GCN Circular 18122
Paul Kuin (MSSL/UCL) and P. A. Evans (U Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150811A 147 s after the BAT trigger (Evans et al., GCN Circ. 18119). A source consistent with the XRT position and the optical transient reported by Butler et al.(GCN Circ. 18120) and Zheng et al. (GCN Circ. 18121) is detected in the promptly available UVOT exposures. The preliminary UVOT position is: RA (J2000) = 19:25:21.4 Dec (J2000) = -15:25:31.1 with an estimated uncertainty of 0.6 arc sec. (radius, 90% confidence). The transient source is at a distance of 4.6 arcsec of a weak nearby source. Preliminary detections for the White and U finding charts 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 white_FC 147 297 147 20.43 +/- 0.23 u_FC 306 549 241 18.24 +/- 0.11 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.17 in the direction of the burst (Schlegel et al. 1998).