TITLE: GCN CIRCULAR NUMBER: 15309 SUBJECT: GRB 131004A: GROND observations of the afterglow DATE: 13/10/05 08:38:02 GMT FROM: Sebastian Schmidl at TLS Tautenburg S. Schmidl, A. Nicuesa Guelbenzu, S. Klose (all TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 131004A (Swift trigger 573190; Hagen et al., GCN 15303) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at ESO La Silla Observatory (Chile). Observations started on October 4, 2013, at 23:41 UT, 2 hrs after the GRB trigger. They were performed at an average seeing of 0".9 and at an average airmass of 1.1. The optical afterglow, discovered by Xu et al. (GCN 15304), is clearly detected. Based on a total exposure time of 460 s in g'r'i'z' and 480 s in JHK, at a midtime of 2.75 hrs after the burst, we measure the following preliminary AB magnitudes: g' = 22.9 +/- 0.1, r' = 22.1 +/- 0.1, i' = 21.6 +/- 0.1, z' = 21.4 +/- 0.1, J = 20.2 +/- 0.2, H = 20.2 +/- 0.2, and K > 19.8 . 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)=0.29 mag in the direction of the burst (Schlegel et al. 1998).