Gamma-Ray Burst Precursors as the Remnant of the Thermal Radiation Initially Trapped in the Fireball

astro-ph/0703144 [abs, ps, pdf, other] :

Title: Gamma-Ray Burst Precursors as the Remnant of the Thermal Radiation
Initially Trapped in the Fireball


Authors:
Li-Xin Li (MPA)


Comments: 16 pages, including 13 figures

In the standard fireball model of gamma-ray bursts (GRBs), the fireball
starts with an optically thick phase. As it expands, the fireball becomes
optically thin at some stage. The thermal radiation trapped in the originally
opaque fireball then leaks out, producing a transient event. The appearance of
the event is investigated in the framework of a homogeneous, spherically
symmetric, and freely expanding fireball. We find that, generally, the event
has a time-duration shorter than that of the main burst, which is presumably
produced by the internal shock when the fireball is optically thin. In
addition, the event is separated from the main burst by a quiescent
time-interval, and is weaker than the main burst at least in a high energy
band. Hence, the event corresponds to a precursor of a GRB. The precursor event
predicted by our model has a simple and FRED (Fast Rise and Exponential Decay)
shape lightcurve, and a quasi-thermal spectrum. Typically, the characteristic
photon energy of the precursor is in the X-ray band. However, if the distortion
of the blackbody spectrum by electron scattering is considered, the inferred
photon energy could be in the gamma-ray band. Observational aspects of the GRB
precursors derived from our model are discussed.

powered by performancing firefox