Gamma-ray Bursts: Whence the Gamma-rays?

Although the central engines that power most gamma-ray bursts are placed convincingly at cosmological distances, and the emitting material is known to be beamed and in relativistic bulk motion, detailed questions about these most dramatic of astrophysical transients have spawned a theoretical cacophony. The most pressing questions include the nature of the engine (black hole or magnetar?), the composition of the jet (strongly magnetzed or not?), the dominant channel of energy dissipation (shocks, magnetic reconnection, or MHD instabilities driven by changes in radiation pressure?), and the gamma-ray emission mechanism (synchrotron radiation, single or multiple Compton scattering?). Recent work offers a good prospect of breaking this logjam, starting with a Poynting-dominated jet which is heated during its escape from a confining baryonic medium. The simplest mechanism of energy dissipation (gradual, volume-distributed heating) goes a long way to reproducing what is seen during the prompt emission phase, without invoking non-thermal particle distributions or a neutron-rich component of the outflow.