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Astron. Astrophys. 332, L13-L16 (1998)

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1. Introduction

Since their discovery with the Vela satellites by the end of the 1960's (Klebesadel et al. 1973), [FORMULA] -ray bursts have intrigued the astronomical community. Their durations range from about 30 ms to more than 1000 s (see Fishman and Meegan 1995 for a review), and during this short time they can be the most prominent [FORMULA] -ray sources in the sky. However, the lack of angular resolution in the detections and their short lifetime made it impossible to identify their counterparts in other wavelengths.

Previous efforts to detect such counterparts in deep optical imaging of GRB error boxes were not made sufficiently rapid after the GRBs (Vrba et al. 1995, Hudec 1995), partly in the hope to identify quiescent bursters by unusual properties. Observations of GRBs detected with the BATSE experiment using automated telescopes alerted through the BACODINE system (delays of a few seconds) have not yet led to the detection of optical counterparts in excess of [FORMULA] th magnitude (Park et al. 1997). [FORMULA] upper limits for millimetric GRB fluxes from COBE were 645 Jy at 90 GHz for strong bursts (Ali et al. 1997), and 3.6 mJy at 95 GHz with the BIMA for sources with small positional errors (Gruendl et al. 1997).

Recently, the search for counterparts in other wavelengths has been successful. The deciding step was provided by the Italian/Dutch satellite BeppoSAX (Boella et al. 1997). Its Gamma-Ray Burst Monitor (GRBM) (Frontera et al. 1997) can detect GRBs in the range 60-600 keV, for some of which the X-ray emission is detected simultaneously with one of the two Wide Field Cameras (FoV [FORMULA], Jager et al. 1997).

GRB 970228 was the first event for which X-ray (Costa et al. 1997a) and optical (Van Paradijs et al. 1997; Galama et al. 1997a) afterglows were detected. HST observations of the optical counterpart (Sahu et al. 1997) showed that it consisted of a fading point source embedded in an extended ([FORMULA]) object, most likely a host galaxy but with a few percent probability a chance superposition (Van Paradijs et al. 1997).

The first unambiguous evidence for the `cosmological' distance scale of [FORMULA] -ray bursts was found for GRB 970508. The BeppoSAX GRBM detected a weak burst on May 8.904 UT lasting about 15 s, whose position was determined to [FORMULA] with the WFC (Costa et al. 1997b). Follow-up observations of this error box led to the detections of X-ray (Piro et al. 1997) and optical (Bond 1997) afterglows. Metzger et al. (1997a, b) placed the redshift of the source between [FORMULA] from absorption line spectra. On May 13, the source was detected at 8.46 GHz with the VLA ([FORMULA] Jy, Frail et al. 1997a,b). In the following weeks, GRB 970508 was closely monitored by numerous observations in all wavebands.

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998