TITLE: GCN CIRCULAR NUMBER: 18461 SUBJECT: GRB 151023A DATE: 15/10/24 01:12:00 GMT FROM: Fabian Knust at MPE/GROND F. Knust (MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 151023A (Swift trigger 661253; D'Avanzo et al., GCN #18450) 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 23:52 UT on Oct. 23rd 2015, 10 hrs after the GRB trigger, and are continuing. They were performed at an average seeing of 1.6" and at an average airmass of 1.6 We found a single point source within the 2.3" Swift-XRT error circle reported by Evans et al. (GCN #18459) at RA (J2000.0) = 18:03:56.244 Dec. (J2000.0) = -08:18:57.42 with an uncertainty of 0.2" in each coordinate. Based on the first 4.4 min of total exposures in g'r'i'z' and 4.0 min in JHK, we estimate preliminary magnitudes (all in AB system) of g'> 22.1 mag, r' = 20.2 +/- 0.1 mag, i' = 19.1 +/- 0.1 mag, z' =18.4 +/- 0.1 mag, J = 17.6 +/- 0.2 mag, H = 17.0 +/- 0.1 mag, and K = 17.3 +/- 0.2 mag. Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=1.38 mag in the direction of the burst (Schlegel et al. 1998). [GCN OPS NOTE(24oct15): The suffix "A" was added to the Subject-line.]