in $\pp^3$, general in the sense
of Noether-Lefschetz theory. The vector bundles of rank 2 on this surface which
are $\mu$-stable with respect to the hyperplane section and have $c_1 = K$, the
canonical class of the surface and fixed $c_2$, are parametrized by a moduli
space. This space is known to be irreducible for large $c_2$ (work of K.G.
O'Grady). I give an explicit bound, namely $c_2 \geq 16$.
| Algebraic Geometry | Mathematics |
alg-geom/9705023 | tomohide Teraoms | Hodge and Tate conjectures for hypergeometric sheaves | alg-geom math.AG | A constructible sheaf corresponding to Gel'fand Zelevinski hypergeometric
functions on a torus is called hypergeometric sheaf. We consider Hodge and Tate
conjectrue for hypergeomtric sheaves. Hodge conjecture is formulated in terms
of variation of Hodge strucure and Tate conjecture is done for l-adic sheaves
on an open set of torus. We prove Hodge and Tate conjecture up to Hodge and
Tate cycle of Fermat motifes. We use cohomological Mellin transform to get the
main theorem. This is the final revision for preprint.
| Algebraic Geometry | Mathematics |
alg-geom/9711021 | Gerard Laumon | A geometric approach to the fundamental lemma for unitary groups | alg-geom math.AG | We consider from a geometric point of view the conjectural fundamental lemma
of Langlands and Shelstad for unitary groups over a local field of positive
characteristic. We introduce projective algebraic varieties over the finite
residue field $k$ and interpret the conjecture in this case as a remarkable
identity between the number of $k$-rational points of them. We prove the
corresponding identity for the numbers of $k_f$-rational points, for any
extension of even degree $f$ of $k$. The proof uses the local intersection
theory on a regular surface and Deligne's theory of intersection multiplicities
with weights. We also discuss a possible descent argument that uses $\ell$-adic
cohomology to treat extensions of odd degree as well.
| Algebraic Geometry | Mathematics |
alg-geom/9510015 | Steven L. Kleiman | Rational curves of degree at most 9 on a general quintic threefold | alg-geom math.AG | We prove the following form of the Clemens conjecture in low degree. Let
$d\le9$, and let $F$ be a general quintic threefold in $\IP^4$. Then (1)~the
Hilbert scheme of rational, smooth and irreducible curves of degree $d$ on $F$
is finite, nonempty, and reduced; moreover, each curve is embedded in $F$ with
normal bundle $\O(-1)\oplus\O(-1)$, and in $\IP^4$ with maximal rank. (2)~On
$F$, there are no rational, singular, reduced and irreducible curves of degree
$d$, except for the 17,601,000 six-nodal plane quintics (found by Vainsencher).
(3)~On $F$, there are no connected, reduced and reducible curves of degree $d$
with rational components.
| Algebraic Geometry | Mathematics |
alg-geom/9510007 | Yekutieli Amnon | Smooth Formal Embeddings and the Residue Complex | alg-geom math.AG | Let \pi : X -> S be a finite type morphism of noetherian schemes. A smooth
formal embedding of X (over S) is a bijective closed immersion X -> \frak{X},
where \frak{X} is a noetherian formal scheme, formally smooth over S. An
example of such an embedding is the formal completion \frak{X} = Y_{/X} where X
\subset Y is an algebraic embedding. Smooth formal embeddings can be used to
calculate algebraic De Rham (co)homology. Our main application is an explicit
construction of the Grothendieck residue complex when S is a regular scheme. By
definition the residue complex is the Cousin complex of \pi^{!} \cal{O}_{S}. We
start with Huang's theory of pseudofunctors on modules with 0-dimensional
support, which provides a graded sheaf \cal{K}^{.}_{X/S}. We then use smooth
formal embeddings to obtain the coboundary operator on \cal{K}^{.}_{X / S}. We
exhibit a canonical isomorphism between the complex (\cal{K}^{.}_{X/S}, \delta)
and the residue complex of Grothendieck. When \pi is equidimensional of
dimension n and generically smooth we show that H^{-n} \cal{K}^{.}_{X/S} is
canonically isomorphic to the sheaf of regular differentials of Kunz-Waldi.
Another issue we discuss is Grothendieck Duality on a noetherian formal scheme
\frak{X}. Our results on duality are used in the construction of
\cal{K}^{.}_{X/S}.
| Algebraic Geometry | Mathematics |
alg-geom/9210004 | Olivier Debarre | Points of Low Degree on Smooth Plane Curves | alg-geom math.AG | The purpose of this note is to provide some applications of Faltings' recent
proof of S. Lang's conjecture to smooth plane curves. Let $C$ be a smooth plane
curve defined by an equation of degree $d$ with integral coefficients. We show
that for $d\ge 7$, the curve $C$ has only finitely many points whose field of
definition has degree $\le d-2$ over $Q$, and that for $d\ge 8$, all but
finitely many points of $C$ whose field of definition has degree $\le d-1$ over
$Q$ arise as points of intersection of rational lines through rational points
of $C$.
| Algebraic Geometry | Mathematics |
alg-geom/9403015 | Richard Hain | Torelli Groups and Geometry of Moduli Spaces of Curves | alg-geom math.AG | In this paper we give an exposition of Dennis Johnson's work on the first
homology of the Torelli groups and show how it can be applied, alone and in
concert with Saito's theory of Hodge modules, to study the geometry of moduli
spaces of curves. For example, we show that the picard groups of moduli spaces
of curves with a fixed level structure are finitely generated, classify all
"natural" normal functions defined over moduli spaces of curves with a fixed
level, and also "compute" the height paring between cycles over moduli spaces
of curves which are homologically trivial and disjoint over the generic point.
Several new sections have been added. These apply the results on normal
functions to prove generalizations of the classical Franchetta conjecture for
curves and abelian varieties. In one section, the monodromy group of nth roots
of the canonical bundle is computed.
| Algebraic Geometry | Mathematics |
alg-geom/9202027 | Kapil and Sudeshna | Cohomological and Cycle-theoretic connectivity | alg-geom math.AG | One of the themes in algebraic geometry is the study of the relation between
the ``topology'' of a smooth projective variety and a (``general'') hyperplane
section. Recent results of Nori produce cohomological evidence for a conjecture
that a general hypersurface of sufficently large degree should have no
``interesting'' cycles. We compute precise bounds for these results and show by
example that there are indeed interesting cycles for degrees that are not high
enough. In a different direction Esnault, Nori and Srinivas have shown
connectivity for intersections of small multidegree. We show analogous
cycle-theoretic connectivity results.
| Algebraic Geometry | Mathematics |
alg-geom/9611006 | Harry Tamvakis | Arithmetic Intersection Theory on Flag Varieties | alg-geom math.AG | Let F be the complete flag variety over Spec(Z) with the tautological
filtration 0 \subset E_1 \subset E_2 \subset ... \subset E_n=E of the trivial
bundle E over F. The trivial hermitian metric on E(\C) induces metrics on the
quotient line bundles L_i(\C). Let \hat{c}_1(L_i) be the first Chern class of
L_i in the arithmetic Chow ring \hat{CH}(F) and x_i = -\hat{c}_1(L_i). Let
h(X_1,...,X_n) be a polynomial with integral coefficients in the ideal
<e_1,...,e_n> generated by the elementary symmetric polynomials e_i. We give an
effective algorithm for computing the arithmetic intersection h(x_1,...,x_n) in
\hat{CH}(F), as the class of a SU(n)-invariant differential form on F(\C). In
particular we show that all the arithmetic Chern numbers one obtains are
rational numbers. The results are true for partial flag varieties and
generalize those of Maillot for grassmannians. An `arithmetic Schubert
calculus' is established for an `invariant arithmetic Chow ring' which
specializes to the Arakelov Chow ring in the grassmannian case.
| Algebraic Geometry | Mathematics |
alg-geom/9503022 | Luca Barbieri-Viale | Roitman's theorem for singular complex projective surfaces | alg-geom math.AG | Let $X$ be a complex projective surface with arbitrary singularities. We
construct a generalized Abel--Jacobi map $A_0(X)\to J^2(X)$ and show that it is
an isomorphism on torsion subgroups. Here $A_0(X)$ is the appropriate Chow
group of smooth 0-cycles of degree 0 on $X$, and $J^2(X)$ is the intermediate
Jacobian associated with the mixed Hodge structure on $H^3(X)$. Our result
generalizes a theorem of Roitman for smooth surfaces: if $X$ is smooth then the
torsion in the usual Chow group $A_0(X)$ is isomorphic to the torsion in the
usual Albanese variety $J^2(X)\cong Alb(X)$ by the classical Abel-Jacobi map.
| Algebraic Geometry | Mathematics |
alg-geom/9505030 | Ziv Ran | How to tell you're hearing a Calabi-Yau: Universal variations of Hodge
structure and local Schottky relations for Calabi-Yau manifolds | alg-geom math.AG | A revised version with a number of corrections and refinements.
| Algebraic Geometry | Mathematics |
alg-geom/9412011 | Gerard van der Geer | Quadratic forms, generalized Hamming weights of codes and curves with
many points | alg-geom math.AG | We use the relations between quadrics, trace codes and algebraic curves to
construct algebraic curves over finite fields with many points and to compute
generalized Hamming weights of codes.
| Algebraic Geometry | Mathematics |
alg-geom/9610023 | Fernando Torres | On maximal curves | alg-geom math.AG | We study arithmetical and geometrical properties of maximal curves, that is,
curves defined over the finite field F_{q^2} whose number of F_{q^2}-rational
points reaches the Hasse-Weil upper bound. Under a hypothesis on non-gaps at a
rational point, we prove that maximal curves are F_{q^2}-isomorphic to y^q + y
= x^m, for some $m \in Z^+$. As a consequence we show that a maximal curve of
genus g=(q-1)^2/4 is F_{q^2}-isomorphic to the curve y^q + y = x^{(q+1)/2}.
| Algebraic Geometry | Mathematics |
alg-geom/9712030 | Terrence Napier | The L^2 dbar method, weak Lefschetz theorems, and the topology of Kahler
manifolds | alg-geom math.AG | A new approach to Nori's weak Lefschetz theorem is described. The new
approach, which involves the dbar-method, avoids moving arguments and gives
much stronger results. In particular, it is proved that if X and Y are
connected smooth projective varieties of positive dimension and f is a
holomorphic immersion of Y into X with ample normal bundle, then the image of
the fundamental group of Y in that of X is of finite index. This result is
obtained as a consequence of a direct generalization of Nori's theorem. The
second part concerns a new approach to the theorem of Burns which states that a
quotient of the unit ball in complex Euclidean space (of dimension at least 3)
by a discrete group of automorphisms which has a strongly pseudoconvex boundary
component has only finitely many ends. The following generalization is
obtained. If a complete Hermitian manifold X of dimension at least 3 has a
strongly pseudoconvex end E and the Ricci curvature of X is bounded above by a
negative constant, then, away from E, X has finite volume.
| Algebraic Geometry | Mathematics |
alg-geom/9608011 | Pandharipande Rahul | Notes on stable maps and quantum cohomology | alg-geom math.AG | These are notes from a jointly taught class at the University of Chicago and
lectures by the first author in Santa Cruz. Topics covered include:
construction of moduli spaces of stable maps, Gromov-Witten invariants, quantum
cohomology, and examples. These notes will appear in the proceedings of the
1995 Santa Cruz conference.
| Algebraic Geometry | Mathematics |
alg-geom/9606009 | Francisco Jose Plaza Martin | The algebraic formalism of soliton equations over arbitrary base fields | alg-geom math.AG | The aim of this paper is to offer an algebraic construction of
infinite-dimensional Grassmannians and determinant bundles (and therefore valid
for arbitrary base fields). As an application we construct the $\tau$-function
and formal Baker-Akhiezer functions over arbitrary fields, by proving the
existence of a ``formal geometry'' of local curves analogous to the geometry of
global algebraic curves.
