Gamma Ray Bursts: open problems

The internal/external synchrotron shock scenario has proved very successful in interpreting the key observations about gamma–ray bursts. There still remains, however, some big uncertainties. The hottest issue concerns the nature of the progenitor, but there are also other problems concerning the global energetics, coupled with the issue of the degree of the collimation of the fireball. To be efficient, internal shocks within the relativistic wind must occur with large contrasts of their bulk Lorentz factors, and it is not clear yet the role of the Compton drag process in limiting the velocity differences. The fireball itself can be “hot” or “cold” according to what accelerates it to ultrarelativistic bulk speeds. In this respect the recent observations of a black body shape of the early phases of a few bursts shed new light on this issue. The most popular radiation process thought to explain the prompt emission is synchrotron, but it faces severe problems when comparing the expected spectrum with observations. Alternatives are called for. Emission features in the X–ray afterglow and absorption features in the prompt spectra are a powerful diagnostical tool. Besides shedding light on the nature of the progenitor, they can constrain the total energy release in a beaming–independent way. 1. Are internal shocks efficient enough? The light curves of the prompt emission of GRBs are characterized by very fast variability, with timescales as short as milliseconds (the record holder being GRB 920229, with 0.22 ms; Shaefer & Walker 1999). Furthermore, the time–width of the pulses does not increase during the burst duration (Fenimore, Ramirez– Ruiz & Wu 1999). These two observational facts led researchers to propose the “internal shock” scenario for the prompt emission of GRBs: the central engine works intermittently, producing a modulated outflow composed of different parts moving with different Lorentz factors Γ, initially separated by a distance R0. If a relatively slow shell of material is followed by a faster one, they collide at a distance of order R0Γ . Taking R0 of the order of a few Schwarzchild radii (for a 1–10 solar masses black hole) and Γ of the order of a few hundreds, the two shells collide at a distance of the order of ∼ 10 cm, just where (for usual parameters), the shells becomes transparent for Thomson scattering. This collision produces a single pulse of ms duration. The distribution of the time–width of the pulses does not change during the total duration of the burst since we have the same kind of event repeating itself.