5 GRB 980519

Figure 2: Sub-section of the STIS CL-band image centered on the OA for GRB 980519 and GRB 990308. Axis units are in arcseconds, North is up and East is to the left. Annotations as in Fig. 1 in addition to a dotted ellipse which marks the Bloom et al. (2002) host identifications.

GRB 980519 was detected by GRO/BATSE (Muller 1998) on 1998 May 19.51 UT and subsequent BeppoSAX WFI localisation (Piro 1998) enabled the detection of the OA (Jaunsen et al. 1998). Jaunsen et al. (2001) presented a homogeneous optical data sample obtained at the Nordic Optical Telescope (NOT) and found that the light-curve breaks at around t₀ + 0.5 days, with a pre- and post-break power law of $\alpha_{\rm O1}=1.73$ and $\alpha_{\rm O2}=2.22$, respectively. The X-ray (pre-break) power law was approximately, $\alpha_{\rm X}=1.6$ though this estimate is somewhat uncertain as it is based on combining the BeppoSAX WFC and NFI measurements. The maximum measured flux of the OA was I=18.4 and R=19.8 at t₀+0.346 and t₀+0.536 days, respectively. Correcting for the decline, the approximate R-I colour was $\sim$0.6.

We used the NOT observations of the OA (Jaunsen et al. 2001) to derive an accurate position of the burst in the STIS image. As the GRB 980519 afterglow was discovered and followed up at very high airmass, differential color refraction (DCR) affects the astrometry significantly. Following Monet et al. (1992) we correct for DCR by a term which is proportional to $\sec\ ({\rm Zdist})$ and depends on the color of each individual object. Since the colors of the tie objects and the afterglow cover a narrow range, the color dependence was approximated with a linear relation. The afterglow was observed in the R and the I bands, so two different DCR corrections must be applied. To minimize the number of free parameters, we choose to derive a theoretical estimate of the relative amplitude of the DCR correction in the R and I bands. From the different refraction across the filter bandwidth it is found that DCR in R is about 2.65 times DCR in I. By using this factor, only the amplitude of DCR as function of color for the complete data set of six images has to be determined. This is done by minimizing the standard deviation of the six independent localisations. For each localisation between 7 and 10 tie objects were used. The difference between the final averaged positions with and without DCR correction is of the order of 0.7 drizzled STIS pixels, comparable to the 1-$\sigma$ astrometric error. In effect, by applying the DCR correction, the average of the three R-band positions coincide with the average of the three I-band positions. The OA localisation result is given in Table 1.

At the OA position we detect two extended objects, clearly visible in Fig. 2. The photometry yields CL ABMAGs of $27.1\pm0.1$ and $27.9\pm0.1$ for the southern and northern component, respectively. The OA is located in the very outskirts of the northern component ($S/N\sim9$), where a faint blob coinciding with the position of the OA is detected (see also Table 2). A host detection was reported by Sokolov et al. (1998) and Bloom et al. (1998) at an estimated Cousins R VEGAMAG of $26.1\pm0.3$. This detection, however, consisted of the smeared sum of both objects. By using a larger aperture of $1\hbox{$.\!\!^{\prime\prime}$ }88$ (enclosing both objects) we find $\sim$ $26.45\pm0.10$. Assuming a flat spectrum and using the STSDAS SYNPHOT/CALCPHOT we converted the CL ABMAGs to Cousins R VEGAMAG, giving $\sim$26.0, in agreement with the earlier combined detections. It is also worth noting that the combined flux of the two objects amounts to 26.7 (ABMAG) as compared to 26.45 for the large aperture. The difference in flux can be attributed to the very low surface brightness (>29 mag) regions in the vicinity of the two major components. Assuming a redshift larger than 0.5, which is reasonable given the redshift distribution of other GRB hosts and the faintness of the host, we note that the average angular scale of 1 arcsec is $\sim$$7 \pm 2$ kpc for our assumed cosmology. Given this angular scale and the disk-like morphology it is most likely that the two detected objects are galaxies which are in the process of merging. This is supported by the low (<0.003) integrated probability (Gardner et al. 2000) of having two objects of this brightness within $\sim$2 sq. arcsec. The faint neighboring patches are therefore likely to be smaller galaxy fragments belonging to the merging system.