TITLE: GCN CIRCULAR NUMBER: 18688 SUBJECT: GRB 151212A: GROND Optical Afterglow Candidate DATE: 15/12/13 09:07:28 GMT FROM: Philip Wiseman at MPE/Swift P. Wiseman (MPE Garching), S. Schmidl (TLS Tautenburg) and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 151212A (MAXI trigger; Nakahira et al., GCN #18685) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 03:27 UT on December 13, 2015, 13.5 hrs after the GRB trigger. They were performed at an average seeing of 1.7" and at an average airmass of 1.1, and through a significant layer of clouds. We detect a single source at the border of the enhanced Swift-XRT error circle (http://www.swift.ac.uk/xrt_positions/00020570/), which we propose is the afterglow, at RA, Dec 68.150, -3.9626, which is equivalent to: RA (J2000.0) = 04:32:36.1 Dec (J2000.0) = -03:57:45.3 with an uncertainty of 0.5" in each coordinate. Based on the 72 min of total exposures in g'r'i'z'J and H, at a mid-time of 14.1 hrs after the burst, we measure the following preliminary magnitudes and upper limits (AB magnitude system): g' = 24.3 +/- 0.2 mag, r' = 23.6 +/- 0.2 mag, i' = 23.1 +/- 0.2 mag, z' = 23.3 +/- 0.3 mag, J > 21.2 mag, and H > 20.7 mag. We cannot say anything about fading at this time, but the power-law-like magnitudes lead us to suspect this is the afterglow. Given magnitudes and upper limits are calibrated against SDSS as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.03 mag in the direction of the burst (Schlegel et al. 1998).