We begin by defining the functor of points, $\fu{\gr}(V,V^+)$, of the
Grassmannian of a $k$-vector space $V$ in such a way that its rational points
are precisely the points of the Grassmannian defined by Segal-Wilson, although
the points over an arbitrary $k$-scheme $S$ have been not previously
considered. This definition of the functor $\fu{\gr}(V,V^+)$ allows us to prove
that it is representable by a separated $k$-scheme $\gr(V,V^+)$. Using the
theory of determinants of Knudsen and Mumford, the determinant bundle is
constructed. This is one of the main results of the paper because it implies
that we can define ``infinite determinants'' in a completely algebraic way.
| Algebraic Geometry | Mathematics |
alg-geom/9603002 | Alice Silverberg | Connectedness extensions for abelian varieties | alg-geom math.AG | Suppose $A$ is an abelian variety over a field $F$, and $\ell$ is a prime not
equal to the characteristic of $F$. Let $F_{\Phi,\ell}(A)$ denote the smallest
extension of $F$ such that the Zariski closure of the image of the $\ell$-adic
representation associated to $A$ is connected. Serre introduced this field, and
proved that when $F$ is a finitely generated extension of ${\mathbf Q}$,
$F_{\Phi,\ell}(A)$ does not depend on the choice of $\ell$. In this paper we
study extensions $F_{\Phi,\ell}(B)/F$ for twists $B$ of a given abelian
variety, especially when the abelian varieties are of Weil type.
| Algebraic Geometry | Mathematics |
0710.4039 | Mariateresa Fiocchi | SAXJ1712.6-3739: a persistent hard X-ray source as monitored with
INTEGRAL | astro-ph | The X-ray source SAXJ1712.6-3739 is a very weak Low Mass X-ray Binary
discovered in 1999 with BeppoSAX and located in the Galactic Center. This
region has been deeply investigated by the INTEGRAL satellite with an
unprecedented exposure time, giving us an unique opportunity to study the hard
X-ray behavior also for weak objects. The spectral results are based on the
systematic analysis of all INTEGRAL observations covering the source position
performed between February 2003 and October 2006. SAXJ1712.6-3739 did not shows
any flux variation along this period as well as compared to previous BeppoSAX
observation. Hence, to better constrain the physical parameters we combined
both instrument data. Long INTEGRAL monitoring reveals, for the first time,
that this X-ray burster is a weak persistent source, displaying a X-ray
spectrum extended to high energy and spending most of the time in a low
luminosity hard state. The broad-band spectrum is well modeled with a simple
Comptonized model with a seed photons temperature of ~0.5keV and an electron
temperature of ~24keV. The low mass accretion rate (~2x10^{-10} Msun/yr), the
long bursts recurrence time, the small sizes of the region emitting the seed
photons consisting with the inner disk radius and the high luminosity ratio in
the 40-100keV and 20-40keV band, are all features common to the Ultra Compact
source class.
| No Label | No Label |
astro-ph/0201403 | David R. Ballantyne | On the hard X-ray spectra of radio-loud active galaxies | astro-ph | Over the last few years X-ray observations of broad-line radio galaxies
(BLRGs) by ASCA, RXTE and BeppoSAX have shown that these objects seem to
exhibit weaker X-ray reflection features (such as the iron K\alpha line) than
radio-quiet Seyferts. This has lead to speculation that the optically-thick
accretion disc in radio-loud active galactic nuclei (AGN) may be truncated to
an optically-thin flow in the inner regions of the source. Here, we propose
that the weak reflection features are a result of reprocessing in an ionized
accretion disc. This would alleviate the need for a change in accretion
geometry in these sources. Calculations of reflection spectra from an ionized
disc for situations expected in radio-loud AGN (high accretion rate,
moderate-to-high black hole mass) predict weak reprocessing features. This idea
was tested by fitting the ASCA spectrum of the bright BLRG 3C 120 with the
constant density ionized disc models of Ross & Fabian. A good fit was found
with an ionization parameter of \xi ~4000 erg cm s^{-1} and the reflection
fraction fixed at unity. If observations of BLRGs by XMM-Newton show evidence
for ionized reflection then this would support the idea that a high accretion
rate is likely required to launch powerful radio jets.
| No Label | No Label |
astro-ph/0005460 | Michela Uslenghi | Periodic modulation of the optical counterpart of the X-ray pulsator
1WGA J1958.2+3232. A new intermediate polar | astro-ph | Time-resolved observations of a 6' x 6' field, containing the position error
boxes of the X-ray source 1WGA J1958.2+3232, were performed in June 1999 with
the 91 cm Catania telescope, equipped with a Photon Counting Intensified CCD.
The star recently proposed as the optical counterpart of the 1WGA J1958.2+3232,
exhibited a strong optical modulation with a period compatible with that seen
in X-ray (~12 min). The optical modulation was detected again in September and
October 1999. These results confirm the identification of the optical
counterpart and support the classification of 1WGA J1958.2+3232 as a
cataclysmic variable, possibly an Intermediate Polar. Modulation at period
twice as large was also found in one observation run, suggesting that the true
spin period of the White Dwarf could be 24 min rather than 12 min, thus 1WGA
J1958.2+3232 appears to be, among the IPs, the slowest rotator which exhibit
double peaked spin profile.
| No Label | No Label |
astro-ph/0111079 | Valerio Bozza | Microlensing by Compact Objects associated to Gas Clouds | astro-ph | We investigate gravitational microlensing of point-like lenses surrounded by
diffuse gas clouds. Besides gravitational bending, one must also consider
refraction and absorption phenomena. According to the cloud density, the light
curves may suffer small to large deviations from Paczynski curves, up to
complete eclipses. Moreover, the presence of the cloud endows this type of
microlensing events with a high chromaticity and absorption lines recognizable
by spectral analysis. It is possible that these objects populate the halo of
our galaxy, giving a conspicuous contribution to the fraction of the baryonic
dark matter. The required features for the extension and the mass of the cloud
to provide appreciable signatures are also met by several astrophysical
objects.
| No Label | No Label |
0712.1481 | Ganna Ivashchenko | Monitoring of FR Cnc Flaring Activity | astro-ph | Being excited by the detection of the first ever-observed optical flare in FR
Cnc, we decided to continue photometrical monitoring of this object. The
observations were carried out at Crimean Astrophysical Observatory (Crimea,
Ukraine; CrAO - hereafter) and at the Terskol Observatory (Russia, Northern
Caucasus). The obtained lightcurves are presented and discussed. No
distinguishable flares were detected that could imply that flares on FR Cnc are
very rare event.
| No Label | No Label |
astro-ph/0601309 | Grzegorz Pietrzynski | The Araucaria Project. The Distance to the Local Group Galaxy IC 1613
from Near-Infrared Photometry of Cepheid Variables | astro-ph | We have measured accurate near-infrared magnitudes in the J and K bands of 39
Cepheid variables in IC 1613 with well-determined periods and optical VI light
curves. Using the template light curve approach of Soszy{\'n}ski, Gieren and
Pietrzy{\'n}ski, accurate mean magnitudes were obtained from these data which
allowed to determine the distance to IC 1613 relative to the LMC from a
multiwavelength period-luminosity solution in the optical VI and near-IR JK
bands, with an unprecedented accuracy. Our result for the IC 1613 distance is
$(m-M)_{0} = 24.291 \pm 0.014$ (random error) mag, with an additional
systematic uncertainty smaller than 2%. From our multiwavelength approach, we
find for the total (average) reddening to the IC 1613 Cepheids $E(B-V) = 0.090
\pm 0.007$ mag,which is significantly higher than the foreground reddening of
about 0.03 mag,showing the presence of appreciable dust extinction inside the
galaxy. Our data suggest that the extinction law in IC 1613 is very similar to
the galactic one.Our distance result agrees, within the uncertainties, with two
earlier infrared Cepheid studies in this galaxy of Macri et al. (from HST data
on 4 Cepheids), and McAlary et al. (from ground-based H-band photometry of 10
Cepheids), but our result has reduced the total uncertainty on the distance to
IC 1613 (relative to the LMC) to less than 3%. With distances to nearby
galaxies from Cepheid infrared photometry at this level of accuracy, which are
currently being obtained in our Araucaria Project, it seems possible to
significantly reduce the systematic uncertainty of the Hubble constant as
derived from the HST Key Project approach, by improving the calibration of the
metallicity effect on PL relation zero points, and by improving the distance
determination to the LMC.
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astro-ph/9812184 | David M. Smith | Astrophysics with HESSI | astro-ph | In the summer of the year 2000, a NASA Small Explorer satellite, the High
Energy Solar Spectroscopic Imager (HESSI), will be launched. It will consist of
nine large, coaxial germanium detectors viewing the Sun through a set of
Rotation Modulation Collimators and will accomplish high-resolution imaging and
spectroscopy of solar flares in the x-ray and gamma-ray bands. Here we describe
some of the astrophysical observations HESSI will also perform in addition to
its solar mission.
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astro-ph/0502074 | James Herald | Discovery of NeVII in the Winds of Hot Evolved Stars | astro-ph | We show that a strong P-Cygni feature seen in the far-UV spectra of some very
hot (Teff >~ 85 kK) central stars of planetary nebulae (CSPN), which has been
previously identified as CIII lambda 977, actually originates from NeVII lambda
973. Using stellar atmospheres models, we reproduce this feature seen in the
spectra of two [WR]-PG 1159 type CSPN, Abell 78 and NGC 2371, and in one PG
1159 CSPN, K 1-16. In the latter case, our analysis suggests an enhanced neon
abundance. Strong neon features in CSPN spectra are important because an
overabundance of this element is indicative of processed material that has been
dredged up to the surface from the inter-shell region in the ``born-again''
scenario, an explanation of hydrogen-deficient CSPN. Our modeling indicates the
NeVII lambda 973 wind feature may be used to discern enhanced neon abundances
for stars showing an unsaturated P-Cygni profile, such as some PG 1159 stars.
