TITLE: GCN CIRCULAR NUMBER: 13641 SUBJECT: GRB 120811A: Swift/XRT-UVOT Obseravations DATE: 12/08/14 15:57:21 GMT FROM: Binbin Zhang at PSU B.-B. Zhang (PSU), S. R. Oates (UCL-MSSL), K. L. Page (U Leicester), P.A. Evans (U. Leicester), D. N. Burrows (PSU), J. R. Cummings (GSFC/UMBC), J. A. Kennea (PSU),H. A. Krimm (GSFC/USRA), E. Helder (PSU) and S. D. Barthelmy (GSFC) report on behalf of the Swift-XRT and Swift-UVOT team: We report further Swift XRT-UVOT observations of GRB 120811A (Zhang et al, GCN 13618). We performed 9.1 ks follow-up XRT observations of the field at T0+1.6 hr, T0+0.27 day, T0+0.94 days and T0+3.1 days (where T0 = 2012 Aug 11 at 02:35:18 UT ), in order to determine whether or not the X-ray source had faded. We find a fading source with count rates in the 0.3-10 keV band as follows : T-T0 Count Rate (5.8+/-0.2)x10^3 s 0.05+/-0.01 count/s (2.2+/-0.01)x10^4 s 0.02+/-0.01 count/s (8.1+/-0.03)x10^4 s 0.02+/-0.001 count/s (2.7+/-0.2)x10^5 s < 0.005 count/s The data are entirely in Photon Counting (PC) mode. Using 1227 s of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 257.16542, -22.71054 , which is equivalent to: RA (J2000): 17h 08m 39.70s Dec(J2000): -22d 42' 38.0" with an uncertainty of 2.8 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=0.31 (+0.13, -0.12). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+/-0.5). The best-fitting absorption column is 7.4 (+4.2, -3.0) x 10^21 cm^-2, in excess of the Galactic value of 1.5 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.5 x 10^-11 (9.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.4 (+4.2, -3.0) x 10^21 cm^-2 Galactic foreground: 1.5 x 10^21 cm^-2 Excess significance: 3.2 sigma Photon index: 1.9 (+/-0.5) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00530581. The fact that the XRT source is fading suggests that Swift trigger 530581 (Zhang et al., GCN Circ. 13618) is most likely a GRB. The Swift/UVOT began settled observations of the field of GRB 120811A 5627 s after the BAT trigger (Zhang et al., GCN Circ. 13618). No optical afterglow consistent with the XRT position is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system(Breeveld et al. 2011,AIP Conf. Proc. 1358, 373) for the initial exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 6243 6443 197 > 20.7 v 6654 6853 197 > 19.1 b 6038 7593 313 > 20.2 u 5832 7468 393 > 20.0 uvw1 5627 7263 393 > 19.9 uvw2 6449 6649 197 > 19.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.77 in the direction of the burst (Schlegel et al. 1998). This circular is an official product of the Swift-XRT and Swift-UVOT team.