TITLE: GCN CIRCULAR NUMBER: 10813 SUBJECT: Swift UVOT and UVOT-enhanced XRT position for GRB 100528A DATE: 10/05/28 22:03:49 GMT FROM: Dirk Grupe at PSU/Swift-XRT D. Grupe (PSU), M. Siegel (PSU), J. Gelbord (PSU), P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift team: We report of the Swift UVOT and XRT detections of a bright source within the 5' error radius of the SuperAGILE detected GRB 100528A (Del Monte et al., GCN Circ 10810) at a level of 2e-12 ergs/s/cm2 in the 0.3-10 keV band. Swift started observing the field of GRB 100528A 42.5 ks after the AGILE detection and has observed it so far for 5 ks (two orbits). At this point it is not obvious if the X-ray source is fading. However, this X-ray source was not present during the ROSAT All-Sky Survey. There is also an archival XMM data set that covers this position (OBSID 0037990101, a 17ks exposure taken 2002-11-21). There is no source detected at the position of the counter part. Measuring the flux within a 30" radius of this position, the 4-sigma upper limit of 5e-14 ergs/s/cm2 in the 0.2-12 keV band. Most likely the X-ray source found by the XRT is the X-ray afterglow of GRB 100528A. Using 2497 s of XRT Photon Counting mode data and 2 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 311.14130, 27.80660 which is equivalent to: RA (J2000): 20 44 33.92 Dec (J2000): +27 48 23.8 with an uncertainty of 2.1 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). Using 4985 s of UVOT data take in the u, v and White filters, we identify a faint uncataloged source within the XRT error circle at a position in the white images of RA,DEC=311.14130, 27.80660 which is equivalent to: RA (J2000): 20 44 33.92 Dec (J2000): +27 48 23.8 with a 90%-confidence interval of about 0.60 arc sec. Preliminary magnitudes using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) are: FILTER T_start(s) T_stop Exposure Mag ========================================================= white 43296 49881 1632 20.96+-0.13 v 44124 50777 1721 20.01+-0.35 u 42469 49036 1632 20.63+-0.24 ========================================================= The above magnitudes are not corrected for the Galactic extinction corresponding to a reddening of E_{B-V} = 0.29 (Schlegel et al., 1998, ApJS, 500, 525). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627). This circular is an official product of the Swift team.