We explore the potential of this strong feature as a wind diagnostic in stellar
atmospheres analyses for evolved objects. For the [WR]-PG 1159 objects, the
line is present as a P-Cygni line for Teff >~ 85 kK, and becomes strong for 100
<~ Teff <~ 155 kK when the neon abundance is solar, and can be significantly
strong beyond this range for higher neon abundances. When unsaturated, i.e.,
for very high Teff and/or very low mass-loss rates, it is sensitive to Mdot and
very sensitive to the neon abundance. The NeVII classification is consistent
with recent identification of this line seen in absorption in many PG 1159
spectra.
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astro-ph/9812251 | Chris Carilli | The Radio-to-Submm Spectral Index as a Redshift Indicator | astro-ph | We present models of the 1.4 GHz to 350 GHz spectral index, alpha(350/1.4),
for starburst galaxies as a function of redshift. The models include a
semi-analytic formulation, based on the well quantified radio-to-far infrared
correlation for low redshift star forming galaxies, and an empirical
formulation, based on the observed spectrum of the starburst galaxies M82 and
Arp 220. We compare the models to the observed values of alpha(350/1.4) for
starburst galaxies at low and high redshift. We find reasonable agreement
between the models and the observations, and in particular, that an observed
spectral index of alpha(350/1.4) > +0.5 indicates that the target source is
likely to be at high redshift, z > 1. The evolution of alpha(350/1.4) with
redshift is mainly due to the very steep rise in the Raleigh-Jeans portion of
the thermal dust spectrum shifting into the 350 GHz band with increasing
redshift. We also discuss situations where this relationship could be violated.
We then apply our models to examine the putative identifications of submm
sources in the Hubble Deep Field, and conclude that the submm sources reported
by Hughes et al. are likely to be at high redshifts, z > 1.5.
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astro-ph/9807290 | James C. Lombardi | Tests of Spurious Transport in Smoothed Particle Hydrodynamics | astro-ph | We have performed a series of systematic tests to evaluate the effects of
spurious transport in three-dimensional smoothed particle hydrodynamics (SPH)
calculations. Our tests investigate (i) particle diffusion, (ii) shock heating,
(iii) numerical viscosity, and (iv) angular momentum transport. The results are
useful for quantifying the accuracy of the SPH scheme, especially for problems
where shear flows or shocks are present, as well as for problems where true
hydrodynamic mixing is relevant.
We examine the different forms of artificial viscosity (AV) which have been
proposed by Monaghan, by Hernquist & Katz, and by Balsara. For each form, our
tests suggest a single set of values for the AV parameters $\alpha$ and $\beta$
(coefficients of the linear and quadratic terms) which are appropriate in a
large number of situations. We also discuss how these parameters should be
adjusted depending on the goals of the particular application. We find that
both the Hernquist & Katz and Balsara forms introduce relatively small amounts
of numerical viscosity. Furthermore, both Monaghan's and Balsara's AV do well
at treating shocks and at limiting the amount of spurious mixing. For these
reasons, we endorse the Balsara AV for use in a broad range of applications.
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astro-ph/0508309 | Parviz Ghavamian | Exploring the Kinematics of the Oxygen-Rich Supernova Remnant
G292.0+1.8: Ejecta Shells, Fast-Moving Knots and Shocked Circumstellar
Material | astro-ph | We present results of an in-depth optical study of the core collapse
supernova remnant G292.0+1.8 using the Rutgers Fabry-Perot (RFP) imaging
spectrometer. Our observations provide a detailed picture of the supernova
remnant in the emission lines of [O III] 5007, Halpha and [N II] 6548. The [O
III] Fabry-Perot scans reveal a bright crescent-shaped spur of previously known
high-velocity (V_radial ~ 1500 km/s) O-rich ejecta located on the eastern side
of the remnant. The spur consists of a semi-coherent structure of mostly
redshifted material, along with several clumps that have apparently broken out
of the more orderly shell-like expansion. The high velocity (>= 600 km/s)
component of the spur also displays a scalloped morphology characteristic of
Rayleigh-Taylor instabilities. We also find a large number of fast-moving knots
(FMKs) of O-rich ejecta undetected in prior photographic plate images and
similar to features seen in Cas A. The position-velocity distribution of the
FMKs can be kinematically described as a shell 3.4' in radius expanding at a
velocity of 1700 km/s. Another feature apparent in the [O III] scans is an
equatorial belt consisting of both a bar-like structure at zero radial velocity
and a clumpy, high velocity ejecta component seen in projection along the line
of sight. The bar is also detected in our Halpha RFP images at zero radial
velocity, providing further evidence that this structure is of circumstellar
origin. We find that the optical and X-ray properties of the bar are consistent
with incomplete (partially radiative) shocks in material of moderate densities.
Assuming a distance of 6 kpc for G292.0+1.8, we estimate a kinematic age of
(3000-3400) d_6 years for this remnant (Abridged).
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astro-ph/0606215 | Steven Diehl | The Hot Interstellar Medium of Normal Elliptical Galaxies. I. A Chandra
Gas Gallery and Comparison of X-ray and Optical Morphology | astro-ph | We present an X-ray analysis of 54 normal elliptical galaxies in the Chandra
archive and isolate their hot gas component from the contaminating point source
emission, allowing us to conduct, for the first time, a morphological analysis
on the gas alone. A comparison with optical images and photometry shows that
the hot gas morphology has surprisingly little in common with the shape of the
stellar distribution. We observe no correlation between optical and X-ray
ellipticities in the inner regions where stellar mass dominates over dark
matter. A shallow correlation would be expected if the gas had settled into
hydrostatic equilibrium with the gravitational potential. Instead, observed
X-ray ellipticities exceed optical ellipticities in many cases. We exclude
rotation as the dominant factor to produce the gas ellipticities. The gas
appears disturbed, and hydrostatic equilibrium is the exception rather than the
rule. Nearly all hydrostatic models can be ruled out at 99% confidence, based
of their inability to reproduce the optical-X-ray correlation and large X-ray
ellipticities. Hydrostatic models not excluded are those in which dark matter
either dominates over stellar mass inside the inner half-light radius or has a
prominently cigar-shaped distribution, both of which can be ruled out on other
grounds. We conclude that, even for rather X-ray faint elliptical galaxies, the
gas is at least so far out of equilibrium that it does not retain any
information about the shape of the potential, and that X-ray derived radial
mass profiles may be in error by factors of order unity.
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astro-ph/0402112 | Juan Lara | Deuterium and Li7 Concordance in Inhomogeneous Big Bang Nucleosynthesis
Models | astro-ph | Recent observational constraints on primodial deuterium and Li7 correspond to
different values of the baryon-to-photon ratio when applied to the Standard Big
Bang Nucleosynthesis (SBBN) model. In this article these constraints are
applied to baryon Inhomogeneous (IBBN) models. A depletion factor of 3.4
applied to the Li7 constraints will bring Li7, deuterium and He4 constraints in
concordance for both the SBBN and IBBN models. A depletion factor of 6.1 will
bring concordance for the IBBN model alone.
| No Label | No Label |
astro-ph/0303034 | Kristof Petrovay | A Consistent One-Dimensional Model for the Turbulent Tachocline | astro-ph | The first consistent model for the turbulent tachocline is presented, with
the turbulent diffusivity computed within the model instead of being specified
arbitrarily. For the origin of the 3D turbulence a new mechanism is proposed.
Owing to the strongly stable stratification, the mean radial shear is stable,
while the horizontal shear is expected to drive predominantly horizontal,
quasi-2D motions in thin slabs. Here I suggest that a major source of 3D
overturning turbulent motions in the tachocline is the secondary shear
instability due to the strong, random vertical shear arising between the
uncorrelated horizontal flows in neighbouring slabs. A formula for the vertical
diffusivity due to this turbulence, equation (9), is derived and applied in a
simplified 1D model of the tachocline. It is found that Maxwell stresses due to
an oscillatory poloidal magnetic field of a few hundred gauss are able to
confine the tachocline to a thickness below 5 Mm. The integral scale of the 3D
overturning turbulence is the buoyancy scale, on the order of 10 km and its
velocity amplitude is a few m/s, yielding a vertical turbulent diffusivity on
the order of 10^8 cm^2/s.
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astro-ph/0609244 | Uma P. Vijh | Optical Emission Band Morphologies of the Red Rectangle | astro-ph | We present narrow-band images of the Red Rectangle (RR) nebula which reveal
the distinct morphologies of this intriguing nebula in different optical
emission bands. The morphology of the RR nebula in blue luminescence (BL) and
extended red emission (ERE) are almost mutually exclusive. We also present the
optical detection of the circum-binary disk of the RR in the light of the BL.
The total intensities from the two optical band emissions (BL and ERE) when
summed over the nebula are of comparable magnitude. Their spatial distributions
with respect to the embedded illumination sources lead us to suggest that they
may be attributed to different ionization stages of the same family of
carriers.
| No Label | No Label |
astro-ph/0412553 | Sven de Rijcke | Formation and evolution of dwarf elliptical galaxies I. Structural and
kinematical properties | astro-ph | This paper is the first in a series in which we present the results of an ESO
Large Program on the kinematics and internal dynamics of dwarf elliptical
galaxies (dEs). We investigate the relations between the parameters that
quantify the structure and internal dynamics of dEs such as the Faber_Jackson
relation and the Fundamental Plane (FP).
We show that the dE sequences in the various diagrams are disjunct from those
traced by bright and intermediate-luminosity elliptical galaxies and bulges of
spirals. It appears that semi-analytical models (SAMs) are able to reproduce
the position of the dEs in those diagrams.
While these findings are clearly a success for the hierarchical-merging
picture of galaxy formation, they do not necessarily invalidate the alternative
``harassment'' scenario, which posits that dEs stem from perturbed and stripped
late-type disk galaxies that entered clusters and groups of galaxies about 5
Gyr ago.
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astro-ph/0202523 | Csaba Kiss | Extending the limits of globule detection -- ISOPHOT Serendipity Survey
Observations of interstellar clouds | astro-ph | A faint $I_{\rm 170}=4$ MJysr$^{-1}$ bipolar globule was discovered with the
ISOPHOT 170 $\mu$m Serendipity Survey (ISOSS). ISOSS J 20246+6541 is a cold
($T_{\rm d}\approx 14.5$ K) FIR source without an IRAS pointsource counterpart.
