Constraints to the nature of the central GRB engine from a comparative analysis of X-ray properties of afterglows.

Afterglows are a powerful source of information on GRBs. In this paper we present a systematic study of 11 afterglows observed with the Narrow Field Instruments on board on BeppoSAX. The intent of this analysis is to: i) disclose the nature of GRB progenitors by the temporal and spectral features imprinted by the nearby environment; ii) disentangle the jet versus spherical expansion of the fireball; iii) test and constrain the fireball model relations. We find no strong evidence of iron lines, except for the cases of GRB970508 (BeppoSAX) and GRB970828 (ASCA). From the average spectral and temporal properties we estimate a lower limit on the opening angle of the jet: θ>12°. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]