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Astron. Astrophys. 364, 655-659 (2000)

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1. Introduction

The prompt localization of gamma-ray bursts (GRBs) by BeppoSAX led to the discovery of X-ray/optical/radio afterglows and associated host galaxies. Subsequent detections of absorption and emission features at high redshifts ([FORMULA]) in optical afterglows of GRB and their host galaxies clearly demonstrate that at least some of the GRB sources lie at cosmological distances (for reviews, see Piran 1999; Vietri 1999).

A common feature of all acceptable models of cosmological [FORMULA]-ray bursters is that a relativistic wind is a source of GRB radiation. The Lorentz factor, [FORMULA], of such a wind is about [FORMULA] or even more (e.g., Fenimore et al. 1993; Baring & Harding 1997). A very strong magnetic field may be in the plasma outflowing from cosmological [FORMULA]-ray bursters (Usov 1994; Blackman, Yi, & Field 1996; Vietri 1996; Katz 1997; Mészáros & Rees 1997; Wheeler et al. 2000). A relativistic, strongly magnetized wind interacts with an ambient medium (e.g. an ordinary interstellar gas) and decelerates. It was pointed out (Mészáros & Rees 1992) that such an interaction, assumed to be shock-like, may be responsible for generation of cosmological GRBs.

The interaction between a relativistic, strongly magnetized wind and an ambient plasma was studied numerically by Smolsky & Usov (1996, 2000) and Usov & Smolsky (1998). These studies showed that about 70% of the wind energy is transferred to the ambient plasma protons that are reflected from the wind front. The other [FORMULA]% of the wind energy losses is distributed between high-energy electrons and low-frequency electromagnetic waves that are generated at the wind front because of nonstationarity of the wind-ambient plasma interaction (see below). High-energy electrons, accelerated at the wind front and injected into the region ahead of the front, generate synchro-Compton radiation in the fields of the low-frequency waves. This radiation closely resembles synchrotron radiation and can reproduce the non-thermal radiation of GRBs observed in the Ginga and BATSE ranges (from a few KeV to a few MeV).

Ginzburg (1973) and Palmer (1993) suggested that GRBs might be sources of radio emission, and that it might be used to determine their distances and, through the dispersion measure, the density of the intergalactic plasma. Here we consider some properties of the low-frequency waves generated at the wind front. We argue that coherent emission by the high-frequency tail of these waves may be detected, in addition to the high-frequency (X-ray and [FORMULA]-ray) emission of GRBs, as a short pulse of low-frequency radio emission (Katz 1994, 1999).

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001