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Astron. Astrophys. 332, L29-L32 (1998)

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1. Introduction

The comprehension of the nature of the Gamma-Ray Bursts (GRBs) is a long-standing problem of a world-wide scientific community since the announcement of their discovery (Klebesadel et al. 1973). Many observational (Fishman & Meegan 1995) and theoretical (Lamb 1995; Paczynski 1995) efforts did not succeed in understanding the origin of GRBs. The launch of the BeppoSAX satellite (Boella et al. 1997a) revolutionized the field, opening a new observational window soon after the GRB event. Due to its Gamma Ray Burst Monitor (GRBM, 40-700 keV, Frontera et al. 1997a; Feroci et al. 1997a) and its Wide Field Cameras (WFCs, 2-26 keV, Jager et al. 1997) this satellite is capable of detecting GRBs in the gamma-ray band and accurately localizing them in X-rays through a coded mask proportional counter.

Five GRBs, amongst those simultaneously detected by the GRBM and the WFCs, were promptly analyzed, allowing multiwavelength follow-up observational campaigns. The first result is the BeppoSAX discovery of the X-ray afterglow of GRB970228 (Costa et al. 1997, Costa et al. 1997a) and the discovery of a related optical transient by ground-based telescopes (van Paradijs et al. 1997). Further results have been achieved with the detection of the X-ray afterglows of GRB970402 (Feroci et al. 1997b, Piro et al. 1997a), GRB970508 (Costa et al. 1997c, Piro et al. 1997b) and GRB971214 (Heise et al. 1997a, Antonelli et al. 1997). From GRB970508 an indication of an extragalactic origin has been derived through the detection of an optical transient (Bond, 1997; Djorgovski et al. 1997) and the measurement of its optical spectrum (Metzger et al. 1997), providing a lower limit of 0.835 for the redshift of the possible GRB optical afterglow.

One of the most intriguing mysteries of GRB emitters is possibly solved, but the overall picture is far from clear. In fact, out of the five events for which BeppoSAX performed rapid follow up searches of a X-ray counterpart, one (GRB970111) has given a result that is significantly different from the other four. The celestial location of GRB970111 was observed by BeppoSAX just 16 hours after the GRB event, and no unambiguous evidence for an X-ray afterglow was found. A new faint source (1SAX J1528.1+1937) was detected at a flux level that is much lower than that expected on the basis of the properties of the other GRBs later observed by BeppoSAX. Here we present this detection, discuss its association with GRB970111 and the diversity from the other four BeppoSAX GRBs.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998