Magnetically powered prompt radiation and flow acceleration in GRB

TL;DR

This paper reviews magnetic energy flux models for GRBs, highlighting how magnetic reconnection efficiently produces prompt emission and accelerates the flow, with implications for observed GRB properties and transition phenomena.

Contribution

It introduces a magnetic dissipation model for GRBs that explains prompt emission and flow acceleration, providing analytical and numerical insights into the transition between X-ray flashes and GRBs.

Findings

Magnetic reconnection outside the photosphere leads to efficient prompt emission.

Flow acceleration is highly efficient due to magnetic dissipation.

The transition between X-ray flashes and GRBs occurs at a Lorentz factor of about 100.

Abstract

The physics of GRB powered by a magnetic energy flux is reviewed. Magnetic fields are natural for transmitting the energy from the central compact object to the small amount of baryons required for a GRB. When dissipation of the flux of magnetic energy by reconnection inside the flow is taken into account, the magnetic model assumes several more convincing properties. For baryon loading typical of observed GRB, most of the dissipation takes place just outside photosphere, so that prompt emission is produced efficiently, and the magnetic field strength in this region is high, resulting in efficient synchrotron emission. Remarkably, the dissipation also causes very efficient acceleration of the bulk flow. This effect is illustrated with a classical hydrodynamic equivalent. In this context, the distinction between the flux of magnetic energy $cB^2/8\pi$ and the Poynting flux $cB^2/4\pi$ is important, and an interpretation of the Poynting flux as a `magnetic enthalpy flux' illuminating. Numerical and analytical results for flow acceleration and the relative contribution of photospheric (thermal) and nonthermal emission as functions of the asymptotic bulk Lorentz factor are given. The transition between X-ray flashes and true GRB is predicted at $\Gamma\approx 100$.