TITLE: GCN CIRCULAR NUMBER: 25978 SUBJECT: GRB 191004B: Swift/UVOT Detection DATE: 19/10/07 20:45:09 GMT FROM: Sam LaPorte at PSU GRB 191004B: Swift/UVOT Detection S. J. LaPorte (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 191004B 69 s after the BAT trigger (Cenko et al., GCN Circ. 25947). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 25951) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 03:16:49.10 = 49.20459 (deg.) Dec (J2000) = -39:38:03.9 = -39.63441 (deg.) with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence), consistent with the positions reported by LCO (Strausbaugh et al. GCN Circ. 25949), NOT (Heintz et al. GCN Circ. 25954), VLT/X-Shooter (D'Elia et al. GCN Circ. 25956), COATLI (Watson et al. GCN Circ. 25958), and GROND (Schady et al. GCN Circ. 25960). 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 b 38697 38997 295 >20.42 uvm2 44827 45727 885 >20.25 u 294 387 91 >19.04 v 43916 44822 885 >20.07 uvw1 45734 46281 538 >19.96 uvw2 39915 40544 618 >20.09 white 82 232 147 18.01+-0.05 white 39003 39909 885 20.51+-0.13 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).