Astron. Astrophys. 364, L54-L61 (2000)

3. Localization and identification of the afterglow

The late localization of the GRB complicated the detection of the optical afterglow, expected to be significantly fainter than the limit of existing sky surveys. Furthermore, the (presumed power-law) decay of the afterglow would be so slow that observations separated by several days would have to be compared before any probable candidate source could be established.

Images of the Inter Planetary Network (IPN) error box were obtained with the FORS1 instrument on Antu (ESO VLT UT1), starting 84 h after the burst. Four 120 s exposures in the B,V and R bands of each of two FORS1 fields, covering the error box (see Fig. 2), were acquired under good seeing conditions (see Table 2 for a log of the observations). On the same night we acquired I-band exposures with a total integration time of 3600 s, using DFOSC on the Danish 1.54-m telescope on La Silla. From this first set of exposures no candidate optical transient could be identified. A subsequent set of images with the same exposure times was acquired 135 h after the burst, under less favorable seeing conditions. Comparing these two epochs, one source located at R.A. = 613, Dec. = -51^o 56´ (J2000) was found to have declined by about 1.1 mag in the R-band. In the first epoch images this source was also detected in the V-band, but not in the B-band. Its proximity () to the center of the error box, in combination with a fading of about half a magnitude per day, which is typical for GRB afterglows after some days, made this object the likely afterglow of GRB 000131 (Pedersen et al. 2000). The afterglow of GRB 000131 was the second to be identified based solely on an IPN localization (Hurley 2000a).

Fig. 2. The rhombus shaped IPN error box with the two square FORS1 fields superimposed. The location of the OT (not present in this Digital Sky Survey image) is marked.

Table 2. Journal of our observations of GRB 000131.

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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