1207.2707 (Andrei M. Beloborodov)

Regulation of the spectral peak in gamma-ray bursts    [PDF]

Andrei M. Beloborodov

Observations of gamma-ray bursts (GRBs) indicate that the peak of the burst spectrum forms in an opaque region of an ultra-relativistic jet. Recent radiative transfer calculations support this picture and show that the spectral peak is inherited from an initially thermal photon distribution that has been modified by heating into a non-blackbody spectrum with a high-energy tail. We discuss processes that regulate the position of the spectral peak E_p and compare the expected range of E_p with observations. The opaque region of a GRB jet has three radial zones: (1) Planck zone r>1 where radiation has a Wien spectrum, and (3) Comptonization zone r>R_W where radiation spectrum is broadened into a Band shape. Besides the initial jet temperature, an important factor regulating E_p is internal dissipation (of bulk motions and magnetic energy) at large distances from the central engine. Dissipation in the Planck zone reduces E_p, and dissipation in the Wien zone increases E_p. In jets with weak magnetic fields, the predicted E_p approaches but does not exceed the thermal photon energy near the central engine; it is expected to vary around 1 MeV up to a maximum value of about 10 MeV. If the jet carries an energetically important magnetic field, E_p can be increased by dissipation of magnetic energy in the Wien zone. Existing data shows a cutoff in the distribution of E_p at about 20 MeV. This suggests that Poynting flux dissipation indeed contributes to heating in the Wien zone, at least in the brightest bursts with the highest E_p.

View original: http://arxiv.org/abs/1207.2707