GCN Circular 9699
A. Cucchiara (PSU), S. B. Cenko (UC Berkeley), D. B. Fox (PSU), E. Ofek (Caltech), N. Tanvir (U. Leicester) report: "On April 21.26 UT and 22.24 UT we obtained two R-band images of the Fermi/Swift GRB 090720 (GCN 9688, 9694) with GMOS on the Gemini-North telescope for a total exposure time, at each epoch, of 15 minutes. In the coadded images we identify a possible R counterpart inside the XRT error circle (Grupe et al., GCN 9697) at the coordinates: RA (J2000): 13:34:43.33 Dec (J2000): -10:20:04.9 with an uncertainty of 0.5". The estimated magnitude at the two epochs are: R = 21.50 +- 0.03 mag R = 22.34 +- 0.04 mag The implied fading, by 0.84 +- 0.05 mag between our two epochs, is consistent with a power-law decay of index alpha=1.12 +- 0.11 referenced to the burst trigger time. We conclude from this fading behavior that the source is the optical afterglow of GRB 090720. Magnitude values are calibrated using the USNO B1 star at RA= 13:34:39.9 Dec= -10:20:17.06 which has tabulated magnitude R = 19.79. We thank the Gemini-N staff for the support in performing this observations, in particular Tom Geballe and Paul Hirst."