2 Discovery of optical afterglows

2.1 GRB 980519

The afterglow of GRB 980519 was discovered at NOT less than 9 hours after the burst occurred as a rapidly fading $R \sim 19.5$ source that was not present in the Digital Sky Survey (Fig. 1). A GCN circular announcing the position of the optical transient (OT) [7, (Jaunsen et al. 1998)] was issued less than 6 hours after the first NOT images were obtained, making it the earliest reported GRB OT discovery to date (December 1998). We obtained a high-quality multi-colour lightcurve showing that the OT brightness decayed with a power-law exponent of $\alpha = -2.0 \pm 0.1$. An optical spectrum of the OT was also obtained. These results will be reported elsewhere.

  

Figure 1: GRB 980519. The upper panel shows the discovery image of the OT (V). The lower panel shows the host galaxy, including a possible contribution from the OT (R). North is up, east is to the left. The images are $45\hbox{$^{\prime\prime}$}\times45\hbox{$^{\prime\prime}$}$

  

Figure 2: GRB 980613. The upper panel shows the discovery image of the OT (R). The lower panel shows the host galaxy imaged after the OT had faded away (R). North is up, east is to the left. The images are $45\hbox{$^{\prime\prime}$}\times45\hbox{$^{\prime\prime}$}$

2.2 GRB 980613

In the discovery image for the OT associated with GRB 980613 (Fig. 2), obtained 16.5 hours after the burst [5, (Hjorth et al. 1998)], the OT was 3.5 mag fainter ($R\approx23$) than that of GRB 980519. Consequently, very little is known about the spectral and decay properties of this system which was the faintest burst detected so far by SAX (Costa et al. 1998). However, the location of the OT/GRB led to the discovery of the host galaxy (Fig. 2) and its redshift, z=1.096 [4, (Djorgovski et al. 1999)].