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A Compact Binary Merger Model for the Short, Hard GRB 050509b

Description: The first X-ray afterglow for a short ({approx}30ms), hard {gamma}-ray burst was detected by Swift on 9 May 2005 (GRB 050509b). No optical or radio counterpart was identified in follow-up observations. The tentative association of the GRB with a nearby giant elliptical galaxy at redshift z = 0.2248 would imply the progenitor had traveled several tens of kpc from its point of origin, in agreement with expectations linking these events to the final merger of compact binaries driven by gravitational wave emission. We model the dynamical merger of such a system and the time-dependent evolution of the accretion tori thus created. The resulting energetics, variability, and expected durations are consistent with GRB 050509b originating from the tidal disruption of a neutron star by a stellar mass black hole, or of the merger of two neutron stars followed by prompt gravitational collapse of the massive remnant. We discuss how the available {gamma}-ray and X-ray data provides a probe for the nature of the relativistic ejecta and the surrounding medium.
Date: June 15, 2005
Creator: Lee, William H.; /Princeton, Inst. Advanced Study /UNAM, Inst. Astron.; Ramirez-Ruiz, Enrico; /Princeton, Inst. Advanced Study; Granot, Jonathan & /Princeton, Inst. Advanced Study /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department

Smooth Light Curves from a Bumpy Ride: Relativistic Blast Wave Encounters a Density Jump

Description: Some gamma-ray burst (GRB) afterglow light curves show significant variability, which often includes episodes of rebrightening. Such temporal variability had been attributed in several cases to large fluctuations in the external density, or density ''bumps''. Here we carefully examine the effect of a sharp increase in the external density on the afterglow light curve by considering, for the first time, a full treatment of both the hydrodynamic evolution and the radiation in this scenario. To this end we develop a semi-analytic model for the light curve and carry out several elaborate numerical simulations using a one dimensional hydrodynamic code together with a synchrotron radiation code. Two spherically symmetric cases are explored in detail--a density jump in a uniform external medium, and a wind termination shock. The effect of density clumps is also constrained. Contrary to previous works, we find that even a very sharp (modeled as a step function) and large (by a factor of a >> 1) increase in the external density does not produce sharp features in the light curve, and cannot account for significant temporal variability in GRB afterglows. For a wind termination shock, the light curve smoothly transitions between the asymptotic power laws over about one decade in time, and there is no rebrightening in the optical or X-rays that could serve as a clear observational signature. For a sharp jump in a uniform density profile we find that the maximal deviation {Delta}{alpha}{sub max} of the temporal decay index {alpha} from its asymptotic value (at early and late times), is bounded (e.g, {Delta}{alpha}{sub max} < 0.4 for {alpha} = 10); {Delta}{alpha}{sub max} slowly increases with {alpha}, converging to {Delta}{alpha}{sub max} {approx} 1 at very large {alpha} values. Therefore, no optical rebrightening is expected in this case as well. In the X-rays, while the asymptotic flux is ...
Date: June 6, 2006
Creator: Nakar, Ehud; /Caltech; Granot, Jonathan & /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department