The role of photon scattering in shaping the lightcurves and spectra of gamma-ray bursts

We analyze the power spectra of the lightcurves of long gamma-ray bursts, dividing the sample in bins of luminosity, using the recently discovered variability-luminosity correlation. We find that the value of the variability parameter strongly correlates with the frequency that contains most of the power in the burst comoving frame. We compute the average power spectra in luminosity bins. The average power spectrum is well described by a broken power-low and the break frequency is a function of the variability parameter, while the two slopes are roughly constant. This allow us to conclude that scattering processes do not play a relevant role in modelling the lightcurves. We finally discuss in which conditions scattering may still play a relevant role in shaping the spectra of GRBs.Comment: Minor changes according to referee comments. Accepted for publication in MNRA

Gamma-Ray Burst Progenitors Confront Observations

The discovery of a supernova emerging at late times in the afterglow of GRB 030329 has apparently settled the issue on the nature of the progenitor of gamma-ray bursts. We now know that at least a fraction of cosmological GRBs are associated with the death of massive stars, and that the two explosions are most likely simultaneous. Even though the association was already suggested for GRB 980425, the peculiarity of that burst did not allow to extend the association to all GRBs. The issue is now to understand whether GRB 030329 is a "standard burst" or not. I will discuss some peculiarities of GRB 030329 and its afterglow lightcurve showing how, rather than a classical cosmological GRB, it looks more like a transition object linking weak events like GRB 980425 to the classical long duration GRBs. I will also discuss the problems faced by the Hypernova scenario to account for the X-ray features detected in several GRBs and their afterglows.Comment: 17 pages, 9 postscript figures. Invited review at the Xth Marcel Grossmann Meeting on General Relativity, Rio de Janeiro, Brazil, July 200