28,353 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
Non-axisymmetric oscillations of rapidly rotating relativistic stars by conformal flatness approximation
We present a new numerical code to compute non-axisymmetric eigenmodes of
rapidly rotating relativistic stars by adopting spatially conformally flat
approximation of general relativity. The approximation suppresses the radiative
degree of freedom of relativistic gravity and the field equations are cast into
a set of elliptic equations. The code is tested against the low-order f- and
p-modes of slowly rotating stars for which a good agreement is observed in
frequencies computed by our new code and those computed by the full theory.
Entire sequences of the low order counter-rotating f-modes are computed, which
are susceptible to an instability driven by gravitational radiation.Comment: 3 figures. To appear in Phys.Rev.
Uniting the Quiescent Emission and Burst Spectra of Magnetar Candidates
Spectral studies of quiescent emission and bursts of magnetar candidates
using XMM-Newton, Chandra and Swift data are presented. Spectra of both the
quiescent emission and the bursts for most magnetar candidates are reproduced
by a photoelectrically absorbed two blackbody function (2BB). There is a strong
correlation between lower and higher temperatures of 2BB (kT_LT and kT_HT) for
the magnetar candidates of which the spectra are well reproduced by 2BB. In
addition, a square of radius for kT_T (R_LT^2) is well correlated with a square
of radius for kT_HT (R_HT^2). A ratio kT_LT/kT_HT ~ 0.4 is nearly constant
irrespective of objects and/or emission types (i.e., the quiescent emission and
the bursts). This would imply a common emission mechanism among the magnetar
candidates. The relation between the quiescent emission and the bursts might be
analogous to a relation between microflares and solar flares of the sun. Three
AXPs (4U 0142+614, 1RXS J170849.0-400910 and 1E 2259+586) seem to have an
excess above ~7 keV which well agrees with a non-thermal hard component
discovered by INTEGRAL.Comment: 17 pages, 5 figures, 12 tables, Accepted for publication in PAS
Ag and N acceptors in ZnO: ab initio study of acceptor pairing, doping efficiency, and the role of hydrogen
Efficiency of ZnO doping with Ag and N shallow acceptors, which substitute
respectively cations and anions, was investigated. First principles
calculations indicate a strong tendency towards formation of nearest neighbor
Ag-N pairs and N-Ag-N triangles. Binding of acceptors stems from the formation
of quasi-molecular bonds between dopants, and has a universal character in
semiconductors. The pairing increases energy levels of impurities, and thus
lowers doping efficiency. In the presence of donors, pairing is weaker or even
forbidden. However, hydrogen has a tendency to form clusters with Ag and N,
which favors the Ag-N aggregation and lowers the acceptor levels of such
complexes.Comment: 10 pages, 4 figure
Angularly excited and interacting boson stars and Q-balls
We study angularly excited as well as interacting non-topological solitons,
so-called Q-balls and their gravitating counterparts, so-called boson stars in
3+1 dimensions. Q-balls and boson stars carry a non-vanishing Noether charge
and arise as solutions of complex scalar field models in a flat space-time
background and coupled minimally to gravity, respectively.
We present examples of interacting Q-balls that arise due to angular
excitations, which are closely related to the spherical harmonics. We also
construct explicit examples of rotating boson stars that interact with
non-rotating boson stars. We observe that rotating boson stars tend to absorb
the non-rotating ones for increasing, but reasonably small gravitational
coupling. This is a new phenomenon as compared to the flat space-time limit and
is related to the negative contribution of the rotation term to the energy
density of the solutions. In addition, our results indicate that a system of a
rotating and non-rotating boson star can become unstable if the direct
interaction term in the potential is large enough. This instability is related
to the appearance of ergoregions.Comment: 20 pages including 9 figures; for higher quality figures please
contact the authors; v2: minor changes, final version to appear in Phys. Rev.
A search for x-ray counterparts of gamma-ray bursts with the ROSAT PSPC
We search for faint X-ray bursts with duration 10--300 seconds in the ROSAT
PSPC pointed observations with a total exposure of 1.6e7 seconds. We do not
detect any events shorter than ~100s, i.e. those that could be related to the
classic gamma-ray bursts. At the same time, we detect a number of long flares
with durations of several hundred seconds. Most, but not all, of the long
flares are associated with stars. If even a small number of those long flares,
that cannot identified with stars, are X-ray afterglows of GRB, the number of
X-ray afterglows greatly exceeds the number of BATSE GRB. This would imply that
the beaming factor of gamma-rays from the burst should be >100. The
non-detection of any short bursts in our data constrains the GRB counts at the
fluences 1--2.5 orders of magnitude below the BATSE limit. The constrained
burst counts are consistent with the extrapolation of the BATSE log N - log S
relation. Finally, our results do not confirm a reality of short X-ray flashes
found in the Einstein IPC data by Gotthelf, Hamilton and Helfand.Comment: Accepted to ApJ Letters. 4 pages with 3 figures, LaTeX2
Bubbling 1/2 BPS Geometries and Penrose Limits
We discuss how to take a Penrose limit in bubbling 1/2 BPS geometries at the
stage of a single function z(x_1,x_2,y). By starting from the z of the AdS_5 x
S^5 we can directly derive that of the pp-wave via the Penrose limit. In that
time the function z for the pp-wave with 1/R^2-corrections is obtained. We see
that it surely reproduces the pp-wave with 1/R^2 terms. In addition we consider
the Penrose limit in the configuration of the concentric rings.Comment: 15 pages, 3 figures, LaTeX, v2:higher order corrections are added,
typos corrected and references added, version to appear in PR
in generalized supergravity
We showed in previous work that for homogeneous Yang-Baxter (YB) deformations
of AdSS, the open string metric and coupling, and as a result the
closed string density , remain undeformed. In this work,
in addition to extending these results to the deformation associated with the
modified CYBE, or -deformation, we identify the Page forms as the open
string counterpart for RR fields and demonstrate case by case that the non-zero
Page forms remain invariant under YB deformations. We give a physical meaning
to the Killing vector of generalized supergravity and show for all YB
deformations: 1) appears as a current for center of mass motion on the
worldvolume of a D-branes probing the background, 2) is equal to the
divergence of the noncommutativity parameter, 3) exhibits "holographic"
behavior, where the radial component of vanishes at the AdS boundary, and
4) in pure spinor formalism is related to a certain state in the BRST
cohomology.Comment: 11 pages, 2 column; v2 references updated; v3 to appear in EPJ
Twisted topological structures related to M-branes II: Twisted Wu and Wu^c structures
Studying the topological aspects of M-branes in M-theory leads to various
structures related to Wu classes. First we interpret Wu classes themselves as
twisted classes and then define twisted notions of Wu structures. These
generalize many known structures, including Pin^- structures, twisted Spin
structures in the sense of Distler-Freed-Moore, Wu-twisted differential
cocycles appearing in the work of Belov-Moore, as well as ones introduced by
the author, such as twisted Membrane and twisted String^c structures. In
addition, we introduce Wu^c structures, which generalize Pin^c structures, as
well as their twisted versions. We show how these structures generalize and
encode the usual structures defined via Stiefel-Whitney classes.Comment: 20 page
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
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