TITLE: GCN CIRCULAR NUMBER: 17078 SUBJECT: GRB 141121A: GROND Afterglow candidate DATE: 14/11/21 07:15:06 GMT FROM: Alexander Kann at TLS Tautenburg M. Tanga (MPE Garching), D. A. Kann (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 141121A (Swift trigger 619182; A. Y. Lien et al., GCN 17075) 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 05:52 UT on November 21, 2014, 2.02 hours after the GRB trigger. They were performed at an average seeing of 1".4 and at an average airmass of 2.3. Within the XRT error circle reported by Lien et al., we detect a bright source in all bands. This source is not detected in the SDSS, therefore we consider it to be the afterglow of GRB 141121A. The source lies at: RA (J2000) = 08:10:40.62 Dec. (J2000) = +22:13:01.8 with an error of 0".3 in each coordinate. Based on total exposures of 7.7 minutes in g'r'i'z' and 8.0 minutes in JHK, at a midtime of 2.34 hrs after the burst, we measure the following preliminary magnitudes (AB magnitude system): g' = 20.0 +/- 0.1 mag, r' = 19.8 +/- 0.1 mag, i' = 19.6 +/- 0.1 mag, z' = 19.4 +/- 0.1 mag, J = 19.4 +/- 0.2 mag, and H = 19.2 +/- 0.2 mag. The magnitudes are calibrated against SDSS and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.05 in the direction of the burst (Schlegel et al. 1998).