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Astron. Astrophys. 321, 229-235 (1997)

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1. Introduction

Observations with the Burst And Transient Source Experiment (BATSE), aboard the Compton Gamma-Ray Observatory (CGRO), have shown that the distribution of gamma-ray bursts (GRBs) is isotropic on the sky and inhomogeneous in space. The main problem in studying the nature of GRBs is that their distance scale is not known; some argue that GRBs originate from a large halo around our Galaxy (e.g., Brainerd 1992; Eichler and Silk 1992), others that they come from "cosmological" distances (e.g., Paczyski 1986, 1991, 1995; Piran 1995). The identification of a counterpart to a GRB at other wavelengths could provide an answer to this question. So far, no firm counterpart to a GRB at other wavelengths has been identified. Two strategies have so far been employed for counterpart searches. Deep searches for quiescent counterparts to accurately localized events, i.e., for low energy emission long after the burst, have been made; but no quiescent counterparts have so far been detected, at all photon energies (e.g., Schaefer 1993). Another strategy has been to search for flaring counterparts in simultaneous wide-field monitoring experiments in the hope to have a GRB in the field of view of the instrument at the moment of the event. Recently, searches for faint counterparts have been made which started within hours or days of the event (e.g., Frail et al. 1994). The GRB detection is used as a trigger to point the telescope in the appropriate direction and search for a flaring and/or fading counterpart on timescales much longer than the burst itself. The incentive for such a counterpart search is provided by fireball models (e.g., Rees and Mészáros 1992; Mészáros and Rees 1993; Paczyski and Rhoads 1993; Piran 1995), that predict a transient delayed radio counterpart. For bursts at cosmological distances the delay at cm-dm wavelengths could be weeks to months or even years. Radio observations provide a good possibility to identify a GRB with its transient radio counterpart (and accurately determine its position). In this paper we present the results of a radio search for variability in the Compton Telescope (COMPTEL) error region of GRB 940301. Information on this GRB is given in Sect. 2. The observations and their reduction are described in Sect. 3. The analysis of the data and search for variability is described in Sect. 4. The observational results on two variable sources are given in Sect. 5, and the more general and speculative aspects of the interpretation are discussed in Sect. 6. Sect. 7 contains our conclusions.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998