TITLE: GCN CIRCULAR NUMBER: 13725 SUBJECT: GRB 120907A: Swift/UVOT Detection DATE: 12/09/07 19:17:57 GMT FROM: Samantha Oates at MSSL S. R. Oates (MSSL-UCL) and J. L. Racusin (NASA/GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 120907A 142 s after the BAT trigger (Racusin et al., GCN Circ. 13716). A source consistent with the enhanced XRT position (Goad et al. GCN Circ. 13722) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 04:59:00.00 = 74.75002 (deg.) Dec (J2000) = -09:18:54.0 = -9.31501 (deg.) with an estimated uncertainty of 0.52 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag v 449 6811 471 >19.5 b 398 567 39 19.02 +/- 0.29 u 142 392 246 17.67 +/- 0.08 w1 498 666 40 18.27 +/- 0.32 m2 473 1122 79 18.68 +/- 0.36 w2 424 642 39 18.56 +/- 0.37 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.09 in the direction of the burst (Schlegel et al. 1998).