Thursday, July 11, 2013

1307.2663 (Andrei M. Beloborodov et al.)

On the origin of GeV emission in gamma-ray bursts    [PDF]

Andrei M. Beloborodov, Romain Hascoet, Indrek Vurm
The most common progenitors of gamma-ray bursts (GRBs) are massive stars with strong stellar winds. We show that the GRB blast wave in the wind should emit a bright GeV flash. It is produced by inverse Compton scattering of the prompt MeV radiation (emitted at smaller radii) which streams through the external blast wave. Some of the prompt photons are scattered and many scattered photons convert to electron-positron pairs. The inverse-Compton flash is bright due to the huge e+- enrichment of the medium. GeV emission generated by this mechanism lasts much longer than the prompt GRB because of a broader angular distribution of scattered photons. At late times, the blast wave switches to normal synchrotron-self-Compton cooling. The mechanism is demonstrated by a detailed transfer simulation. The observed prompt MeV radiation is taken as an input of the simulation; we use GRB 080916C as an example. The result reproduces the GeV flash observed by the Fermi telescope. It explains the delayed onset, the steep rise, the peak flux, the time of the peak, the long smooth decline, and the spectral slope of GeV emission. The wind density required to reproduce all these features is typical of Wolf-Rayet stars. Our simulation predicts strong TeV emission 1 min after the burst trigger; then the cutoff of the observed high-energy spectrum must be shaped by extragalactic background light absorption. In addition, a bright optical counterpart of the GeV flash is expected for plausible values of the magnetic field; such double (optical+GeV) flashes may be observed with Fermi and optical robotic telescopes.
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