TITLE: GCN CIRCULAR NUMBER: 13025 SUBJECT: GRB 120308A: Swift/UVOT Detection DATE: 12/03/08 19:32:24 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (MSSL-UCL) and W. H. Baumgartner (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 120308A 158 s after the BAT trigger (Baumgartner et al., GCN Circ. 13017). A source consistent with the optical position reported by Virgili et al. (GCN Circ. 13018) is detected in the initial UVOT exposures, but only in the v and b band. In u there is no evidence for a source, which suggests that the source is at a redshift of z < 4, and in case there is no contamination of dust, z ~ 3.2. The best UVOT position is RA(J2000) = 14h 36m 20.05s Dec(J2000) = 79d 41' 12.2" with an uncertainty of 0.55" (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 463 483 20 17.02 +/- 0.17 b 413 432 19 17.65 +/- 0.13 u 158 7690 1020 >20.9 w1 512 13962 1165 >21.3 m2 636 13409 1415 >21.3 w2 438 8103 646 >21.5 b 711 730 19 18.40 +/- 0.25 v 761 1082 39 18.14 +/- 0.29 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).