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Astron. Astrophys. 328, L13-L16 (1997)

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1. Introduction

Gamma-Ray Bursts are strong high-energy flashes lasting a few seconds on average and showing an isotropic sky distribution (Fishman & Meegan 1995). Their emission in the other parts of electromagnetic spectrum is very elusive. Their distance is currently unknown, since no optical or radio counterpart has been detected so far. This situation seems to be rapidly changing thanks to the reduced error boxes now available. Therefore, it is of paramount importance to observe the error boxes of these bursts in the optical bands as soon as possible after the event, in order to catch the afterglow proposed by several theories. GRB 970228 offered for the first time the possibility of searching for an optical event shortly after a burst and to follow its evolution. Indeed an Optical Transient (OT) was suggested (Groot et al. 1997a, van Paradijs et al. 1997) as the optical counterpart of this Gamma-Ray Burst.

GRB 970228 was detected on February 28, 1997, by the X-ray satellite BeppoSAX (Costa et al. 1997a) as a four-peaked [FORMULA] -ray burst at [FORMULA], [FORMULA] (equinox 2000.0; error box radius = 3'), with total duration 80 seconds. Eight hours after the event, Costa et al. (1997b) observed a transient X-ray source, now labelled as SAX J0501.7+1146, located at the edge of the GRB 970228 error box. Palmer et al. (1997), Matz et al. (1997) and Liang et al. (1997) analyzed the spectral and the decay behaviours of both GRB 970228 and SAX J0501.7+1146; in particular, it was found that the [FORMULA] -ray spectrum was consistent with that of a classical burst. Preliminary comparison with the Digital Sky Survey showed no brightening of sources to [FORMULA] (Groot et al. 1997b), [FORMULA] and [FORMULA] (Guarnieri et al. 1997). Hurley et al. (1997), using the Ulysses satellite data, reduced the error box of GRB 970228 and found that the SAX J0501.7+1146 error box overlapped partially that of the [FORMULA] -ray burst. Within the intersection of the former error boxes Groot et al. (1997a) discovered an OT, which they indicated as probably related to GRB 970228. A thorough presentation of this result was given by van Paradijs et al. (1997).

In this paper we present the optical photometric analysis of the error box of GRB 970228, started [FORMULA] 17 hours after the gamma-ray event, from February 28 to March 18, 1997. Section 2 will present the data along with the reduction and calibration techniques, while Sect. 3 will discuss the results. Finally, Sect. 4 will draw the conclusions.

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998