TITLE: GCN CIRCULAR NUMBER: 11774 SUBJECT: GRB 110305A: GROND Detection of Optical Afterglow DATE: 11/03/05 11:56:58 GMT FROM: Patricia Schady at MPE/Swift A. Nicuesa Guelbenzu (TLS Tautenburg), V. Sudilovsky, P. Schady and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 110305A (Swift trigger 448229; Grupe et al., GCN #11773) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 06:40 UT on March 5th, 2 minutes after the GRB trigger. They were performed at an average seeing of 0.8" and at an average airmass of 1.8. We found a fading source within the 2.2" Swift/XRT error circle reported by Grupe et al. (GCN #11773) at RA (J2000.0) = 17h 23m 31.3s DEC (J2000.0) = -15d 48' 08.8" with an uncertainty of 0.3" in each coordinate. The source decayed by 0.8mag in 60.5 minutes, suggesting that it is the optical afterglow of GRB110305A. At T+116.9 minutes the source had an r'-band magnitude of 23.7 +/- 0.1. These measurements are not corrected for the considerable Galactic foreground extinction corresponding to a reddening of E(B-V)=0.390 mag in the direction of the burst (Schlegel et al. 1998).