Physical parameters and emission mechanism in gamma-ray bursts

Detailed information on the physical parameters in the sources of cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions consistent with observations. We consider monoenergetic injection of electrons and let them cool self-consistently, taking into account Klein-Nishina cut-off in electron-photon scattering. The general requirements posed by the assumptions on the emission mechanism in GRBs are formulated. It is found that the observed radiation in the sub-MeV energy range is generated by the synchrotron emission mechanism, though about ten per cent of the total GRB energy should be converted via the inverse Compton (IC) process into the ultra-hard spectral domain (above 100 GeV). We estimate the magnetic field strength in the emitting region, the Lorentz factor of accelerated electrons, and the typical energy of IC photons. We show that there is a synchrotron-self-Compton constraint which limits the parameter space available for GRBs that are radiatively efficient in the sub-MeV domain. This concept is analogous to the line-of-death relation existing for pulsars and allows us to derive the lower limits on both GRB duration and the timescale of GRB variability. The upper limit on the Lorentz factor of GRB fireballs is also found. We demonstrate that steady-state electron distribution consistent with the Compton losses may produce different spectral indices, e.g., 3/4 as opposed to the figure 1/2 widely discussed in the literature. It is suggested that the changes in the decline rate observed in the lightcurves of several GRB afterglows may be due to either a transition to efficient IC cooling or the time evolution of Klein-Nishina and/or Compton spectral breaks, which are the general features of self-consistent electron distribution.