GCN Circular 11141
P. D'Avanzo (INAF-OAB), A.J. Levan (U. Warwick), D. Malesani (DARK/NBI), N.R. Tanvir (U. Leicester), E. Palazzi (INAF-IASFBo), A. Fiorenzano (INAF-TNG), report: We observed the field of GRB 100823A (Mangano et al. GCN 11135) using the Italian 3.6m TNG telescope, located in the Canary Islands. A set of J-band images were acquired on Aug 24.03490 UT (~7.4 hours after the burst; mid exposure time) for a total exposure time of 16 minutes. We detect the two objects reported by Levan et al. (GCN 11138) and Guziy et al. (GCN 11140) with magnitudes J=21.0 for object "A" (the one within the refined XRT error circle; Goad et al. GCN 11136) and J=19.4 for object "B", calibrated against the 2MASS catalogue. Although we have yet to photometrically calibrate the z-band image, from a comparison of the TNG and WHT images (Levan et al. GCN 11138) we note that object "B" has a z-J color significantly redder with respect to the average color of field stars. As suggested by Levan et al (GCN 11138) object "B" can thus be a red source not visible in the DSS2 red image. In light of this, its location outside the XRT refined error circle would make it likely unrelated to the GRB.