TITLE: GCN CIRCULAR NUMBER: 13516 SUBJECT: GRB 120724A: Swift/UVOT Detection of the Optical Afterglow DATE: 12/07/25 13:11:42 GMT FROM: Stephen Holland at STScI S. T. Holland (STScI) and P. D'Avanzo (INAF-OAB) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 120724A starting 99 s after the BAT trigger (D'Avanzo et al., 2012, GCNC 13510). Settled observations started at 120 s. We detect a faint source with preliminary coordinates RA (J2000) 16:20:43.35 = 245.18062 (deg) Dec (J2000) +03:30:27.8 = +3.50772 (deg) and an estimated uncertainty of 0.77 arcsec (radius, 90% confidence, statistical + systematic). This position is consistent with the optical source discovered by Guidorzi et al. (2012, GCNC 13511) and is slightly outside the UVOT-enhanced XRT position (D'Avanzo et al., 2012, GCNC 13514). Preliminary magnitiudes for this source, and 3-sigma upper limits for the finding charts (FC) and the co-added images, are Filter TSTART TSTOP EXPOSURE Mag Err Sigma ---------------------------------------------------------------- white (FC) 117 267 147 >21.3 u (FC) 330 580 246 >20.5 white (FC) 609 629 20 >19.8 ---------------------------------------------------------------- v 832 1404 72 >19.2 b 585 1331 58 20.01 0.41 2.7 u 330 1461 288 20.50 0.37 3.0 uvw1 1088 1453 59 >19.2 uvm2 1237 1429 39 >18.4 uvw2 635 1380 78 >19.5 white 117 1355 199 21.54 0.45 2.5 ---------------------------------------------------------------- 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} = 0.05 mag (Schlafly et al. 2011, ApJS, 737, 103).