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Astron. Astrophys. 344, L37-L40 (1999)

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1. Introduction

Gamma Ray Bursts (GRBs) have been a mystery for almost 30 years. Recently, thanks to the Italian-Dutch satellite BeppoSax, it has been possible to detect for the first time their X-ray, optical, and radio afterglows. Many articles have been published to report the main characteristics of GRBs, (e.g., Guarnieri & al. 1997; Piro & al. 1998; Frail & al. 1997; Metzger & al. 1997; Gorosabel & al. 1998; Kulkarni & al. 1998). By now we know that GRBs are isotropically distributed over the sky, at least three of them are cosmological, they show a time variability in the [FORMULA]-emission of the order of milliseconds, and long complex bursts. Many, but not all of them have an X-ray afterglow, and it seems that for most of them a host galaxy can be found.

Modern theoretical attempts to interpret the data are based on ideas by Mészáros and Rees (1993), Mészáros (1994), Panaitescu and Mészáros (1998), as well as Paczynski (1986), and Paczynski and Rhoads (1993). In these models a relativistic shock is caused by a relativistic fireball in a pre-existing gas, such as the interstellar medium or a stellar wind, producing and accelerating electrons/positrons to very high energies, which produce the gamma-emission and the various afterglows observed. The low level of associated radiation at other wavelengths limits the baryonic load of the emitting regions to very low amounts, and constrains the scale of the emitting region to lengths much larger than a neutron star.

One serious question is whether the overall energetics of the fireball-assumed to be isotropic-are reasonable (Sari and Piran 1997; Dar 1997) or actually exceed the level given by any conceivable model of neutron star mergers or other stellar collapses. Another question is whether a fireball model that uses external or internal shock waves can solve the baryonic mass load problem to explain the [FORMULA]-emission, starting with an initial energy of [FORMULA] erg in the spherical shell rest frame.

Many authors suppose the validity of the fireball model and provide evidence for its agreement with the observations (Waxman 1997a, Waxman 1997b, Vietri 1997), but the observational results for GRB971214, requiring an initial energy [FORMULA] erg (if the emission is isotropic), pose a serious challenge to existing models.

Here we propose an anisotropic model where the [FORMULA]-ray emission and the afterglow is produced inside a preexisting jet and calculate the temporal evolution of the corresponding flux.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999