TITLE: GCN CIRCULAR NUMBER: 12308 SUBJECT: GRB110825A: Swift target of opportunity observation DATE: 11/08/26 21:29:20 GMT FROM: Binbin Zhang at PSU B.-B. Zhang (PSU) and D. Grupe (PSU) reports on behalf of the Swift-XRT team: Swift began a target of opportunity observation of the IPN-detected burst: GRB110825A (Hurley et al. et al. GCN Circ. 12307) on August 26, 2011 at 01:05 UT, approximately 22.7 hours after the burst was detected by IPN. Swift data for these observations utilize Target ID 20183. We have analysed 2.9 ks of XRT data for this burst, from 81.3 ks to 94.0 ks after the IPN trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the XRT field-of-view. The refined XRT position is RA, Dec = 44.79846, +15.42733 which is equivalent to: RA (J2000): 02 59 11.63 Dec(J2000): +15 25 38.4 with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is 346 arcsec from the IPN position and about 180 arcsec from the edge of IPN error box (see http://www.personal.psu.edu/buz12/IPN_ERRORBOX_VS_XRT_GRB110825A.png). We cannot determine at the present time whether the source is fading. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.8, -0.4). The best-fitting absorption column is consistent with the Galactic value of 9.9 x 10^20 cm^-2 (Kalberla et al. 2005). The count rate for the X-ray source is about 0.01 count/sec, which is relatively bright when comparing to other GRB's X-ray afterglow at t-T0=10^5 seconds. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10^-11 (4.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 0 (+1.7, -0) x 10^21 cm^-2 Galactic foreground: 9.9 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.8 (+0.8, -0.4) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020183. No source was found from UVOT data within the XRT error circle. Preliminary 3-sigma up-limits are : Filter EXPOSURE Mag --------------------------------------------------- v & 579 & >19.19 u & 1365 & >20.79 white & 895.63 & >21.40 --------------------------------------------------- We notice that there is no known source at the X-ray source position in Simbad or the NED. We also notice that the RASS data for this position were not deep enough to detect a source of this flux (0.01 count/sec). We have scheduled further Swift TOO for this target. This circular is an official product of the Swift-XRT team. Burst Advocate for this burst is B.-B. Zhang (bbzhang AT psu.edu). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)