145 research outputs found
Distribution of binary mergers around galaxies
We use a stellar binary population synthesis code to find the lifetimes and
velocities of several types of possible GRB progenitors: double neutron stars,
black hole neutron stars, black hole white dwarfs, helium star mergers.
Assuming that they are born in different types of galaxies we compute their
spatial distribution and compare it with the observed locations of GRB
afterglows within their hosts. We discuss constraints on the compact object
merger model of GRBs imposed by this comparison and find that the observations
of afterglows and their host galaxies appear inconsistent with the GRB compact
object merger model.Comment: 5 pages, 2 figures, Latex with aipproc.sty, Proc. of the 5th
Huntsville Gamma Ray Burst Symposium, Oct. 1999, ed. R.M. Kippen, AI
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
Spectral Effects of the Vacuum Resonance in Soft Gamma-Ray Repeaters
The association of all three soft gamma-ray repeaters (SGRs) with supernova
remnants has established that SGRs are young neutron stars, and has given us a
starting point for detailed modeling. One of the most popular classes of models
involves strongly magnetised neutron stars, with surface dipole fields B~
10^{14}-10^{15} Gauss. In such strong magnetic fields, many otherwise
negligible processes can play an important role. Here we consider the effects
of vacuum polarisation on Compton scattering. Vacuum polarisation introduces a
characteristic density-dependent photon frequency at which the normal modes of
polarisation become nonorthogonal and the mean free path of photons decreases
sharply. Our analytic results and Monte Carlo simulations of photon propagation
through a magnetised plasma show that this effect leads, under a wide range of
physical conditions, to a broad absorption-like feature in the energy range ~5
keV---40 keV. We discuss this effect in light of the spectra from SGR 1806-20.Comment: 13 pages Latex , uses mn.sty. Submitted to MNRA
Looking for GRB progenitors
Using stellar binary population synthesis code we calculate the production
rates and lifetimes of several types of possible GRB progenitors. We consider
mergers of double neutron stars, black hole neutron stars, black hole white
dwarfs and helium star mergers. We calibrate the results with the measured star
formation rate history. We discuss the viability of each GRB model, and
alternatively assuming that all bursts are connected with one model we
constrain the required collimation of GRBs. We also show the importance of
widely used evolutionary parameters on the merger rates of calculated binary
populations.Comment: 5 pages, 2 figures, Latex with aipproc.sty, Proc. of the 5th
Huntsville Gamma Ray Burst Symposium, Oct. 1999, ed. R.M. Kippen, AI
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