In the Digitized Sky Survey B band it is seen as a 3\arcmin size bipolar
nebulosity with an average excess surface brightness of $\approx 26$
mag/$\square $\arcsec . The CO column density distribution determined by
multi-isotopic, multi-level CO measurements with the IRAM-30m telescope agrees
well with the optical appearance. An average hydrogen column density of
$\approx 10^{21}$cm$^{-2}$ was derived from both the FIR and CO data. Using a
kinematic distance estimate of 400 pc the NLTE modelling of the CO, HCO$^+$,
and CS measurements gives a peak density of $\approx 10^4$cm$^{-3}$. The
multiwavelength data characterise ISOSS 20246+6541 as a representative of a
class of globules which has not been discovered so far due to their small
angular size and low 100$\mu $m brightness. A significant overabundance of
$^{13}$CO is found $X(^{13}CO) \ge 150\times X(C^{18}O)$. This is likely due to
isotope selective chemical processes.
| No Label | No Label |
astro-ph/0405328 | Claudia V. Rodrigues | Circular polarimetry of magnetic cataclysmic variables | astro-ph | Magnetic cataclysmic variables are complex accreting binary systems with
short orbital periods. Here we present circular polarimetry of five magnetic
cataclysmic variable candidates. 1RXS J161008.0+035222, V1432 Aql, and 1RXS
J231603.9-052713 have cyclotron emission, which confirms them as AM Her
systems. Our data are consistent with zero values for the circular polarization
of 1RXS J042555.8-194534 and FIRST J102347.6+003841 imposing some constraints
to the polar classification of these objects.
| No Label | No Label |
astro-ph/9905046 | Paolo Tozzi | The connection between X-ray Clusters and Star Formation | astro-ph | The properties of X-ray clusters of galaxies can be well understood in terms
of a competition between shock heating and adiabatic compression. Strong shocks
are expected to be important for massive clusters, while adiabatic compression
is dominant for small clusters and groups. The scale of the shock/adiabatic
transition is marked by a change of slope of the L-T relation and in the global
properties of the emitting plasma. This scale is connected to star formation
processes. Two quantities are crucial: the average energy injected in the IGM
from stars and SNe, and the epoch of the energy injection. We show how these
quantities can be synthesized in terms of specific entropy, which ultimately
determines the X-ray emission from groups and clusters.
| No Label | No Label |
astro-ph/0308416 | Riccardo Scarpa | Using Globular Clusters to Test Newton?s Law of Gravityy | astro-ph | New measurements of the velocity dispersion of the globular cluster omega
Centauri allow to trace its gravitational potential down to an acceleration of
8e-9 cm/s/s. It is found that the dispersion profile remains flat well inside
the tidal radius as soon as the acceleration of gravity approaches a_0, a
result that finds its simplest explanation within the contest of Modified
Newtonian dynamics. A similar behavior is observed in the globular cluster M15
showing this is not a peculiar features of omega Centauri. This result is
surprising and suggestive of a failure of Newton's law at low accelerations.
| No Label | No Label |
astro-ph/9701045 | Judy Cohen | Blue Horizontal Branch Stars in M92 | astro-ph | We have analyzed high dispersion and high precision spectra of 5 blue
horizontal branch stars in the globular cluster M92 to establish that the
projected rotational velocity for these stars ranges from 15 to 40 \kms. This
is larger than that expected based on the rotation of their main sequence
progenitors, the spin down of rotation with age, and the conservation of
angular momentum. Possible explanations include a rapidly rotating stellar
core.
An abundance analysis of these spectra of these blue HB stars in M92 yields
the same results as have been obtained from the giants in this cluster. There
is a hint of a trend of higher abundance as the projected surface rotational
velocity increases, which could be chance and requires confirmation.
| No Label | No Label |
0812.1210 | Rodger Thompson Prof. | Alternative Data Reduction Procedures for UVES: Wavelength Calibration
and Spectrum Addition | astro-ph | This paper addresses alternative procedures to the ESO supplied pipeline
procedures for the reduction of UVES spectra of two quasar spectra to determine
the value of the fundamental constant mu = Mp/Me at early times in the
universe. The procedures utilize intermediate product images and spectra
produced by the pipeline with alternative wavelength calibration and spectrum
addition methods. Spectroscopic studies that require extreme wavelength
precision need customized wavelength calibration procedures beyond that usually
supplied by the standard data reduction pipelines. An example of such studies
is the measurement of the values of the fundamental constants at early times in
the universe. This article describes a wavelength calibration procedure for the
UV-Visual Echelle Spectrometer on the Very Large Telescope, however, it can be
extended to other spectrometers as well. The procedure described here provides
relative wavelength precision of better than 3E-7 for the long-slit
Thorium-Argon calibration lamp exposures. The gain in precision over the
pipeline wavelength calibration is almost entirely due to a more exclusive
selection of Th/Ar calibration lines.
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astro-ph/0506310 | Xin Zhang | Constraints on holographic dark energy from type Ia supernova
observations | astro-ph | In this paper, we use the type Ia supernovae data to constrain the
holographic dark energy model proposed by Li. We also apply a cosmic age test
to this analysis. We consider in this paper a spatially flat
Friedmann-Robertson-Walker Universe with matter component and holographic dark
energy component. The fit result shows that the case $c<1$ ($c=0.21$) is
favored, which implies that the holographic dark energy behaves as a
quintom-type dark energy. Furthermore, we also perform a joint analysis of
SNe+CMB+LSS to this model; the result is well improved, and still upholds the
quintom dark energy conclusion. The best fit results in our analysis are
$c=0.81$, $\Omega_m^0=0.28$, and $h=0.65$, which lead to the present equation
of state of dark energy $w_0=-1.03$ and the deceleration/acceleration
transition redshift $z_T=0.63$. Finally, an expected SNAP simulation using
$\Lambda$CDM as a fiducial model is performed on this model, and the result
shows that the holographic dark energy model takes on $c<1$ ($c=0.92$) even
though the dark energy is indeed a cosmological constant.
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astro-ph/0602185 | Simon Vaughan | X-ray reflection in the nearby Seyfert 2 galaxy NGC 1068 | astro-ph | We use the full broad-band XMM-Newton EPIC data to examine the X-ray spectrum
of the nearby Seyfert 2 galaxy NGC 1068, previously shown to be complex with
the X-ray continuum being a sum of components reflected/scattered from cold
(neutral) and warm (ionised) matter, together with associated emission line
spectra. We quantify the neutral and ionised reflectors in terms of the
luminosity of the hidden nucleus. Both are relatively weak, a result we
interpret on the Unified Seyfert Model by a near side-on view to the putative
torus, reducing the visibility of the illuminated inner surface of the torus
(the cold reflector), and part of the ionised outflow. A high inclination in
NGC 1068 also provides a natural explanation for the large (Compton-thick)
absorbing column in the line-of-sight to the nucleus. The emission line fluxes
are consistent with the strength of the neutral and ionised continuum
components, supporting the robustness of the spectral model.
| No Label | No Label |
astro-ph/9904180 | Antonio Fernandes da F. Teixeira | Cosmic crystallography: three multi-purpose functions | astro-ph | A solid sphere is considered, with a uniformly distributed infinity of
points. Two points being pseudorandomly chosen, the analytical probability
density that their separation have a given value is computed, for three types
of the underlying geometry: $E^3, H^3$ and $S^3$. Figures, graphs and
histograms to complement this short note are given.
| No Label | No Label |
astro-ph/0307339 | Steinn Sigurdsson | A Young White Dwarf Companion to Pulsar 1620-26: Evidence for Early
Planet Formation | astro-ph | The pulsar B1620-26 has two companions, one of stellar mass and one of
planetary mass. We detected the stellar companion using Hubble Space Telescope
observations. The color and magnitude of the stellar companion indicate that it
is an undermassive white dwarf ($0.34 \pm 0.04 M_{\odot}$) of age $480\pm 140$
Myr. This places a constraint on the recent history of this triple system and
supports a scenario in which the current configuration arose through a
dynamical exchange interaction in the cluster core. This implies that planets
may be relatively common in low-metallicity globular clusters and that planet
formation is more widespread and happened earlier than previously believed.
| No Label | No Label |
astro-ph/0201197 | Sebastian Wolf | Detecting planets in protoplanetary disks: A prospective study | astro-ph | We investigate the possibility to find evidence for planets in circumstellar
disks by infrared and submillimeter interferometry. We present simulations of a
circumstellar disk around a solar-type star with an embedded planet of 1
Jupiter mass. The three-dimensional (3D) density structure of the disk results
from hydrodynamical simulations. On the basis of 3D radiative transfer
simulations, images of this system were calculated. The intensity maps provide
the basis for the simulation of the interferometers VLTI (equipped with the
mid-infrared instrument MIDI) and ALMA. While MIDI/VLTI will not provide the
possibility to distinguish between disks with or without a gap on the basis of
visibility measurements, ALMA will provide the necessary basis for a direct gap
detection.
| No Label | No Label |
astro-ph/0103008 | Moshe Carmeli | Lengths of the First days of the Universe | astro-ph | The early stage of the Universe is discussed and the time lengths of its
first days are given. If we denote the Hubble time in the zero-gravity limit by
T (approximately 12.16 billion years), and T(n) denotes the length of the n-th
day, then we have the very simple relation T(n)=T/(2n-1). Hence we obtain for
the first days the following lenghts of time: T(1)=T, T(2)=T/3, T(3)=T/5, etc.
| No Label | No Label |
astro-ph/0501328 | Phil Marshall | The SNAP Strong Lens Survey | astro-ph | Basic considerations of lens detection and identification indicate that a
wide field survey of the types planned for weak lensing and Type Ia SNe with
SNAP are close to optimal for the optical detection of strong lenses. Such a
``piggy-back'' survey might be expected even pessimistically to provide a
catalogue of a few thousand new strong lenses, with the numbers dominated by
systems of faint blue galaxies lensed by foreground ellipticals. After
sketching out our strategy for detecting and measuring these galaxy lenses
using the SNAP images, we discuss some of the scientific applications of such a
large sample of gravitational lenses: in particular we comment on the partition
of information between lens structure, the source population properties and
cosmology. Understanding this partitioning is key to assessing strong lens
cosmography's value as a cosmological probe.
| No Label | No Label |
astro-ph/0505254 | Ruth Lazkoz | Duality extended Chaplygin cosmologies with a big rip | astro-ph | We consider modifications to the Friedmann equation motivated by recent
proposals along these lines pursuing an explanation to the observed late time
acceleration. Here we show those modifications can be framed within a theory
with self-interacting gravity, where the term self-interaction refers here to
the presence of functions of $\rho$ and $p$ in the right hand side of the
Einstein equations. We then discuss the construction of the duals of the
cosmologies generated within that framework. After that we investigate the
modifications required to generate generalized and modified Chaplygin
cosmologies and show that their duals belong to a larger family of cosmologies
we call extended Chaplygin cosmologies. Finally, by letting the parameters of
those models take values not earlier considered in the literature we show some
representatives of that family of cosmologies display sudden future
singularities, which indicates their behavior is rather different from
generalized or modified Chaplygin gas cosmologies. This reinforces the idea
that modifications of gravity can be responsible for unexpected evolutionary
features in the universe.
| No Label | No Label |
astro-ph/0401613 | Manukovskij Konstantin Viktorovich | A Rotating Collapsar and Possible Interpretation of the LSD Neutrino
Signal from SN 1987A | astro-ph | We consider an improved rotational mechanism of the explosion of a collapsing
supernova. We show that this mechanism leads to two-stage collapse with a phase
difference of \sim 5 h. Based on this model, we attempt a new interpretation of
the events in underground neutrino detectors on February 23, 1987, related to
the supernova SN 1987A.
