GCN Circular 11830
G. Leloudas, D. Malesani (DARK/NBI), N.R. Tanvir (U. Leicester), Dong Xu (WIS), A.J. Levan (U. Warwick), P. Jakobsson (U. Iceland), T. Pursimo (NOT), report on behalf of a larger collaboration. We used the Nordic Optical Telescope (La Palma, Spain) equipped with ALFOSC to observe the field of GRB 110328A / Swift J164449.3+573451 (Cummings et al., GCN 11823; Barthelmy et al., GCN 11824). Observations started at 01:47 UT (12.8 and 12.1 hr after the first and second trigger, respectively). We obtained 2x600 s in BVR and 4x300 s in z. In all filters we detect an object at the coordinates (J2000): RA: 16:44:50.0 DEC: +57:34:59.13 with an astrometric calibration error of approximately 0.3 arcsec. This position is within the enhanced XRT error circle (Osborne et al., GCN 11826) and is consistent with the coordinates of the object reported by Cenko et al. (GCN 11827) from pre-outburst images. At the moment we have no good photometric calibration of the field so photometry should be considered preliminary (see also GCN 11827). Assuming R = R1 = 19.40 and B = B2 = 21.30 for the USNO B1.0 star 1475-0312998, we estimate for the counterpart B = 24.0 +- 0.2 and R = 22.6 +- 0.05 (statistical errors only). At this stage it is not possible to verify whether the source is variable.