Physics of Gamma-Ray Bursts

Gamma-Ray Bursts (GRBs) are emitted by ultra-relativistic jets from compact objects. The jets may be strongly magnetized outflows driven by rotation of the central object. Alternatively, the jets may be driven by thermal pressure due to neutrino heating around the compact object. Both scenarios can supply the observed luminosities, but only under extreme conditions that will be discussed.

A long-debated problem is how the jet emits the observed GRB radiation. A simplest model resembles the big bang: the opaque hot plasma expands to transparency and releases its thermal photons. The expected spectrum from a radiation-dominated jet is Planckian with a peak around 1 MeV. The observed GRB spectra do peak around 1 MeV, however they have non-Plankian shapes, with extended high-energy tails. Physical processes generating non-Plankian radiation will be discussed. The popular phenomenological model of synchrotron emission from internal shocks has serious problems -- it contradicts both shock physics and observations. I will describe a different emission mechanism: internal motions in the neutron-proton jet generate electron heat via nuclear and Coulomb collisions, and electrons radiate the received energy. This mechanism invokes no phenomenological parameters, and the produced radiation is calculated from first principles. Remarkably, the predicted spectra agree with observations.