| No Label | No Label |
astro-ph/0511813 | Ashley Ruiter | The Nature of the Faint Chandra X-ray Sources in the Galactic Centre | astro-ph | Recent Chandra observations have revealed a large population of faint X-ray
point sources in the Galactic Centre. The observed population consists of about
2000 faint sources in the luminosity range ~10^31-10^33 erg/s. The majority of
these sources (70%) are described by hard spectra, while the rest are rather
soft. The nature of these sources still remains unknown. Belczynski & Taam
(2004) demonstrated that X-ray binaries with neutron star or black hole
accretors may account for most of the soft sources, but are not numerous enough
to account for the observed number and X-ray properties of the faint hard
sources. A population synthesis calculation of the Galactic Centre region has
been carried out. Our results indicate that the numbers and X-ray luminosities
of intermediate polars are consistent with the observed faint hard Galactic
Centre population.
| No Label | No Label |
astro-ph/9812024 | Kevin Hurley | The Ulysses Supplement to the BATSE 4Br Catalog of Cosmic Gamma-Ray
Bursts | astro-ph | We present Interplanetary Network localization information for 147 gamma-ray
bursts observed by the Burst and Transient Source Experiment between the end of
the 3rd BATSE catalog and the end of the 4th BATSE catalog, obtained by
analyzing the arrival times of these bursts at the Ulysses and Compton
Gamma-Ray Observatory (CGRO) spacecraft. For any given burst observed by these
two spacecraft, arrival time analysis (or "triangulation") results in an
annulus of possible arrival directions whose half-width varies between 7
arcseconds and 2.3 degrees, depending on the intensity and time history of the
burst, and the distance of the Ulysses spacecraft from Earth. This annulus
generally intersects the BATSE error circle, resulting in an average reduction
of the error box area of a factor of 25.
| No Label | No Label |
astro-ph/0402290 | Jeremy Dunning-Davies | Results concerning the centre of our galaxy | astro-ph | For many years it was felt that, when a star collapsed, a white dwarf
resulted if the mass of the original star was below the Chandrasekhar limit, a
neutron star if the mass was somewhat larger but still less than four or five
solar masses, but after that black holes were felt to provide the only possible
final state. The extension of this hierarchy to include the possibility of
quark, and even sub-quark, stars has been proposed and here is used to offer an
alternative explanation for the recently published photograph, credited to
Eckart and Genzel, purporting to show stars near the centre of our Galaxy
moving at very high speeds. The same basic results are used also to consider
the even more recent results of Schodel and collaborators concerning the
detailed observations of a stellar orbit very close to the centre of our
galaxy.
| No Label | No Label |
astro-ph/0609436 | Guinevere Kauffmann | Ongoing Formation of Bulges and Black Holes in the Local Universe: New
Insights from GALEX | astro-ph | We analyze a volume-limited sample of massive bulge-dominated galaxies with
data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer
(GALEX) satellite. The galaxies have central velocity dispersions greater than
100 km/s and stellar surface mass densities that lie above the value where
galaxies transition from actively star forming to passive systems. The sample
is limited to redshifts 0.03<z<0.07. At these distances, the SDSS spectra
sample the light from the bulge-dominated central regions of the galaxies. The
GALEX NUV data provide high sensitivity to low rates of global star formation
in these systems. Our sample of bulge-dominated galaxies exhibits a much larger
dispersion in NUV-r colour than in optical g-r colour. Nearly all of the
galaxies with bluer NUV-r colours are AGN. Both GALEX images and SDSS colour
profiles demonstrate that the excess UV light is associated with an extended
disk. We find that galaxies with red outer regions almost never have a young
bulge or a strong AGN. Galaxies with blue outer regions have bulges and black
holes that span a wide range in age and accretion rate. Galaxies with young
bulges and strongly accreting black holes almost always have blue outer disks.
Our suggested scenario is one in which the source of gas that builds the bulge
and black hole is a low mass reservoir of cold gas in the disk.The presence of
this gas is a necessary, but not sufficient condition for bulge and black hole
growth. Some mechanism must transport this gas inwards in a time variable way.
As the gas in the disk is converted into stars, the galaxies will turn red, but
further gas infall can bring them back into the blue NUV-r sequence.(Abridged)
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astro-ph/0203390 | Isaac Shlosman | Dark Halo Shapes and the Fate of Stellar Bars | astro-ph | We investigate the stability of trajectories in barred galaxies with mildly
triaxial halos by means of Liapunov exponents. This method is perfectly
suitable for time-dependent 3D potentials where surfaces of sections and other
simple diagnostics are not applicable. We find that when halos are
centrally-concentrated most trajectories starting near the plane containing the
bar become chaotic. Moreover, the shape of many of the remaining regular
trajectories do not match the bar density distribution, being too round.
Therefore, time-independent self-consistent solutions are highly unlikely to be
found. When the non-rotating non-axisymmetric perturbation in the potential
reaches 10%, almost all trajectories integrated are chaotic and have large
Liapunov exponents. No regular trajectories aligned with the bar have been
found. Hence, if the evolution of the density figure is directly related to the
characteristic timescale of orbital instability, bar dissolution would take
place on a timescale of few dynamical times. The slowly rotating
non-axisymmetric contribution to the potential required for the onset of
widespread chaotic behavior is remarkably small. Systems consisting of
centrally-concentrated axisymmetric halos and stellar bars thus appear to be
structurally unstable, and small (1%) deviations from perfect axisymmetry
should result in a bar dissolution on a timescale significantly smaller than
the Hubble time. Since halos found in CDM simulations of large scale structure
are both centrally-concentrated and triaxial it is unlikely that stellar bars
embedded in such halos would form and survive unless the halos are modified
during the formation of the baryonic component.
| No Label | No Label |
0806.1735 | Steen Hannestad | Are cosmological neutrinos free-streaming? | astro-ph | Precision data from cosmology suggest neutrinos stream freely and hence
interact very weakly around the epoch of recombination. We study this issue in
a simple framework where neutrinos recouple instantaneously and stop streaming
freely at a redshift z_i. The latest cosmological data imply z_i < 1500, the
exact constraint depending somewhat on the assumed prior on z_i. This bound
translates into a limit on the coupling strength between neutrinos and
majoron-like particles phi, implying tau > 1 x 10^10 s (m_2/50 meV)^3 for the
decay nu_2 -> nu_1+phi.
| No Label | No Label |
astro-ph/0601025 | Jason Harris | Spectroscopic Survey of Red Giants in the SMC. I: Kinematics | astro-ph | We present a spectroscopic survey of 2046 red giant stars, distributed over
the central 4x2 kpc of the Small Magellanic Cloud (SMC). After fitting and
removing a small velocity gradient across the SMC (7.9 km/s/deg oriented at 10
deg E of N), we measure an rms velocity scatter of 27.5+-0.5 km/s. The line of
sight velocity distribution is well-characterized by a Gaussian and the
velocity dispersion profile is nearly constant as a function of radius. We find
no kinematic evidence of tidal disturbances. Without a high-precision
measurement of the SMC's proper motion, it is not possible to constrain the
SMC's true rotation speed from our measured radial-velocity gradient. However,
even with conservative assumptions, we find that v < sigma and hence that the
SMC is primarily supported by its velocity dispersion. We find that the shape
of the SMC, as measured from the analysis of the spatial distribution of its
red giant stars, is consistent with the degree of rotational flattening
expected for the range of allowed v/sigma values. As such, the properties of
the SMC are consistent with similar low luminosity spheroidal systems. We
conclude that the SMC is primarily a low luminosity spheroid whose irregular
visual appearance is dominated by recent star formation. A simple virial
analysis using the measured kinematics implies an enclosed mass within 1.6 kpc
of between 1.4 and 1.9x10^9 Mo, and a less well constrained mass within 3 kpc
of between 2.7 and 5.1x10^9 Mo.
| No Label | No Label |
0709.0114 | Andrea Dieball | Unveiling the core of the Globular Cluster M15 in the Ultraviolet | astro-ph | We have obtained deep far- (FUV) and near-ultraviolet (NUV) images of the
inner region of the dense globular cluster M15 with the Advanced Camera for
Surveys on board the Hubble Space Telescope. The FUV-NUV colour-magnitude
diagram shows a well defined track of horizontal branch stars, as well as a
trail of blue stragglers and white dwarfs. The main sequence turn-off is
clearly visible at FUV~23.5 mag and FUV-NUV~3 mag, and the main sequence stars
form a prominent track that extends at least two magnitudes below the main
sequence turn-off. As such, this is the deepest FUV-NUV colour-magnitude
diagram of a globular cluster presented so far. Cataclysmic variable and blue
straggler candidates are the most centrally concentrated stellar populations,
which might either be an effect of mass segregation or reflect the preferred
birthplace in the dense cluster core of such dynamically-formed objects. We
find 41 FUV sources that exhibit significant variability. We classify the
variables based on an analysis of their UV colours and variability properties.
We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one
known Cepheid and six further candidates, six cataclysmic variables, one known
and one probable SX Phoenicis star, and the well known low-mass X-ray binary
AC211. Our analysis represents the first detection of SX Phoenicis pulsations
in the FUV. We find that Cepheids, RR Lyraes and SX Phoenicis exhibit massive
variability amplitudes in this waveband (several mags).
| No Label | No Label |
astro-ph/9704086 | null | A Massive Cluster of Galaxies at z = 0.996 | astro-ph | We report the identification of a cluster of galaxies around the
high-redshift radio galaxy 3CR184 at z = 0.996. The identification is supported
by an excess of galaxies observed in projection in I band images (both in
ground-based and HST data), a peak in the redshift distribution comprising 11
galaxies (out of 56 with measured redshifts) in a ~2000 km/s velocity interval,
and the observation on HST WFPC2 frames of a gravitational arc seen projected
at 42kpc away from the central radio galaxy. We thus have strong evidence for
the presence of a massive cluster at z~1.
| No Label | No Label |
astro-ph/0411172 | Sofia Feltzing | Abundance trends in the thin and thick disks | astro-ph | The Milky Way harbours two disks that appear distinct concerning
scale-heights, kinematics, and elemental abundance patterns. Recent years have
seen a surge of studies of the elemental abundance trends in the disks using
high resolution spectroscopy. Here I will review and discuss the currently
available data. Special focus will also be put on how we define stars to be
members of either disk, and how current models of galaxy formation favour that
thick disks are formed from several accreted bodies. The ability for the
stellar abundance trends to test such predictions are discussed.
| No Label | No Label |
astro-ph/0302580 | Yutaka Fujita | Infant Galaxy Clusters at Low Redshifts? | astro-ph | The population and population composition of galaxies in galaxy clusters at
present reflect the mass of the clusters and the mass growth of the galaxies in
the past. We investigate them for six clusters. We show that galaxies in
massive clusters stopped growing at redshifts of ~4. Moreover, we find that
some small galaxy clusters (groups) have too many massive galaxies for their
apparent masses. One possibility is that these groups are much more massive and
in a phase just before virialization. If this is the case, they should be
called `infant galaxy clusters' that will be matured clusters in the dynamical
time-scale (~10^9 yrs).
