Fireball loading and the blast wave model
of gamma-ray bursts

C. D. Dermer, J. Chiang, and M. Böttcher,
Astrophysical Journal, 513, 656 (1999)

Abstract

A simple function for the spectral power is proposed to model, with 9 parameters, the spectral and temporal evolution of the observed nonthermal synchrotron power flux from GRBs in the blast wave model. Here is the observed dimensionless photon energy and t is the observing time. Assumptions and an issue of lack of self-consistency are spelled out. The spectra are found to be most sensitive to the baryon loading, expressed in terms of the initial bulk Lorentz factor , and an equipartition term q which is assumed to be constant in time and independent of . Expressions are given for the peak spectral power at the photon energy of the spectral power peak. A general rule is that the total fireball particle kinetic energy , where is the deceleration time scale and is the maximum measured bolometric power output in radiation, during which it is carried primarily by photons with energy .

This rule governs the general behavior of fireballs with different baryon loading. Clean fireballs with small baryon loading ( » 300) are intense, subsecond, medium-to-high energy gamma-ray events, and are difficult to detect because of deadtime and sensitivity limitations of previous gamma-ray detectors such as EGRET on CGRO. Dirty fireballs with large baryon loading ( « 300) produce transient emissions which are longer lasting and most luminous at X-ray energies and below, but these events are lost behind the glow of the X-ray and lower-energy background radiations except for rare serendipitous detections by pointed instruments. The correlation between hardness and duration of loaded GRB fireballs (100    1000) follows from this rule.

Postscript file

Recent publications
Archive