Astron. Astrophys. 335, L5-L8 (1998)

1. Introduction

After 31 years since the discovery of gamma-ray bursts (GRBs), the origin of such brief gamma-ray flashes remains unknown. The observed isotropy of GRBs in the sky, could only be explained by theoretical models where GRBs originate either in a extended halo around the Galaxy or arise from sources at cosmological distances. Before the launch of the BeppoSAX satellite, the poor localization capability of the GRB detectors made the searches at other wavelengths unfruitful.

The breakthrough took place in 1997 when the first X-ray afterglows were observed by the BeppoSAX, RXTE, ROSAT and ASCA satellites (Costa et al. 1997, Heise et al. 1997a, Marshall et al. 1997, Greiner et al. 1998, Murakami 1998). They were able to localize the fading X-ray emission that followed the more energetic gamma-ray photons once the GRB event had ended. This emission (the afterglow) extends to longer wavelengths, and the good accuracy in the position determination by BeppoSAX (typically radius error boxes) has led to the discovery of the first optical counterparts for GRB 970228 (van Paradijs et al. 1997, Guarnieri et al. 1997), and GRB 970508 (Bond 1997, Djorgovski et al. 1997, Castro-Tirado et al. 1998), greatly improving our understanding of these puzzling sources. The measurement of the redshift for the GRB 970508 optical counterpart (Metzger et al. 1997) has established that one GRB, maybe all, lie at cosmological distances.

GRB 971214 is the third GRB with a known optical counterpart. It was detected by the BeppoSAX Gamma-ray Burst Monitor (GBM, Frontera et al. 1997) on Dec 14.97 1997, as a 25 s long-structured gamma-ray burst. Simultaneous to the detection of the GBM, the Wide Field Cameras (WFC, Jager et al. 1997) on board BeppoSAX provided an accurate position (a radius error box at a 3 confidence level, Heise et al. 1997b) that allowed deep optical, infrared and radio observations. The position was also consistent with the one given by the all-sky monitor on RXTE (Bradt et al. 1993) and by the BATSE/Ulysses annulus (Kippen et al. 1997). When BeppoSAX pointed its Narrow-Field Instruments (NFI) to the GRB position, on Dec 15.25 ( 6.5 hours after the burst), a previously unknown variable X-ray source was found inside the WFC error box (Antonelli et al. 1997) which was identified as the X-ray afterglow of GRB 971214. Soon after, Halpern et al. (1997) reported the presence of a fading object inside the WFC GRB error box, based on two I-band images separated 24 hours. The object was afterwards confirmed as the counterpart of GRB 971214 by means of additional observations at other wavelengths: R-band (Castander et al. 1997, Diercks et al. 1998), I-band (Rhoads 1997) and J-band (Tanvir et al. 1997). No detections in the K-band were reported in the literature, although observations performed on Dec 15.54 imposed an upper limit of K 18.5 (Garcia et al. 1997). As it will be explained later, this upper limit will be used to constraint the power-law index and the position of the possible maximum of the light curve.

We report here the detection of the GRB 971214 counterpart in the near infrared (IR) by means of two -band images taken at Calar Alto on Dec 15.12 and 15.18 (mean observing time, only 3.5 and 5 hours after the gamma-ray event). The second image is almost simultaneous to the beginning of the observations performed by the BeppoSAX narrow-field instruments. We discuss whether our observations are in agreement with the extrapolation of the power-law seen at other bands in later epochs.

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

helpdesk.link@springer.de