20,684 research outputs found
Statistical Analysis of Spectral Line Candidates in Gamma-Ray Burst GRB870303
The Ginga data for the gamma-ray burst GRB870303 exhibit low-energy dips in
two temporally distinct spectra, denoted S1 and S2. S1, spanning 4 s, exhibits
a single line candidate at ~ 20 keV, while S2, spanning 9 s, exhibits
apparently harmonically spaced line candidates at ~ 20 and 40 keV. We evaluate
the statistical evidence for these lines, using phenomenological continuum and
line models which in their details are independent of the distance scale to
gamma-ray bursts. We employ the methodologies based on both frequentist and
Bayesian statistical inference that we develop in Freeman et al. (1999b). These
methodologies utilize the information present in the data to select the
simplest model that adequately describes the data from among a wide range of
continuum and continuum-plus-line(s) models. This ensures that the chosen model
does not include free parameters that the data deem unnecessary and that would
act to reduce the frequentist significance and Bayesian odds of the
continuum-plus-line(s) model. We calculate the significance of the
continuum-plus-line(s) models using the Chi-Square Maximum Likelihood Ratio
test. We describe a parametrization of the exponentiated Gaussian absorption
line shape that makes the probability surface in parameter space
better-behaved, allowing us to estimate analytically the Bayesian odds. The
significance of the continuum-plus-line models requested by the S1 and S2 data
are 3.6 x 10^-5 and 1.7 x 10^-4 respectively, with the odds favoring them being
114:1 and 7:1. We also apply our methodology to the combined (S1+S2) data. The
significance of the continuum-plus-lines model requested by the combined data
is 4.2 x 10^-8, with the odds favoring it being 40,300:1.Comment: LaTeX2e (aastex.cls included); 41 pages text, 10 figures (on 11
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
Gamma-Ray Burst and Relativistic Shells: The Surface Filling Factor
The variability observed in many complex gamma-ray bursts (GRBs) is
inconsistent with causally connected variations in a single, symmetric,
relativistic shell interacting with the ambient material ("external shocks").
Rather, the symmetry of the shell must be broken on an angular scale much
smaller than Gamma^{-1} where Gamma is the bulk Lorentz factor for the shell.
The observed variability in the external shock models arises from the number of
causally connected regions that (randomly) become active. We define the
"surface filling factor" to be the ratio of the area of causally connected
regions that become active to the observable area of the shell. From the
observed variability in 52 BATSE bursts, we estimate the surface filling factor
to be typically 0.005 although some values are near unity. We find that the
surface filling factor is about 0.1 Delta T/T in both the constant Gamma phase
(which probably produces the GRB) and the deaccelerating phase (which probably
produces the x-ray afterglows). Here, \Delta T is a typical time scale of
variability and T is the time since the initial signal. We analyze the 2 hr
flare seen by ASCA 36 hr after the GRB and conclude that the surface filling
factor must be small (0.001) in the x-ray afterglow phase as well. Explanations
for low surface filling factor can either require more or less energy (by a
factor of about 1000) compared to that expected for a symmetric shell.Comment: 26 pages, 5 embedded figures, Latex, revised version as in press,
ApJ, added figure to show the possible expanding shell geometries that can
give low filling facto
Constraints on Cold Dark Matter in the Gamma-ray Halo of NGC 253
A gamma-ray halo in a nearby starburst galaxy NGC 253 was found by the
CANGAROO-II Imaging Atmospheric Cherenkov Telescope (IACT). By fitting the
energy spectrum with expected curves from Cold Dark Matter (CDM) annihilations,
we constrain the CDM-annihilation rate in the halo of NGC 253. Upper limits for
the CDM density were obtained in the wide mass range between 0.5 and 50 TeV.
Although these limits are higher than the expected values, it is complementary
important to the other experimental techniques, especially considering the
energy coverage. We also investigate the next astronomical targets to improve
these limits.Comment: 13 pages, 5 figures, aastex.cls, natbib.sty, To appear in ApJ v596n1,
Oct. 10, 200
Classical instability of Kerr-AdS black holes and the issue of final state
It is now established that small Kerr-Anti-de Sitter (Kerr-AdS) black holes
are unstable against scalar perturbations, via superradiant amplification
mechanism. We show that small Kerr-AdS black holes are also unstable against
gravitational perturbations and we compute the features of this instability. We
also describe with great detail the evolution of this instability. In
particular, we identify its endpoint state. It corresponds to a Kerr-AdS black
hole whose boundary is an Einstein universe rotating with the light velocity.
This black hole is expected to be slightly oblate and to co-exist in
equilibrium with a certain amount of outside radiation.Comment: 11 pages, RevTex4. v2: small typos corrected. Version to appear in
Phys. Rev.
Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303
We demonstrate that models of resonant cyclotron radiation transfer in a
strong field (i.e. cyclotron scattering) can account for spectral lines seen at
two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a
generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we
model line formation by injecting continuum photons into a static
plane-parallel slab of electrons threaded by a strong neutron star magnetic
field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the
slab normal. We examine two source geometries, which we denote "1-0" and "1-1,"
with the numbers representing the relative electron column densities above and
below the continuum photon source plane. We compare azimuthally symmetric
models, i.e. models in which the magnetic field is parallel to the slab normal,
with models having more general magnetic field orientations. If the bursting
source has a simple dipole field, these two model classes represent line
formation at the magnetic pole, or elsewhere on the stellar surface. We find
that the data of S1 and S2, considered individually, are consistent with both
geometries, and with all magnetic field orientations, with the exception that
the S1 data clearly favor line formation away from a polar cap in the 1-1
geometry, with the best-fit model placing the line-forming region at the
magnetic equator. Within both geometries, fits to the combined (S1+S2) data
marginally favor models which feature equatorial line formation, and in which
the observer's orientation with respect to the slab changes between the two
epochs. We interpret this change as being due to neutron star rotation, and we
place limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
Non-Abelian Localization for Supersymmetric Yang-Mills-Chern-Simons Theories on Seifert Manifold
We derive non-Abelian localization formulae for supersymmetric
Yang-Mills-Chern-Simons theory with matters on a Seifert manifold M, which is
the three-dimensional space of a circle bundle over a two-dimensional Riemann
surface \Sigma, by using the cohomological approach introduced by Kallen. We
find that the partition function and the vev of the supersymmetric Wilson loop
reduces to a finite dimensional integral and summation over classical flux
configurations labeled by discrete integers. We also find the partition
function reduces further to just a discrete sum over integers in some cases,
and evaluate the supersymmetric index (Witten index) exactly on S^1x\Sigma. The
index completely agrees with the previous prediction from field theory and
branes. We discuss a vacuum structure of the ABJM theory deduced from the
localization.Comment: 50 pages, 3 figures, typos corrected, references adde
Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification
Based on Rossi X-ray Timing Explorer data, we discuss the classification of
the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of
its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray
colors. The color-color diagram shows one elongated ("banana") structure and
several "islands" of data points. The power spectra of the island are best
represented by a broken power-law, whereas those of the banana by a power-law
below ~ 1 Hz plus an exponentially cut-off component at intermediate
frequencies (30-60 Hz). The parameters of these two components change in
correlation with the position of the source in the color-color diagram. Based
on the pattern that the source shows in the color-color diagram and its
aperiodic variability we conclude that Aquila X-1 is an atoll source. We have
also investigated the possible correlation between the frequency of the kHz QPO
and the position of the source in the color-color diagram. The complexity seen
in the frequency versus count rate diagram is reduced to a single track when
the frequency is plotted against hard or soft color.Comment: 9 pages, 5 figures. Accepted for publication in Ap
Galaxy clustering constraints on deviations from Newtonian gravity at cosmological scales II: Perturbative and numerical analyses of power spectrum and bispectrum
We explore observational constraints on possible deviations from Newtonian
gravity by means of large-scale clustering of galaxies. We measure the power
spectrum and the bispectrum of Sloan Digital Sky Survey galaxies and compare
the result with predictions in an empirical model of modified gravity. Our
model assumes an additional Yukawa-like term with two parameters that
characterize the amplitude and the length scale of the modified gravity. The
model predictions are calculated using two methods; the second-order
perturbation theory and direct N-body simulations. These methods allow us to
study non-linear evolution of large-scale structure. Using the simulation
results, we find that perturbation theory provides reliable estimates for the
power spectrum and the bispectrum in the modified Newtonian model. We also
construct mock galaxy catalogues from the simulations, and derive constraints
on the amplitude and the length scale of deviations from Newtonian gravity. The
resulting constraints from power spectrum are consistent with those obtained in
our earlier work, indicating the validity of the previous empirical modeling of
gravitational nonlinearity in the modified Newtonian model. If linear biasing
is adopted, the bispectrum of the SDSS galaxies yields constraints very similar
to those from the power spectrum. If we allow for the nonlinear biasing
instead, we find that the ratio of the quadratic to linear biasing
coefficients, b_2/b_1, should satisfy -0.4 < b_2/b_1<0.3 in the modified
Newtonian model.Comment: 12 pages, 7 figure
Clustering of dark matter halos on the light-cone: scale-, time- and mass-dependence of the halo biasing in the Hubble volume simulations
We develop a phenomenological model to predict the clustering of dark matter
halos on the light-cone by combining several existing theoretical models.
Assuming that the velocity field of halos on large scales is approximated by
linear theory, we propose an empirical prescription of a scale-, mass-, and
time-dependence of halo biasing. We test our model against the Hubble Volume
-body simulation and examine its validity and limitations. We find a good
agreement in two-point correlation functions of dark matter halos between the
phenomenological model predictions and measurements from the simulation for
Mpc both in the real and redshift spaces. Although calibrated on the
mass scale of groups and clusters and for redshifts up to , the model
is quite general and can be applied to a wider range of astrophysical objects,
such as galaxies and quasars, if the relation between dark halos and visible
objects is specified.Comment: 5 pages, 2 figures, ApJL accepted. New references adde
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