Decay properties of the X-ray afterglows of gamma-ray bursts

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: gendre@rm.iasf.cnr.it
² Observatoire de Haute-Provence, 04870 St. Michel l'Observatoire, France e-mail: Michel.Boer@oamp.fr

Received: 20 September 2004
Accepted: 2 November 2004

Abstract

We present a set of seventeen Gamma-Ray Bursts (GRBs) with known redshifts and X-ray afterglow emission. We apply cosmological corrections in order to compare their fluxes normalized at a redshift of 1. Two classes of GRB can be defined using their X-ray afterglow light curves. We show that the brightest afterglows seem to decay faster than the dimmer ones. We also point out evidence for a possible flux limit of the X-ray afterglow depending on the time elapsed since the burst. We try to interpret these observations in the framework of the canonical fireball model of GRB afterglow emission.