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

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2. Observations and analysis

The observations were obtained in the [FORMULA] -band (Wainscoat and Cowie 1992) with the 3.5-m telescope (equipped with Omega) at the German-Spanish Calar Alto observatory. Omega is a near-IR detector with a 1024 [FORMULA] 1024 pixel HgCdTe array. The image scale is [FORMULA] /pixel giving a field of view of 6.8 [FORMULA] 6.8 arcminutes (Bizenberger et al. 1998). Each image consists of 20 different frames forming a mosaic on the sky in order to avoid problems due to bad pixels and cosmic rays. Each frame is the co-addition of 10 images (3-second exposure time each), see Table 1 for details. The two sets of mosaics were started 3.24 and 4.77 hours after the GRB. Unfortunately no observations could be obtained on Dec 16 (the day after our measurements) due to bad weather thus preventing a prompt identification of the near-IR counterpart.


Table 1. Log of Calar Alto observations for GRB 971214.

Images were calibrated using the UKIRT faint standard FS17 (Casali and Hawarden 1992), which was imaged immediately after the first GRB mosaic. Since a K-standard star was used in the calibration, the value of K- [FORMULA] measured for the GRB depends upon the difference in spectral shape between the standard star and the GRB transient. Since the transient is fairly blue (optical spectral index [FORMULA] 0) like the standard star, then [FORMULA] 0 also for the transient. In fact, the maximum error in the value of K- [FORMULA] is [FORMULA] magnitudes (Wainscoat and Cowie 1992), i.e. not significant compared to the other photometric errors. The optical transient reported by Halpern et al. (1997) is clearly detected in both images, [FORMULA] and [FORMULA], respectively, over the limit (see Fig. 1). The short time span between both observations ([FORMULA] 1.5 hours) makes it difficult the calculation of the slope of the light curve. Photometry of the object yields [FORMULA] = [FORMULA] for the first observation and [FORMULA] = [FORMULA] for the second one. Therefore the brightness difference between [FORMULA] 3.5 and [FORMULA] 5 hours after the burst is [FORMULA] magnitudes. Astrometry of the near-IR counterpart gives [FORMULA] (equinox 2000.0), which is consistent with the optical position reported by Halpern et al. (1997).

[FIGURE] Fig. 1. GRB971214 in the [FORMULA] -band as seen by the 3.5-m telescope (+ Omega) at Calar Alto. The image has been obtained co-adding the observations taken 3.5 and 5 hours after the gamma-ray event. The bright star in the upper left is SAO 15663. The field of view is [FORMULA], containing [FORMULA] 75% of the WFC error box (the intersections of the WFC error box with the image are shown in the right upper and lower corners). The circle totally contained in the field of view represents the NFI error box. The position of the GRB counterpart, inside the small square, is only marginally consistent with the NFI error box. A blow-up containing the GRB counterpart is shown as an inset at the bottom left corner. North is at the top and east to the left.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998