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Astron. Astrophys. 337, 43-50 (1998)

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1. Introduction

Emission of TeV/PeV gamma-rays associated with GRBs has been extensively searched. These studies are motivated by both the experimental results obtained with the satellite experiments as e.g. EGRET and by theoretical models (Meszaros et al. 1994) which predict or at least allow TeV emission to be produced in GRBs. So far none of these searches have revealed any convincing evidence for VHE emission (see for example Aglietta et al. 1993; Alexandreas et al. 1994; Borione et al. 1995; Connaughton et al. 1997; Dazeley et al. 1997), although some tentative positive evidence has been found recently in this energy range (Plunkett et al. 1995; Krawczynski et al. 1995; Amenomori et al. 1996).

Up to now five BATSE GRBs have been detected by EGRET with photons of energies up to 18 GeV (Hurley et al. 1994). Those GRBs are among the most intense ones recorded by BATSE in the FOV of EGRET, so observations are compatible with the hypothesis that all GRBs emit GeV photons but only the strongest ones are above the EGRET sensitivity. A simple power law extrapolation of the Superbowl GRB spectrum (Sommer et al. 1994) predicts that [FORMULA]20 photons above 20 TeV should be observed with the extensive air shower (EAS) array of wide angle integrating Cherenkov counters (AIROBICC) within 25 seconds while expecting only 0.1 background events. This would lead to a highly significant detection by several other experiments currently operating (Cherenkov telescopes, EAS arrays). The previous extrapolation neglects source-intrinsic cutoffs and the attenuation through interaction with the low energy cosmic photon background (Wdowczyk et al. 1972; Mannheim et al. 1996; Stecker & de Jager 1997), which is expected for cosmological sources with redshift greater than 0.1. Another interesting feature of strong GRBs is that their high energy emission can be delayed and have longer durations than keV-MeV emission (Hurley et al. 1994). These ideas motivate searches for counterparts at times and with durations independent of those given by the space detectors at lower energies, especially when considering that the latter could miss GRBs out of their FOV, or with a very hard spectrum, as has been already suggested (Piran & Narayan 1995; see also Kommers et al. 1997). Searches for GRBs with the HEGRA experiment using other data periods can be found in Krawczynski (1997), Funk (1997), Padilla et al. (1997), Krawczynski et al. (1998), Padilla (1998).

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998