TITLE: GCN CIRCULAR NUMBER: 12356 SUBJECT: GRB 110915B: GROND Observations of the Optical/NIR Afterglow Candidate DATE: 11/09/19 16:23:34 GMT FROM: Alexander Kann at TLS Tautenburg J. Elliott (MPE Garching), D. A. Kann (TLS Tautenburg), and J. Greiner (also MPE Garching) report on behalf of the GROND team: We observed the field of the AGILE GRB 110915B (E. Del Monte et al., GCN 12341) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP, 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations were taken starting at 06:59 UT on 18 September 2011 (2.5 days after the GRB trigger). They were performed at an average seeing of 1".4 and at an airmass of 2, with the Moon only 22 degrees away. We detect the afterglow candidate reported by A. de Ugarte Postigo et al. (GCN 12350). Based on an exposure time of 4502 s in the optical, we estimate an r' magnitude (in AB) of: r' = 23.2 +/- 0.1. Compared to the magnitudes reported by A. de Ugarte Postigo et al., the candidate shows no decay over the past 2.1 days and so is likely not the GRB afterglow. We find a flux ratio between optical and X-rays of f_x/f_opt = 4500 which indicates an AGN nature if the X-ray emission (P. D'Avanzo et al., GCN 12348) has not faded. Given magnitudes are calibrated against GROND zeropoints and shifted based on the USNO-B1.0 star used by A de Ugarte Postigo., RA = 05:10:09.720, Dec. = +01:56:40.12, and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.07 in the direction of the burst (Schlegel et al. 1998). We are indebted to A. de Ugarte Postigo for providing us a finding chart.