TITLE: GCN CIRCULAR NUMBER: 13666 SUBJECT: GRB 120815A: Swift/UVOT Detection of the Optical Afterglow DATE: 12/08/17 12:25:18 GMT FROM: Stephen Holland at STScI S. T. Holland (STScI) and C. Pagani (U Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 120815A starting 2672 s after the BAT trigger (Pagani et al., 2012, GCNC 13645). Settled observations started at 2690 s. We detect an uncatalogued source inside the UVOT-enhanced XRT error circle (Goad et al., 2012, GCNC 13650) with preliminary coordinates RA (J2000.0) 18:15:49.88 = 273.95783 (deg) Dec (J2000.0) -52:07:52.1 = -52.13114 (deg) and an estimated uncertainty of 0.6 arcsec (radius, 90% confidence, statistical + systematic). The UVOT detection is consistent with the optical afterglow reported by Sudilovsky et al. (2012, GCNC 13648) Source magnitudes and 3-sigma upper limits at this location are given below. The afterglow is 4.27 arcsec northeast of the USNO-B1.0 source 0378-0977505, so the following photometry should be considered preliminary. Filter TSTART TSTOP Exp Time Mag Err Sigma ------------------------------------------------------------ v 2847 3046 100 19.34 0.29 3.9 b 3667 4120 226 20.11 0.24 4.7 u 3462 3661 100 >20.5 uvw1 3257 3457 100 >20.2 uvm2 3052 3252 100 >19.9 uvw2 4330 4530 100 >20.3 white 2690 2840 75 19.94 0.19 5.8 ------------------------------------------------------------ The quoted magnitudes and upper limits have not been corrected for the Galactic extinction along the line of sight to this burst of E_{B-V} = 0.10 mag (Schlafly et al. 2011, ApJS, 737, 103).