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Astron. Astrophys. 330, 14-18 (1998)

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1. Introduction

Gamma-ray bursts (hereafter GRBs) are brief flashes of cosmic high energy photons, and they remain one of the major mysteries for high-energy astrophysicists, the main problem being the lack of knowledge about their distance scale. With the advent of the Italian/Dutch X-ray satellite BeppoSAX, it has been possible for the first time, to find counterparts at other wavelengths for both GRB 970228 (Costa et al. 1997, van Paradijs et al. 1997) and GRB 970508 (Piro et al. 1997a, Bond 1997). This development can be considered as one major step towards the solution of the GRB enigma.

GRB 970402 was detected as a rather weak, highly-structured Gamma-Ray Burst on April 2.93 UT by the PDS and Wide Field Camera instruments aboard BeppoSAX (Feroci et al. 1997, Heise & in't Zand 1997). The coordinates of the burst were: [FORMULA], [FORMULA] (equinox 2000.0) with an error circle of radius 3 [FORMULA]. Eight hours after the burst (April 3.28-3.57 UT), BeppoSAX detected a previously unknown X-ray source, labelled SAX J1450.1-6920, within the field of GRB 970402 (Piro et al. 1997b). The 2-10 keV flux from the source showed a decreasing trend. Only an upper limit was derived from the second X-ray observation (April 4.63-5.21 UT), indicating that the source decreased by at least a factor of three from one observation to the other. The decline in the X-ray emission was attributed to the X-ray afterglow of GRB 970402.

Searches for quiescent counterparts have been conducted in the past in all wavelengths, including the IR. Schaefer et al. (1987) used the IRAS data base at wavelenghts of 12, 25, 60 and 100 [FORMULA] m and looked for candidates within 23 well localized GRB error boxes, but without finding any convincing counterparts. Aiming to detect transient IR-emission from this burst, we requested a target-of-opportunity observation with ISO, the European Space Agency's Infrared Space Observatory (Kessler et al. 1996), that started only 55 h after the gamma-ray event. Soon after, we learned that a fading near-IR counterpart was indeed observed for GRB 970228 (Soifer et al. 1997) and another would subsequently be detected for GRB 970508 (Morris et al. 1997).

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998