| No Label | No Label |
astro-ph/0409667 | Pablo Rodriguez-Gil | The role of the SW Sextantis stars in the picture of CV evolution | astro-ph | The SW Sextantis stars are novalike cataclysmic variables (CVs) which exhibit
intricate behaviour which is still a matter of debate. For years the common
belief has been that these systems were the``outlaws'' of the CVs, mainly
because of the lack of an overall understanding. We are now realising that a
large percentage (~30%) of all novalikes in the ~3-4 hour period range are SW
Sex stars. The study of the dominance of this class just above the period gap
will provide clues on the evolution of CVs. Here we present the discovery of 7
new SW Sex stars from the CVs found in the Hamburg Quasar Survey.
| No Label | No Label |
astro-ph/9808003 | Douglas Scott | Implications of SCUBA observations for the Planck Surveyor | astro-ph | We investigate the implications for the Planck Surveyor of the recent
sub-millimetre number counts obtained using the SCUBA camera. Since it observes
at the same frequency as one of the higher frequency science channels on
Planck, SCUBA can provide constraints on the point-source contribution to the
CMB angular power spectrum, which require no extrapolation in frequency. We
have calculated the two-point function of these sub-millimetre sources, using a
Poisson model normalized to the observed counts. While the current data are
uncertain, under reasonable assumptions the point-source contribution to the
anisotropy is comparable to the noise in the 353GHz channel. The clustering of
these sources is currently unknown, however if they cluster like the z~3
Lyman-break galaxies their signal would be larger than the primary anisotropy
signal on scales smaller than about 10 arcmin. We expect the intensity of these
sources to decrease for wavelengths longward of 850 microns. At the next lowest
Planck frequency, 217GHz, the contribution from both the clustered and Poisson
terms are dramatically reduced. Hence we do not expect these sources to
seriously affect Planck's main science goal, the determination of the
primordial anisotropy power spectrum. Rather, the potential determination of
the distribution of sub-mm sources is a further piece of cosmology that Planck
may be able to tackle.
| No Label | No Label |
0802.3311 | Durgesh Tripathi Dr | Density structure of an active region and associated moss using
Hinode/EIS | astro-ph | Context: Studying the problem of active region heating requires precise
measurements of physical plasma parameters such as electron density,
temperature etc. It is also important to understand the relationship of coronal
structures with the magnetic field. The Extreme-ultraviolet Imaging
Spectrometer (EIS) aboard Hinode provides a rare opportunity to derive electron
density simultaneously at different temperatures.
Aims: MethodsWe study the density structure and characterise plasma in active
regions and associated moss regions. In addition we study its relationship to
the photospheric magnetic field.
Methods: We used data recorded by the EIS, together with magnetic field
measurements from the Michelson Doppler Imager (MDI) aboard SoHO and images
recorded with the Transition Region And Coronal Explorer (TRACE) and X-Ray
Telescope (XRT/Hinode).
Results: We find that the hot core of the active region is densest with
values as high as 10^10.5 cm^-3. The electron density estimated in specific
regions in the active region moss decreases with increasing temperature. The
moss areas were located primarily on one side of the active region, and they
map the positive polarity regions almost exactly. The density within the moss
region was highest at log T=5.8-6.1, with a value around 10^(10.0-10.5) cm^-3.
The moss densities were highest in the strong positive magnetic field region.
However, there was no such correlation for the negative polarity areas, where
there was a large sunspot.
| No Label | No Label |
astro-ph/0703126 | Julia Comerford | The Observed Concentration-Mass Relation for Galaxy Clusters | astro-ph | The properties of clusters of galaxies offer key insights into the assembly
process of structure in the universe. Numerical simulations of cosmic structure
formation in a hierarchical, dark matter dominated universe suggest that galaxy
cluster concentrations, which are a measure of a halo's central density,
decrease gradually with virial mass. However, cluster observations have yet to
confirm this correlation. The slopes of the run of measured concentrations with
virial mass are often either steeper or flatter than predicted by simulations.
In this work, we present the most complete sample of observed cluster
concentrations and masses yet assembled, including new measurements for 10
strong lensing clusters, thereby more than doubling the existing number of
strong lensing concentration estimates. We fit a power law to the observed
concentrations as a function of virial mass, and find that the slope is
consistent with the slopes found in simulations, though our normalization
factor is higher. Observed lensing concentrations appear to be systematically
larger than X-ray concentrations, a more pronounced effect than found in
simulations. We also find that at fixed mass, the bulk of observed cluster
concentrations are distributed log-normally, with the exception of a few
anomalously high concentration clusters. We examine the physical processes
likely responsible for the discrepancy between lensing and X-ray
concentrations, and for the anomalously high concentrations in particular. The
forthcoming Millennium simulation results will offer the most comprehensive
comparison set to our findings of an observed concentration-mass power law
relation.
| No Label | No Label |
astro-ph/9911455 | Philipp Richter | Discovery of molecular hydrogen in a high-velocity cloud of the Galactic
halo | astro-ph | We report the discovery of molecular hydrogen absorption in a Galactic
high-velocity cloud (HVC) in the direction of the Large Magellanic Cloud. For
the same HVC we derive an iron abundance which is half of the solar value.
Thus, all evidence points to a Galactic origin for high-velocity cloud complex
in front of the LMC.
| No Label | No Label |
astro-ph/0309654 | JaeSub Hong | Instability of the 13.8 day period in X-ray emission from 4U1700-377 | astro-ph | We present a new result on long-term periodicity searches of the X-ray
emission from 4U1700-377 using RXTE ASM and CGRO BATSE data. The hard X-ray
data (20 - 200 keV) from early BATSE observations (1780 days from JD 2448370,
before RXTE observations started) show evidence of a 13.8 day periodicity. The
long term periodicity became substantially less prominent in the data from
later BATSE observations and in the soft X-ray data from the ASM (2 - 10 keV)
observations. We demonstrate that disk precession models can explain the 13.8
day period and its instability in the X-ray emission from 4U1700-377.
| No Label | No Label |
0704.1298 | Cristian Vignali | The obscured quasar population from optical, mid-infrared, and X-ray
surveys | astro-ph | Over the last few years, optical, mid-infrared and X-ray surveys have brought
to light a significant number of candidate obscured AGN and, among them, many
Type 2 quasars, the long-sought after "big cousins'' of local Seyfert 2
galaxies. However, despite the large amount of multi-wavelength data currently
available, a proper census and a panchromatic view of the obscured AGN/quasar
population are still missing, mainly due to observational limitations. Here we
provide a review of recent results on the identification of obscured AGN,
focusing primarily on the population of Type 2 quasars selected in the optical
band from the Sloan Digital Sky Survey.
| No Label | No Label |
astro-ph/0405607 | Marceau Limousin | Constraining the mass distribution of galaxies using galaxy-galaxy
lensing in clusters and in the field | astro-ph | We present a maximum-likelihood analysis of galaxy-galaxy lensing effects in
galaxy clusters and in the field. The aim is to determine the accuracy and
robustness of constraints that can be obtained on galaxy halo properties in
both environments - the high density cluster and the low density field. This
paper is theoretically motivated, therefore, we work exclusively with simulated
data (nevertheless defined to match observations) to study the accuracy with
which input parameters for mass distributions for galaxies can be extracted. We
model galaxies in the cluster and the field using a wide range of mass
profiles: the truncated pseudo isothermal elliptical mass distribution, the
Navarro, Frenk and White profile, and a Power Law model with a core radius. We
find that independent of the choice of profile the mean mass of galaxies (of
the order of 10^{12}Mo) can be estimated to within 15% from ground-based data
and with an error of less than 10% with space observations. Additionally robust
constraints can be obtained on the mean slope of the mass profile. The two
standard parameters that characterise galaxy halo models, the central velocity
dispersion and the truncation radius can also be retrieved reliably from the
maximum-likelihood analysis. Furthermore, going beyond the usual formulation,
we propose a re-parameterisation of the mass models that allows us to put yet
stronger constraints on the aperture mass of a galaxy halo (with less than 10%
error). The gain in signal to noise using space observations, expected for
instance with the proposed SNAP satellite compared to ground based data in
terms of accuracy of retrieving input parameters is highly significant.
| No Label | No Label |
astro-ph/0612197 | Sven Wedemeyer-B\"ohm | Carbon monoxide in the solar atmosphere II. Radiative cooling by CO
lines | astro-ph | The role of carbon monoxide as a cooling agent for the thermal structure of
the mid-photospheric to low-chromospheric layers of the solar atmosphere in
internetwork regions is investigated. The treatment of radiative cooling via
spectral lines of carbon monoxide (CO) has been added to the radiation
chemo-hydrodynamics code CO5BOLD. [...] The CO opacity indeed causes additional
cooling at the fronts of propagating shock waves in the chromosphere. There,
the time-dependent approach results in a higher CO number density compared to
the equilibrium case and hence in a larger net radiative cooling rate. The
average gas temperature stratification of the model atmosphere, however, is
only reduced by roughly 100 K. Also the temperature fluctuations and the CO
number density are only affected to small extent. A numerical experiment
without dynamics shows that the CO cooling process works in principle and
drives the atmosphere to a cool radiative equilibrium state. At chromospheric
heights, the radiative relaxation of the atmosphere to a cool state takes
several 1000 s. The CO cooling process thus would seem to be too slow compared
to atmospheric dynamics to be responsible for the very cool temperature regions
observed in the solar atmosphere. The hydrodynamical timescales in our solar
atmosphere model are much too short to allow for the radiative relaxation to a
cool state, thus suppressing the potential thermal instability due to carbon
monoxide as a cooling agent. Apparently, the thermal structure and dynamics of
the outer model atmosphere are instead determined primarily by shock waves.
| No Label | No Label |
astro-ph/0204271 | Jian Ge | Fixed-delay Interferometry for Doppler Extra-solar Planet Detection | astro-ph | We present a new technique based on fixed-delay interferometry for high
throughput, high precision and multi-object Doppler radial velocity (RV)
surveys for extra-solar planets. The Doppler measurements are conducted through
monitoring the stellar fringe phase shifts of the interferometer. High Doppler
sensitivity is achieved through optimizing the optical delay in the
interferometer and reducing photon noise by measuring multiple fringes over a
broadband. This broadband operation is performed through coupling the
interferometer with a low to medium resolution post-disperser. The total
Doppler sensitivity of this approach is, in theory, independent of dispersing
power of the post-disperser, which allows development of new generation RV
machines with much reduced size, high stability and low cost compared to
echelles. This technique has the potential to improve RV survey efficiency by
2-3 orders of magnitude over cross-dispersed echelle spectroscopy approach to
allow a full sky RV survey for planets once the instrument is operated as a
multi-object instrument and optimized for high throughput.
