{
  "bibcode": "2006GCN..4796....1P",
  "body": "K.L. Page (U. Leicester), D.N. Burrows (PSU) and E. Rol (U. Leicester)\nreport on behalf of the Swift-XRT team:\n \nIt has recently been pointed out (Halpern 2006, GCN 4614) that the nature\nof GRB 051221B has still not been definitively settled (GRB vs hard X-ray\ntransient). Prompted by this report, we are providing further analysis\ndetails of the Swift-XRT observation of this source.\n\nThe XRT began observing the field of the possible GRB 051221B (GCN 4376, \nBoyd et al. 2005) 281 seconds after the trigger. 7 seconds of data in\nWindowed Timing (WT) mode were obtained, before switching into Photon\nCounting (PC) mode.\n \nThe X-ray source was only detected during the first orbit, with 14 counts\nin the WT data and 51 counts in 385 seconds of PC data (starting 290\nseconds after the trigger). The second orbit, covering 1319 seconds of\nexposure time (starting 3.6 ks after the trigger) shows only 1 count at\nthe position of the source. These later data give a 3-sigma upper limit of\n1.5e-3 ct s^-1.\n \nThe start of the light-curve falls extremely steeply (alpha ~ 9,\ncalculated with respect to the trigger time). There is an indication of\nflattening at 400 - 500 seconds after the trigger, though this is based on\nonly 2 bins of data, each containing 5 counts.\n \nUsing Cash statistics because of the low number of counts, the spectrum\nfor the first orbit of PC data (290 - 680 seconds after the trigger) can\nbe modelled with a power-law of Gamma = 1.27 +/- 0.37, with the estimated\nGalactic column fixed at 5.6e21 cm^-2 (Dickey & Lockman 1990). This\nspectrum has a mean count rate of 0.15 ct s^-1 and an observed\n(unabsorbed) flux of 1.43e-11 (1.77e-11) erg cm^-2 s^-1.\n\nThe XRT observations of this source were terminated early in order to\nallow followup of the short GRB 051221A.  Therefore, no additional data\nare available.  However, the rapid decline in X-ray flux appears to\nsupport a GRB origin.\n\nThis circular is an official product of the Swift-XRT Team.",
  "circularId": 4796,
  "createdOn": 1140428113000,
  "email": "kpa@star.le.ac.uk",
  "subject": "GRB 051221B: Swift-XRT Team Refined Analysis",
  "submitter": "Kim Page at U.of Leicester  <kpa@star.le.ac.uk>",
  "eventId": "GRB 051221B"
}