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Astron. Astrophys. 348, L5-L8 (1999)

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1. Introduction

On 1999 May 10.36743 UT the BATSE detectors on board CGRO, and the Gamma-Ray Burst Monitor (GRBM) and the Wide Field Cameras (WFCs) on board the Italian-Dutch satellite Beppo SAX detected a gamma ray burst, GRB 990510, with a fluence of 2.5[FORMULA] erg cm-2 above 20 keV (Kippen 1999; Amati et al. 1999; Dadina et al. 1999). The first optical follow-up observations began only [FORMULA]3.5 hours after the [FORMULA]-ray event and revealed a relatively bright optical transient (R=17.54, Axelrod, Mould & Schmidt 1999; Vreeswijk et al. 1999a) at [FORMULA], [FORMULA] (equinox 2000; Hjorth et al. 1999b). When compared to previously studied afterglows, the OT showed initially a fairly slow flux decay ([FORMULA]; Galama et al. 1999), that steepened after one day ([FORMULA]; Stanek et al. 1999a) and further steepened after 4 days ([FORMULA]; Pietrzyski & Udalski 1999; Bloom et al. 1999) and 5 days ([FORMULA]; Marconi et al. 1999a,b). Such a progressive and smooth steepening had not been observed before. Vreeswijk et al. (1999), using the VLT, detected red-shifted absorption lines in the OT spectrum corresponding to a redshift lower limit of [FORMULA].

In this letter we report on deep observations of the OT performed with the ESO VLT, NTT and 3.6 m telescopes. These observations allowed to extend the coverage of the OT light curve up to [FORMULA]10 days from the burst onset and to search for an underlying host galaxy.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999