Relativistic Beam--Magnetic Barrier Collision and Nonthermal Radiation of Cosmological gamma -Ray Bursters

Abstract

We study the interaction between a magnetized wind that flows away from the gamma-ray burster at relativistic speeds and an external medium.In the frame of the outflowing gas, this problem is identical to theproblem of collision between a wide relativistic beam of cold plasma anda strong magnetic field. Such a collision is considered numerically. Itis assumed that the plasma beam is initially homogeneous and impactswith the Lorentz factor Gamma0 ≫ 1 on the magnetic field region,which is called a magnetic barrier. The magnetic field of the barrier ishomogeneous and perpendicular to the beam velocity. It is shown that inthe process of such a collision, strong electric fields are generatednear the front of the magnetic barrier, and electrons are accelerated inthese fields. The mean energy of reversed electrons that move away fromthe barrier is ˜0.05mpc2Gamma0within a factor of 2. The maximum energy of these electrons may be atleast ˜½mpc2Gamma0. Theradiation of accelerated electrons in the vicinity of the barrier frontis calculated. In the case of the magnetic field and beam parametersthat are relevant to both cosmological gamma-ray bursters and anexternal medium around them, this radiation is in the range ofgamma-rays. The total energy that can escape from the region of therelativistic beam-magnetic barrier interaction in both gamma-rays andhigh-energy electrons is ˜10%-20% of the beam energy. The fractionof this energy that is radiated in gamma-rays immediately at themagnetic barrier may be as high as ˜15% of the beam energy. Theresults of simulations are used for explanation of observational data onthe nonthermal radiation of gamma-ray bursters.