210 research outputs found
The Role of Dust in GRB Afterglows
We show that the clumpy structure of star-forming regions can naturally
explain the fact that 50-70% of GRB afterglows are optical``dark.'' We also
show that dust echos from the GRB and its afterglow, produced by the clumpy
structure of the star-forming region in which the GRB occurs, can lead to
temporal variability and peaks in the NIR, optical, and UV lightcurves of GRB
afterglows.Comment: To appear in Procs. of Gamma-Ray Bursts in the Afterglow Era: 2nd
Workshop, 3 pages, 2 figures, LaTe
Constraints on the Galactic Corona Models of Gamma-Ray Bursts From the 3B Catalogue
We investigate the viability of Galactic corona models of gamma-ray bursts by
calculating the spatial distribution expected for a population of high-velocity
neutron stars born in the Galactic disk and moving in a gravitational potential
that includes the Galactic bulge, disk, and a dark matter halo. We consider
models in which the bursts radiate isotropically and in which the radiation is
beamed. We place constraints on the models by comparing the resulting
brightness and angular distributions with the data in the BATSE 3B catalog. We
find that, if the burst sources radiate isotropically, the Galactic corona
model can reproduce the BATSE peak flux and angular distributions for neutron
star kick velocities > 800 km s, source turn-on ages > 20 Myrs, and
BATSE sampling distances 130 kpc < d_{max} < 350 kpc. If the radiation is
beamed, no turn-on age is required and agreement with the BATSE data can be
found provided that the width of the beam is less than 20 deg.Comment: 5 pages latex uses aip macros. To be published in the Proceedings of
the 3rd Huntsville Gamma-Ray Burst Symposium, AIP editors C. Kouvelietou,
M.S. Briggs, G.J. Fishma
Likelihood Methods and Classical Burster Repetition
We develop a likelihood methodology which can be used to search for evidence
of burst repetition in the BATSE catalog, and to study the properties of the
repetition signal. We use a simplified model of burst repetition in which a
number of sources which repeat a fixed number of times are superposed upon a number of non-repeating sources. The
instrument exposure is explicitly taken into account. By computing the
likelihood for the data, we construct a probability distribution in parameter
space that may be used to infer the probability that a repetition signal is
present, and to estimate the values of the repetition parameters. The
likelihood function contains contributions from all the bursts, irrespective of
the size of their positional errors --- the more uncertain a burst's position
is, the less constraining is its contribution. Thus this approach makes maximal
use of the data, and avoids the ambiguities of sample selection associated with
data cuts on error circle size. We present the results of tests of the
technique using synthetic data sets.Comment: 5 pages, Revtex (aipbook.sty included), 2 PostScript figures included
using psfig. To appear in the Proceedings of the 1995 La Jolla Workshop "High
Velocity Neutron Stars and Gamma-Ray Bursts," eds. R. Rothschild and R.
Lingenfelter, AIP, New Yor
The Detectability of Gamma-Ray Bursts and Their Afterglows at Very High Redshifts
There is increasingly strong evidence that gamma-ray bursts (GRBs) are
associated with star-forming galaxies, and occur near or in the star-forming
regions of these galaxies. These associations provide indirect evidence that at
least the long GRBs detected by BeppoSAX are a result of the collapse of
massive stars. The recent evidence that the light curves and the spectra of the
afterglows of GRB 970228 and GRB 980326 appear to contain a supernova
component, in addition to a relativistic shock wave component, provide more
direct clues that this is the case. Here we establish that GRBs and their
afterglows are both detectable out to very high redshifts (z > 5).Comment: To appear in Proc. of the 10th Annual October Astrophysics Conference
in Maryland: Cosmic Explosions, 4 pages, LaTe
Construction of the Variability -> Luminosity Estimator
We present a possible Cepheid-like luminosity estimator for the long-duration
gamma-ray bursts based on the variability of their light curves.Comment: To appear in Procs. of Gamma-Ray Bursts in the Afterglow Era: 2nd
Workshop, 3 pages, 2 figures, LaTe
Gamma-Ray Bursts as a Probe of Cosmology
We show that, if the long GRBs are produced by the collapse of massive stars,
GRBs and their afterglows may provide a powerful probe of cosmology and the
early universe.Comment: 6 pages, 5 PostScript figures. To appear in the proceedings of the
October 2000 Rome Workshop on Gamma-Ray Bursts in the Afterglow Er
Gamma-Ray Bursts as a Probe of Cosmology
We show that, if the long GRBs are produced by the collapse of massive stars,
GRBs and their afterglows may provide a powerful probe of cosmology and the
early universe.Comment: 5 pages, 7 figures, to appear in AIP proc. "Gamma-Ray Burst and
Afterglow Astronomy 2001" Woods Hole, Massachusett
Determining the Gamma-Ray Burst Rate as a Function of Redshift
We exploit the 14 gamma-ray bursts (GRBs) with known redshifts and the 7
GRBs for which there are constraints on to determine the GRB rate , using a method based on Bayesian inference. We find that, despite the
qualitative differences between the observed GRB rate and estimates of the SFR
in the universe, current data are consistent with being
proportional to the SFR.Comment: 3 pages, 3 figures, to appear in AIP proc. "Gamma-Ray Burst and
Afterglow Astronomy 2001" Woods Hole, Massachusett
Extracting Neutron Star Properties from X-ray Burst Oscillations
Many thermonuclear X-ray bursts exhibit brightness oscillations. The
brightness oscillations are thought to be due to the combined effects of
non-uniform nuclear burning and rotation of the neutron star. The waveforms of
the oscillations contain information about the size and number of burning
regions. They also contain substantial information about the mass and radius of
the star, and hence about strong gravity and the equation of state of matter at
supranuclear densities. We have written general relativistic ray-tracing codes
that compute the waveforms and spectra of rotating hot spots as a function of
photon energy. Using these codes, we survey the effect on the oscillation
waveform and amplitude of parameters such as the compactness of the star, the
spot size, the surface rotation velocity, and whether there are one or two
spots. We also fit phase lag versus photon energy curves to data from the
millisecond X-ray pulsar, SAX J1808--3658.Comment: To appear in Proc. of the 10th Annual October Astrophysics Conference
in Maryland: Cosmic Explosions, 4 page
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