Z. Cano (U Iceland), S. Schulze (PUC, MCSS), D. Malesani (DARK/NBI), P. Jakobsson (U Iceland), D. J. Watson (DARK/NBI), N. R. Tanvir (U Leicester), D. Xu, J. P. U. Fynbo (DARK/NBI) and M. Hayes (Stockholm University) report on behalf of a larger collaboration:
We observed the field of GRB 131103A (Cummings et al., GCN Circ. 15440) with the 2.5-m Nordic Optical Telescope (NOT) equipped with ALFOSC, and subsequently with the ESO VLT using the optical acquisition camera of the X-shooter spectrograph, as well as with the HAWK-I near-infrared imager. A brief summary of our observations is reported below. For the NOT observation, the object was very low on the horizon (airmass = 4.56), hence we could only achieve a poor S/N.
At the position of the UVOT afterglow (Kuin et al., GCN Circ. 15443), a source is detected in all our images. It is clearly elongated in both R and J as seen in the VLT data, and we thus suggest that this is the host galaxy, though some afterglow contribution cannot be excluded. We note that a marginally significant object is visible in the DSS (R band) at the same position.
Instrument Time (hr) Filter Magnitude
NOT/ALFOSC 1.29 R 20.07 +/- 0.12
VLT/X-shooter 4.48 R 20.48 +/- 0.01
VLT/HAWK-I 5.94 J 19.63 +/- 0.02
While the comparison of the two R-band magnitudes provides some evidence for fading, we note that both R-band measurements have an uncertain calibration, since no common star is available between the ALFOSC and X-shooter images. The two measurements are therefore tied to different USNO stars, which can introduce an error of ~0.1-0.2 mag. Specifically, we used the R1 magnitudes of stars 0453-0803362 and 0453-0803335 for ALFOSC and X-shooter, respectively. The HAWK-I observation was calibrated against five nearby 2MASS stars.
We kindly acknowledge excellent support by the observing staff at the VLT, in particular Claudia Reyes, Dimitri Gadotti, Julien Girard and Roger Wesson.
[GCN OPS NOTE(12nov13): Per author's request, the observing time for the VLT/X-shooter
was changed from 5.84 to 4.48.]