{
  "bibcode": "2012GCN.13641....1Z",
  "body": "B.-B. Zhang (PSU), S. R. Oates (UCL-MSSL),  K. L. Page (U Leicester), P.A.\nEvans (U. Leicester), D. N. Burrows (PSU), J. R. Cummings (GSFC/UMBC),  J. A.\nKennea (PSU),H. A. Krimm (GSFC/USRA), E. Helder (PSU) and S. D. Barthelmy\n(GSFC) report on behalf of the Swift-XRT and Swift-UVOT team: \n\nWe report further Swift XRT-UVOT observations of GRB 120811A (Zhang et al,  GCN\n13618).\n\nWe performed 9.1 ks follow-up XRT observations of the field at T0+1.6 hr,\nT0+0.27 day, T0+0.94 days and T0+3.1 days (where T0 = 2012 Aug 11 at 02:35:18\nUT ), in order to determine whether or not the X-ray source had faded. We find\na fading source with count rates in the 0.3-10 keV band as follows :\n\nT-T0                     Count Rate\n(5.8+/-0.2)x10^3 s   0.05+/-0.01 count/s\n(2.2+/-0.01)x10^4 s   0.02+/-0.01 count/s\n(8.1+/-0.03)x10^4 s  0.02+/-0.001 count/s\n(2.7+/-0.2)x10^5 s   < 0.005 count/s\n\nThe data are entirely in Photon Counting (PC) mode. Using 1227 s of PC mode\ndata and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT\nalignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =\n257.16542, -22.71054 , which is equivalent to:\n\nRA (J2000): 17h 08m 39.70s\nDec(J2000): -22d 42' 38.0\"\n\nwith an uncertainty of 2.8 arcsec (radius, 90% confidence).\n\nThe light curve can be modelled with  a power-law decay with a decay index of\nalpha=0.31 (+0.13, -0.12).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.9 (+/-0.5). The best-fitting\nabsorption column is  7.4 (+4.2, -3.0) x 10^21 cm^-2, in excess of the Galactic\nvalue of 1.5 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed\n(unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is\n5.5 x 10^-11 (9.7 x 10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     7.4 (+4.2, -3.0) x 10^21 cm^-2\nGalactic foreground: 1.5 x 10^21 cm^-2\nExcess significance: 3.2 sigma\nPhoton index:\t     1.9 (+/-0.5)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00530581.\n\nThe fact that the XRT source is fading suggests that Swift trigger 530581\n(Zhang et al., GCN Circ. 13618) is most likely a GRB.\n\nThe Swift/UVOT began settled observations of the field of GRB 120811A 5627 s\nafter the BAT trigger (Zhang  et al., GCN Circ. 13618). No optical afterglow\nconsistent with the XRT position is detected in the initial UVOT exposures.\nPreliminary 3-sigma upper limits using the UVOT photometric system(Breeveld et\nal. 2011,AIP Conf. Proc. 1358, 373) for the initial exposures are:\n\nFilter         T_start(s)   T_stop(s)      Exp(s)    Mag\n\nwhite           6243        6443         197        > 20.7    \nv               6654        6853         197        > 19.1    \nb               6038        7593         313        > 20.2    \nu               5832        7468         393        > 20.0     \nuvw1            5627        7263         393        > 19.9\nuvw2            6449        6649         197        > 19.8    \n\nThe magnitudes in the table are not corrected for the Galactic extinction due\nto the reddening of E(B-V) = 0.77 in the direction of the burst (Schlegel et\nal. 1998).\n\nThis circular is an official product of the Swift-XRT and Swift-UVOT team.",
  "circularId": 13641,
  "createdOn": 1344959841000,
  "email": "buz12@psu.edu",
  "subject": "GRB 120811A: Swift/XRT-UVOT Obseravations",
  "submitter": "Binbin Zhang at PSU  <buz12@psu.edu>",
  "eventId": "GRB 120811A"
}