The simple interferometer response potentially allows this technique to be
operated at other wavelengths independent of popular iodine reference sources
to search for planets around early type stars, white dwarfs, and M, L and T
dwarfs for the first time.
| No Label | No Label |
astro-ph/0010484 | Gerardo Iovane | Quasar luminosity and twin effects induced by filamentary and planar
structures | astro-ph | We consider filamentary and planar large scale structures as possible
refraction channels for electromagnetic radiation coming from cosmological
structures. By this hypothesis, it is possible to explain the quasar luminosity
distribution and, in particular, the presence of "twin" and "brother" objects.
Method and details of simulation are given.
| No Label | No Label |
astro-ph/0003219 | Thomas H. Reiprich | The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray
Properties of Clusters Detected as Extended X-ray Sources | astro-ph | In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination $\delta \ge
0\deg $ and galactic latitude $|b_{II}| \ge 20\deg $ and comprises sources with
a count rate $\ge 0.06$ counts s$^{-1}$ and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...
| No Label | No Label |
astro-ph/0103432 | Christy Tremonti | Star Formation in the Field and Clusters of NGC 5253 | astro-ph | We investigate the star formation history of both the bright star clusters
and the diffuse `field star' population in the dwarf starburst galaxy NGC 5253
using STIS longslit ultraviolet spectroscopy. Our slit covers a physical area
of 370 x 1.6 pc and includes 8 apparent clusters and several inter-cluster
regions of diffuse light which we take to be the field. The diffuse light
spectrum lacks the strong O-star wind features which are clearly visible in
spectra of the brightest clusters. This discrepancy provides compelling
evidence that the diffuse light is not reflected light from nearby clusters,
but originates in a UV-bright field star population, and it raises the issue of
whether the star formation process may be operating differently in the field
than in clusters. We compare our spectra to STARBURST99 evolutionary synthesis
models which incorporate a new low metallicity atlas of O-star spectra. We
favor a scenario which accounts for the paucity of O-stars in the field without
requiring the field to have a different IMF than the clusters: stellar clusters
form continuously and then dissolve on ~10 Myr timescales and disperse their
remaining stars into the field. We consider the probable contribution of an
O-star deficient field population to the spatially unresolved spectra of high
redshift galaxies. (Abridged)
| No Label | No Label |
astro-ph/0701858 | Nigel Smith | First limits on WIMP nuclear recoil signals in ZEPLIN-II: a two phase
xenon detector for dark matter detection | astro-ph | Results are presented from the first underground data run of ZEPLIN-II, a 31
kg two phase xenon detector developed to observe nuclear recoils from
hypothetical weakly interacting massive dark matter particles. Discrimination
between nuclear recoils and background electron recoils is afforded by
recording both the scintillation and ionisation signals generated within the
liquid xenon, with the ratio of these signals being different for the two
classes of event. This ratio is calibrated for different incident species using
an AmBe neutron source and Co-60 gamma-ray sources. From our first 31 live days
of running ZEPLIN-II, the total exposure following the application of fiducial
and stability cuts was 225 kgxdays. A background population of radon progeny
events was observed in this run, arising from radon emission in the gas
purification getters, due to radon daughter ion decays on the surfaces of the
walls of the chamber. An acceptance window, defined by the neutron calibration
data, of 50% nuclear recoil acceptance between 5 keVee and 20 keVee, had an
observed count of 29 events, with a summed expectation of 28.6+/-4.3 gamma-ray
and radon progeny induced background events. These figures provide a 90% c.l.
upper limit to the number of nuclear recoils of 10.4 events in this acceptance
window, which converts to a WIMP-nucleon spin-independent cross-section with a
minimum of 6.6x10^-7 pb following the inclusion of an energy dependent,
calibrated, efficiency. A second run is currently underway in which the radon
progeny will be eliminated, thereby removing the background population, with a
projected sensitivity of 2x10^-7 pb for similar exposures as the first run.
| No Label | No Label |
0810.1897 | Veronica Sommariva | A search for spectroscopic binaries in the galactic globular cluster M4.
Based on 5973 individual spectra collected at VLT | astro-ph | We present a large multi-epoch high resolution spectroscopic investigation
for the search of binary candidates in the Galactic Globular Cluster (GGC) M4.
The aim of our work is the identification of the binary candidates, and the
determination of the binary fraction and of the binary radial distribution. We
present a large multi-epoch high resolution spectroscopic investigation for the
search of binary candidates in the Galactic Globular Cluster (GGC) M4. The aim
of our work is the identification of the binary candidates, and the
determination of the binary fraction and of the binary radial distribution. The
average radial velocity of the observed cluster members is
$70.29+/-0.07(+/-0.3)(+/-0.1)km/s. The search for variations in radial
velocities among the stars with multi-epoch observations yielded 57 binary star
candidates. Our radial velocity measurement accuracy allowed us to identify at
a 3sigma level binaries with radial velocity variations larger than ~0.3km/s
for the target stars with V<15, and larger than ~0.5km/s for the targets with
V>15. We identified 4 binary star candidates out of 97 observed targets inside
the core radius, and 53 candidates out of 2372 observed stars outside the core
radius. Accounting for the incompleteness affecting our survey, the lower limit
for the total binary fraction is f=3.0+/-0.3%. The lower limit for the binary
fraction in the cluster core is f=5.1+/-2.3%, while outside the core it
decreases to f=3.0+/-0.4%. Similarly, we found f=4.5+/-0.4% and f=1.8+/-0.6%
for the binary fraction inside and outside the half mass radius.
| No Label | No Label |
astro-ph/9905182 | Mark Seaborne | A Comparison of The PSCz and Stromlo-APM Redshift Surveys | astro-ph | We present a direct comparison of the clustering properties of two redshift
surveys covering a common volume of space: the recently completed IRAS Point
Source Catalogue redshift survey (PSCz) containing 14500 galaxies with a
limiting flux of 0.6 Jy at 60 microns, and the optical Stromlo-APM survey
containing 1787 galaxies in a region of 4300 deg^2 in the southern Galactic
cap. We use three methods to compare the clustering properties: the
counts-in-cells comparison of Efstathiou (1995), the two-point cross
correlation function, and the Tegmark (1998) `null-buster' test. We find that
the Stromlo variances are systematically higher than those of PSCz, as expected
due to the deficit of early-type galaxies in IRAS samples. However we find that
the differences between the cell counts are consistent with a linear bias
between the two surveys, with a relative bias parameter b_rel =
b_Stromlo/b_PSCz \approx 1.3 which appears approximately scale-independent. The
correlation coefficient R between optical and IRAS densities on scales \sim 20
Mpc/h is R > 0.72 at 95% c.l., placing limits on types of `stochastic bias'
which affect optical and IRAS galaxies differently.
| No Label | No Label |
astro-ph/9501073 | null | Kinetics of Conversion of Air Bubbles to Air-Hydrate Crystals in
Antarctic Ice | astro-ph | The depth-dependence of bubble concentration at pressures above the
transition to the air hydrate phase and the optical scattering length due to
bubbles in deep ice at the South Pole are modeled using diffusion-growth data
from the laboratory, taking into account the dependence of age and temperature
on depth in the ice. The model fits the available data on bubbles in cores from
Vostok and Byrd and on scattering length in deep ice at the South Pole. It
explains why bubbles and air hydrate crystals co-exist in deep ice over a range
of depths as great as 800 m and predicts that at depths below $\rm \sim$ 1400 m
the AMANDA neutrino observatory at the South Pole will operate unimpaired by
light scattering from bubbles.
| No Label | No Label |
astro-ph/0107416 | Ryan Scranton | Analysis of Systematic Effects and Statistical Uncertainties in Angular
Clustering of Galaxies from Early SDSS Data | astro-ph | The angular distribution of galaxies encodes a wealth of information about
large scale structure. Ultimately, the Sloan Digital Sky Survey (SDSS) will
record the angular positions of order 10^8 galaxies in five bands, adding
significantly to the cosmological constraints. This is the first in a series of
papers analyzing a rectangular stripe 2.5x90 degrees from early SDSS data. We
present the angular correlation function for galaxies in four separate
magnitude bins on angular scales ranging from 0.003 degrees to 15 degrees. Much
of the focus of this paper is on potential systematic effects. We show that the
final galaxy catalog -- with the mask accounting for regions of poor seeing,
reddening, bright stars, etc. -- is free from external and internal systematic
effects for galaxies brighter than r* = 22. Our estimator of the angular
correlation function includes the effects of the integral constraint and the
mask. The full covariance matrix of errors in these estimates is derived using
mock catalogs with further estimates using a number of other methods.
| No Label | No Label |
astro-ph/9707328 | Pascal Fouque | Very accurate Distances and Radii of Open Cluster Cepheids from a
Near-Infrared Surface Brightness Technique | astro-ph | We have obtained the radii and distances of 16 galactic Cepheids supposed to
be members in open clusters or associations using the new optical and
near-infrared calibrations of the surface brightness (Barnes-Evans) method
given by Fouque & Gieren (1997). We discuss in detail possible systematic
errors in our infrared solutions and conclude that the typical total
uncertainty of the infrared distance and radius of a Cepheid is about 3 percent
in both infrared solutions, provided that the data are of excellent quality and
that the amplitude of the color curve used in the solution is larger than ~0.3
mag.
We compare the adopted infrared distances of the Cepheid variables to the
ZAMS-fitting distances of their supposed host clusters and associations and
find an unweighted mean value of the distance ratio of 1.02 +- 0.04. A detailed
discussion of the individual Cepheids shows that the uncertainty of the
ZAMS-fitting distances varies considerably from cluster to cluster. We find
clear evidence that four Cepheids are not cluster members (SZ Tau, T Mon, U Car
and SV Vul) while we confirm cluster membership for V Cen and BB Sgr for which
the former evidence for cluster membership was only weak. After rejection of
non-members, we find a weighted mean distance ratio of 0.969 +- 0.014, with a
standard deviation of 0.05, which demonstrates that both distance indicators
are accurate to better than 5%, including systematic errors, and that there is
excellent agreement between both distance scales.
| No Label | No Label |
0801.3738 | Richard I. Davies | Laser Guide Star Adaptive Optics without Tip-tilt | astro-ph | Adaptive optics (AO) systems allow a telescope to reach its diffraction limit
at near infrared wavelengths. But to achieve this, a bright natural guide star
(NGS) is needed for the wavefront sensing, severely limiting the fraction of
the sky over which AO can be used. To some extent this can be overcome with a
laser guide star (LGS). While the laser can be pointed anywhere in the sky, one
still needs to have a natural star, albeit fainter, reasonably close to correct
the image motion (tip-tilt) to which laser guide stars are insensitive. There
are in fact many astronomical targets without suitable tip-tilt stars, but for
which the enhanced resolution obtained with the Laser Guide Star Facility
(LGSF) would still be very beneficial. This article explores what adaptive
optics performance one might expect if one dispenses with the tip-tilt star,
and in what situations this mode of observing might be needed.
