TITLE: GCN CIRCULAR NUMBER: 13840 SUBJECT: GRB 121001A: GROND Afterglow Confirmation DATE: 12/10/02 23:46:14 GMT FROM: Alexander Kann at TLS Tautenburg S. Schmidl (TLS Tautenburg), D. A. Kann (MPE Garching), S. Klose (TLS Tautenburg), and J.Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 121001A (Swift trigger 535026, D'Elia et al., GCN # 13831) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile). Observations started on October 01, 2012, at 23:37 UT, 5.23 hr after the GRB trigger. They were performed at an average seeing of 2".7 (in r') and at an average airmass of 1.2. We found a source in the z', J, H and K bands at coordinates (J2000): RA = 18:24:07.84 Dec. = -05:39:55.30 with an error of 0".3 in each coordinate. This is consistent with the afterglow positions reported by Andreev et al. (GCN # 13833) and Tello et al. (GCN # 13835), but slightly outside the refined XRT position reported by Evans et al. (GCN # 18837). Based on 1380 s of total exposures in g'r'i'z' (mean time 0.2415 days after the trigger) and 1920 s in JHK (mean time 0.2363 days after the trigger) we estimate preliminary AB magnitudes (upper limits are 3 sigma) of g' > 23.6, r' > 22.9, i' > 22.2, z' = 21.7 +/- 0.2, J = 19.5 +/- 0.4, H = 18.4 +/- 0.1 and K = 17.94 +/- 0.06. Given magnitudes are calibrated against GROND zeropoints in the optical as well as 2MASS field stars in the NIR and are not corrected for the high Galactic foreground reddening of E_(B-V) ~ 1.3 mag (Schlegel et al. 1998). Our r'- and i'-band non-detections vs. the detections reported by Andreev et al. (GCN # 13833) and Tello et al. (GCN # 13835) confirm that this is the afterglow of GRB 121001A. Furthermore, the bright source we detect in K is not detected in 2MASS, while nearby sources of similar magnitude are clearly visible in 2MASS.