TITLE: GCN CIRCULAR NUMBER: 18270 SUBJECT: GRB 150910A: Swift/UVOT Detection of a Brightening Afterglow DATE: 15/09/10 16:11:49 GMT FROM: Marissa McCaule at PSU L. M. McCauley (PSU) and C. Pagani (U Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150910A 154 s after the BAT trigger (Pagani et al., GCN Circ. 18264). A source consistent with the XRT position (Pagani et al. GCN Circ. 18264) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 00:22:40.11 = 5.66713 (deg.) Dec (J2000) = +33:28:21.9 = 33.47274 (deg.) with an estimated uncertainty of 0.58 arc sec. (radius, 90% confidence). Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) show a very rapid and steep rise in the luminosity of the afterglow over the first 1 ks of observation. Further investigation is strongly encouraged. Preliminary detections for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 153 303 147 19.90 white 156 303 144 19.93 white 592 612 19 17.09 white 765 785 19 16.67 white 866 1016 147 16.30 v 135 145 9 >17.23 v 642 662 19 16.64 v 815 835 19 16.91 v 1046 1066 19 16.11 b 568 588 19 17.56 b 741 761 19 16.87 u 312 561 245 18.03 u 312 561 245 18.02 u 716 736 19 16.14 uvw1 692 712 19 16.76 uvw2 1022 1041 19 >17.97 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.05 in the direction of the burst (Schlegel et al. 1998).