| No Label | No Label |
astro-ph/0103265 | Konishchev K. V. | Extragalactic neutrino background from PBHs evaporation | astro-ph | We calculated the energy spectra and the fluxes of electron neutrinos in
extragalactic space emitted in the process of the evaporation of primordial
black holes (PBHs) in the early universe. It was assumed that PBHs are formed
by a blue power-law spectrum of primordial density fluctuations. In the
calculations of neutrino spectra the spectral index of density fluctuations and
the reheating temperature were used as free parameters. The absorption of
neutrinos during propagation in the space was taken into account. We obtained
the bounds on the spectral index assuming validity of the standard picture of
gravitational collapse and using the available data of several experiments with
atmospheric and solar neutrinos. The comparison of our results with the
previous constraints (which had been obtained using diffuse photon background
data) shows that such bounds are quite sensitive to an assumed form of the
initial PBH mass function.
| No Label | No Label |
astro-ph/0410692 | Boris Sbarufatti | ESO-VLT optical spectroscopy of BL Lac objects: I. new redshifts | astro-ph | We report redshift measurements for 12 BL Lacertae objects from a program
aimed at obtaining high signal to noise (up to ~ 500) optical spectroscopy of a
mixed sample of objects. The new observations, gathered with the 8 m ESO Very
Large Telescope, allowed us to detect weak spectral features down to a line
equivalent width as small as ~ 1 Angstrom. The new redshifts fall in the
0.2-1.3 interval. For nine objects we observe emission lines from the active
nucleus. In the remaining three cases absorption lines from the host galaxy are
found. For two objects we also detect absorption lines from intervening
systems.
| No Label | No Label |
0712.1763 | Manuel Guedel | The Sun in Time: Activity and Environment | astro-ph | (abridged) The Sun's magnetic activity has steadily declined during its
main-sequence life. While the solar photospheric luminosity was about 30% lower
4.6 Gyr ago when the Sun arrived on the main sequence compared to present-day
levels, its faster rotation generated enhanced magnetic activity; magnetic
heating processes in the chromosphere, the transition region, and the corona
induced ultraviolet, extreme-ultraviolet, and X-ray emission about 10, 100, and
1000 times, respectively, the present-day levels, as inferred from young
solar-analog stars. Also, the production rate of accelerated, high-energy
particles was orders of magnitude higher than in present-day solar flares, and
a much stronger wind escaped from the Sun, permeating the entire solar system.
The consequences of the enhanced radiation and particle fluxes from the young
Sun were potentially severe for the evolution of solar-system planets and
moons. Interactions of high-energy radiation and the solar wind with upper
planetary atmospheres may have led to the escape of important amounts of
atmospheric constituents. The present dry atmosphere of Venus and the thin
atmosphere of Mars may be a product of early irradiation and heating by solar
high-energy radiation. High levels of magnetic activity are also inferred for
the pre-main sequence Sun. At those stages, interactions of high-energy
radiation and particles with the circumsolar disk in which planets eventually
formed were important. Traces left in meteorites by energetic particles and
anomalous isotopic abundance ratios in meteoritic inclusions may provide
evidence for a highly active pre-main sequence Sun. The present article reviews
these various issues related to the magnetic activity of the young Sun and the
consequent interactions with its environment.
| No Label | No Label |
astro-ph/0703410 | Evgeny Malogolovets Mr. | Nearby low-mass triple system GJ795 | astro-ph | We report the results of our optical speckle-interferometric observations of
the nearby triple system GJ795 performed with the 6-m BTA telescope with
diffraction-limited angular resolution. The three components of the system were
optically resolved for the first time. Position measurements allowed us to
determine the elements of the inner orbit of the triple system. We use the
measured magnitude differences to estimate the absolute magnitudes and spectral
types of the components of the triple: $M_{V}^{Aa}$=7.31$\pm$0.08,
$M_{V}^{Ab}$=8.66$\pm$0.10, $M_{V}^{B}$=8.42$\pm$0.10, $Sp_{Aa}$ $\approx$K5,
$Sp_{Ab}$ $\approx$K9, $Sp_{B}$ $\approx$K8. The total mass of the system is
equal to $\Sigma\mathcal{M}_{AB}$=1.69$\pm0.27\mathcal{M}_{\odot}$. We show
GJ795 to be a hierarchical triple system which satisfies the empirical
stability criteria.
| No Label | No Label |
astro-ph/0106117 | Jean-Louis Rouet | Influence of Expansion on Hierarchical Structure | astro-ph | We study a one dimensional model of gravitational instability in an
Einstein-de Sitter universe. Scaling in both space and time results in an
autonomous set of coupled Poisson-Vlasov equations for the field and phase
space density, and the $N$-body problem. Using dynamical simulation, we find
direct evidence of hierarchical clustering. A multi-fractal analysis reveals a
bifractal geometry similar to that observed in the distribution of galaxies. To
demonstrate the role of scaling, we compare the system to other one dimensional
models recently employed to study structure formation. Finally we show that the
model yields an estimate of the time of galaxy formation of the correct order.
| No Label | No Label |
astro-ph/9809200 | Caroline Terquem | Planet formation, orbital evolution and planet-star tidal interaction | astro-ph | We consider several processes operating during the late stages of planet
formation that can affect observed orbital elements. Disk-planet interactions,
tidal interactions with the central star, long term orbital instability and the
Kozai mechanism are discussed.
| No Label | No Label |
astro-ph/0308032 | Leon Brian Lucy | The formation of helium lines in the spectrum of COM J1740-5340 | astro-ph | The He I 5876A absorption line recently discovered in the spectrum of the
companion to the millisecond pulsar PSR J1740-5340 is tentatively attributed to
electron impact excitations due to the irradiation of its atmosphere by
gamma-rays emitted by the pulsar's magnetosphere. Numerical calculations,
similar to those carried out previously for Type Ib SNe, indicate that a pulsar
beam with photon energies ~ 1 MeV gives rise to a 5876A line of the observed
strength if the beam's spin-down conversion efficiency approaches 1%. However,
a significant difficulty for the proposed mechanism is the strength of the
singlet line at 6678A. Compared to the corresponding triplets, singlet lines
are weak because of the loss of excitation when photons emitted in decays to
the ground state ionize hydrogen atoms, an effect absent in the hydrogen-free
atmospheres of Type Ib SNe.
| No Label | No Label |
astro-ph/0105304 | Jochen Greiner | The X-ray spectrum of LSI+61o303 | astro-ph | It had been proposed earlier that the hard X-ray and gamma-ray radiation of
the Be/X-ray system LSI+61o303 could be due to inverse Compton scattering of
optical photons from the Be star by the same electron population which also
produces the radio emission. Recently, Apparao (2001) has calculated this
inverse Compton emission in more detail, and predicted that the X-ray spectrum
should show a break at around 20 keV. We investigated archival RXTE data, but
do not find such a break in the 2--25 keV range. The implications of this
finding are shortly discussed.
| No Label | No Label |
astro-ph/0204121 | Mirt Gramann | Dynamical state of superclusters of galaxies: do superclusters expand or
have they started to collapse? | astro-ph | We investigate the dynamical state of superclusters in Lambda cold dark
matter ($\Lambda$CDM) cosmological models, where the density parameter
$\Omega_0=0.2-0.4$ and $\sigma_8$ (the rms fluctuation on the $8h^{-1}$Mpc
scale) is $0.7-0.9$. To study the nonlinear regime, we use N-body simulations.
We define superclusters as maxima of the density field smoothed on the scale
$R=10h^{-1}$Mpc. Smaller superclusters defined by the density field smoothed on
the scale $R=5h^{-1}$Mpc are also investigated. We find the relations between
the radially averaged peculiar velocity and the density contrast in the
superclusters for different cosmological models. These relations can be used to
estimate the dynamical state of a supercluster on the basis of its density
contrast. In the simulations studied, all the superclusters defined with the
$10h^{-1}$Mpc smoothing are expanding by the present epoch. Only a small
fraction of the superclusters defined with $R=5h^{-1}$Mpc has already reached
their turnaround radius and these superclusters have started to collapse. In
the model with $\Omega_0=0.3$ and $\sigma_8=0.9$, the number density of objects
which have started to collapse is $5 \times 10^{-6}h^3$Mpc$^{-3}$. The results
for superclusters in the N-body simulations are compared with the spherical
collapse model. We find that the radial peculiar velocities in N-body
simulations are systematically smaller than those predicted by the spherical
collapse model ($\sim 25$% for the $R=5h^{-1}$Mpc superclusters).
| No Label | No Label |
astro-ph/0202148 | David R. Ballantyne | Soft X-ray emission lines from photoionized accretion discs: constraints
on their strength and width | astro-ph | We consider the properties of soft X-ray emission lines in the reprocessed
emission from a photoionized accretion disc. Observations of these lines will
be important in determining the ionization state and metallicity of the
innermost regions of active galaxies. Calculations of reflection from a
constant-density disc with an ionization parameter between 250 and 1000 erg
cm/s show that emission from O VIII Ly\alpha will dominate the soft X-ray
emission spectrum. There is also significant emission from C VI, N VII, O VII,
as well as Fe XVII--XIX. As the ionization parameter is increased these lines
become weaker and are broadened by Compton scattering. Significantly increasing
the O abundance primarily strengthens the O VII line, making it as large or
larger than the O VIII line. The nitrogen and carbon lines are quite weak with
equivalent widths (EWs) <30 eV, even with an increase in the N abundance. A
hydrostatic ionized disc model has a more realistic density structure and shows
a similar spectrum, but with the lines weaker and broader. This is a result of
the hot ionized skin at the surface of the disc. We apply these results to the
controversial claim of soft X-ray relativistic lines in the XMM-Newton spectrum
of MCG--6-30-15. We are unable to find a situation where O VIII has the
required EW without substantial emission from O VII. Furthermore, Compton
scattering results in the blue wing of the O VIII line to be much broader than
the << 10 eV drop observed in the data. We conclude that soft X-ray accretion
disc lines will, in general, be weak and broad features and are unlikely to
produce sharp edges in the data.
| No Label | No Label |
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arXiv Computer Science Dataset Description
Overview
This dataset is a filtered subset of arXiv papers focusing exclusively on Computer Science domains. It contains 46,328 academic papers with their metadata and classifications.
Dataset Structure
The dataset contains 7 columns:
id: Unique identifier for each arXiv papersubmitter: The individual who submitted the paper to arXivtitle: The title of the research papercategories: Original arXiv category codes (e.g., cs.AI, cs.CL)abstract: The paper's abstract textlabels: Transformed category names in human-readable formatdomain: The primary domain classification (Computer Science)
Data Processing
- The dataset was created by filtering the complete arXiv metadata dump for Computer Science papers
- Category codes were mapped to their full names using a category taxonomy
- Each paper's primary domain was extracted from its categories
- Only papers with at least one Computer Science category were included
Data Format
- File format: CSV
- Number of records: 46,328
Usage
This dataset is suitable for:
- Text classification tasks
- Topic modeling
- Research trend analysis
- Bibliometric studies
- Computer Science subdomain classification
Source
The data was derived from the arXiv metadata snapshot and processed using a custom category taxonomy mapping.
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