TITLE: GCN CIRCULAR NUMBER: 17541 SUBJECT: Trigger 633105 (aka GRB150301C): Swift/UVOT Observations of XRT Counterpart DATE: 15/03/04 03:16:06 GMT FROM: Lea Hagen at PSU L. M. Z. Hagen (PSU) and A. Y. Lien (GSFC/UMBC) report on behalf of the Swift/UVOT team: A point source is detected by UVOT that is consistent with the XRT counterpart and 2.5 arcsec from the XMM-Newton source discussed in Evans et al. (GCN Circ. 17536). This source is also seen in archival DSS imaging. It is unclear if this is the same object detected by XRT or a serendipitous source. Given how crowded the field is, it is also likely that the UVOT source is comprised of multiple individual sources. The coordinates of the UVOT source are RA(J2000) = 00:45:14.76 = 11.31150 DEC(J2000) = +41:50:35.2 = 41.84311 Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the available exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 102 4151 369 19.22+-0.13 v 592 4562 216 >18.53 b 518 5241 96 >19.08 u 261 5177 461 18.60+-0.15 uvw1 641 79666 5117 18.64+-0.05 uvm2 617 45953 1237 18.94+-0.13 uvw2 568 4357 216 18.99+-0.27 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.06 in the direction of the source (Schlegel et al. 1998). This field has also been observed previously by UVOT as part of a different observing campaign, primarily in 2013 and 2014. The stacked images have the following detections: Filter Exp(s) Mag v 1406 19.04+-0.19 b 1445 19.77+-0.15 u 27521 18.65+-0.03 uvw1 52236 18.57+-0.02 uvm2 58908 18.65+-0.03 uvw2 65719 18.61+-0.02 These indicate possible variability of the source, but further analysis will be required to confirm this.