Nonthermal Radiation of Cosmological Gamma‐Ray Bursters

Abstract

We use 1.5 dimensional particle-in-cell plasma simulations to study the interaction of a relativistic, strongly magnetized wind with an ambient medium. Such an interaction is a plausible mechanism that leads to generation of cosmological γ-ray bursts. We confirm the idea of Meszaros & Rees that an essential part (about 20%) of the energy that is lost by the wind in the process of its deceleration may be transferred to high-energy electrons and then to high-frequency (X-ray and γ-ray) emission. We show that in the wind frame the spectrum of electrons that are accelerated at the wind front and move ahead of the front is nearly a two-dimensional relativistic Maxwellian with a relativistic temperature T = mec2ΓT/k 6 × 109ΓT K, where ΓT is equal to 200Γ0, with the accuracy of ~20%, and Γ0 is the Lorentz factor of the wind, Γ0 102 for winds outflowing from cosmological γ-ray bursters. Our simulations point to an existence of a high-energy tail of accelerated electrons with a Lorentz factor of more than ~700Γ0. Large-amplitude electromagnetic waves are generated by the oscillating currents at the wind front. The mean field of these waves ahead of the wind front is an order of magnitude less than the magnetic field of the wind. High-energy electrons that are accelerated at the wind front and injected into the region ahead of the front generate synchro-Compton radiation in the fields of large-amplitude electromagnetic waves. This radiation closely resembles synchrotron radiation and can reproduce the nonthermal radiation of γ-ray bursts observed in the Ginga and BATSE ranges (from a few keV to a few MeV). Synchrotron photons that are generated in the vicinity of the wind front may be responsible for the radiation of γ-ray bursts in the EGRET energy range above a few ten MeV. The spectrum of γ-ray bursts in high-energy γ-rays may extend, in principle, up to the maximum energy of the accelerated electrons, which is about 1013(Γ0/102)2 eV in the frame of the γ-ray burster.