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Astron. Astrophys. 364, L54-L61 (2000)

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1. Introduction

Since the first detections of soft X-ray (Costa et al. 1997), optical (van Paradijs et al. 1997), and radio (Frail and Kulkarni 1997) afterglows to gamma-ray bursts (GRBs), 19 optical afterglows have been detected.

For the current sample of 12 GRBs with well-determined redshifts, the median redshift is [FORMULA] = 1.1 with a very large root-mean-square variation of [FORMULA]0.8. While a photometric redshift of [FORMULA] 5 has been proposed for GRB 980329 (Fruchter 1999), the highest spectroscopic redshift determination for a GRB so far is [FORMULA] for GRB 971214 (Kulkarni et al. 1998). This redshift was determined from the spectrum of a faint Lyman Break galaxy that was found at the position of the optical afterglow (see also Odewahn et al., 1998). The highest redshift determined directly from absorption lines in the spectrum of an optical afterglow is [FORMULA] for GRB 000301C (Smette et al. 2000; Jensen et al. 2000). Several authors have speculated about the possibility of detecting GRBs at even higher redshifts than achieved so far. Wijers et al. Wijers et al. (1998) and Lamb & Reichart Lamb & Reichart (2000) estimate that GRBs should be detectable at very high redshifts ([FORMULA]) based on the assumption that GRBs are associated with star formation and by using models for the cosmic star formation rate as a function of redshift. Blain & Natarajan Blain and Natarajan (2000) propose to use the opposite strategy, namely to use the observed redshift distribution of GRBs to measure the (uncertain) cosmic star formation rate as a function of redshift.

Unfortunately, afterglows have been detected in only [FORMULA] percent of all well-localized (SAX, RXTE, IPN) GRBs (e.g., Fynbo et al. 2000). It is not clear why most GRBs with no apparent optical afterglow (`dark bursts') are so abundant. Possible explanations are rapid decay (Groot et al. 1998), reddening in the immediate environment and host galaxy (eg., Jensen et al. Jensen et al. (2000)), intrinsic optical faintness (Taylor et al. 2000), very high redshifts (Lamb & Reichart 2000) or a combination of these effects. To shed light on dark bursts, one obvious strategy is to set deeper optical limits by using large telescopes and to determine the decay slopes and colours (see Fynbo et al. 2000).

In this Letter we present the first attempt to identify the afterglow of a GRB with an 8-m class telescope. The attempt was successful in that the observations led to the discovery of the afterglow and the subsequent determination of the decay slope, spectral energy distribution and record high redshift. In the following we document the gamma-ray, optical, and infrared observations, and discuss the properties of the gamma-ray burst and afterglow.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000