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Distribution of Gamma-ray Burst Ejecta Energy with Lorentz Factor

Description: The early X-ray afterglow for a significant number of gamma-ray bursts detected by the Swift satellite is observed to have a phase of very slow flux decline with time (F{sub {nu}} {proportional_to} t{sup -{alpha}} with 0.2 {approx}< {alpha} {approx}< 0.8) for 10{sup 2.5} s{approx}< t {approx}< 10{sup 4} s, while the subsequent decline is the usual 1 {approx}< {alpha}{sub 3} {approx}< 1.5 behavior, that was seen in the pre-Swift era. We show that this behavior is a natural consequence of a small spread in the Lorentz factor of the ejecta, by a factor of {approx} 2-4, where the slower ejecta gradually catch-up with the shocked external medium, thus increasing the energy of forward shock and delaying its deceleration. The end of the ''shallow'' flux decay stage marks the beginning of the Blandford-McKee self similar external shock evolution. This suggests that most of the energy in the relativistic outflow is in material with a Lorentz factor of {approx} 30-50.
Date: October 7, 2005
Creator: Granot, Jonathan & Kumar, Pawan
Partner: UNT Libraries Government Documents Department

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

The Case for Anisotropic Afterglow Efficiency Within Gamma-Ray Burst Jets

Description: Early X-ray afterglows recently detected by Swift frequently show a phase of very shallow flux decay lasting from a few hundred seconds up to {approx} 10{sup 4} s, followed by a steeper, more familiar decay. We suggest that the flat early part of the light curve may be a combination of the decaying tail of the prompt emission and the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of the jet, as predicted previously. This would imply that a significant fraction of viewers have a very small external shock energy along their line of sight and a very high {gamma}-ray to kinetic energy ratio. The early flat phase in the afterglow light curve implies, according to this or other interpretations, a very large {gamma}-ray efficiency, typically {approx}> 90%, which is very difficult to produce by internal shocks.
Date: October 5, 2005
Creator: Eichler, David; Negev, /Ben Gurion U. of; Granot, Jonathan & /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department