{
  "bibcode": "2008GCN..7399....1B",
  "body": "A.P. Beardmore, J.P. Osborne, R.L.C. Starling, K.L. Page, P.A. Evans \n(U. Leicester) and J.R. Cummings (NASA/UMBC) report on behalf of \nthe Swift-XRT team:\n\nThe Swift-XRT started observing GRB 080310 (trigger no. 305288,\nCummings et al. GCN 7382) at 2008-03-10 08:39:27.85 UT, 89s after the\ntrigger. The observation so far spans 3 snapshots, with Windowed Timing\nmode data from T+95s to T+799s, and Photon Counting mode thereafter.\n\nThe best XRT position is the UVOT-enhanced position from Osborne et al.\n(GCN 7394).\n\nAt first, the 0.3-10keV X-ray light curve decays from 39 count s^-1 at\nT+95s to 18 count s^-1 at T+130s but then shows strong flaring\nactivity. An initial rebrightening occurs from about T+135s to T+420s,\nreaching ~135 count s^-1 at approximate times of 200s, 250s, 280s and\n350s after the trigger.  This is followed by a second flare from T+500\nto T+620s, where it reaches a maximum of ~115 count s^-1 at T+565s.\nFollowing the flaring activity, the X-ray light curve, thus far, shows\na slow decline out to the end of the third snapshot (T+1.0ks to\nT+18.7ks), where it reaches a count rate of 0.1 count s^-1 with an\nunderlying decay slope of alpha~0.5.\n\nThe X-ray data also show strong spectral evolution during the flaring\nintervals.  A 1.5-10keV/0.3-1.5keV hardness ratio reveals the X-ray\nemission initially hardens from T+135s to T+200s as the light curve\nrebrightens, at which point it remains approximately constant until\nT+360s, before softening as the light curve decays.  Similarly, the\nsource hardens again during the second flare from T+500s to T+565s,\nthen softens as this flare declines.\n\nA spectrum of the WT data from T+200s to T+360s, where the hardness\nratio is approximately constant, can be well fit by an absorbed\npowerlaw with photon index 1.45 +/- 0.02 and column density of (7.0\n+/- 1.0)e21 cm^-2 at the redshift of the burst (z=2.43, Prochaska et\nal. GCN 7388; Berger and Rauch GCN 7389; Vreeswijk et al. GCN 7391),\nin addition to the Galactic column density of 3.3e20 cm^2 in this\ndirection. The observed 0.3-10keV flux is (6.1 +/- 0.1)e-9 erg cm^-2 s^-1,\nwhich corresponds to an unabsorbed flux of 6.8e-9 erg cm^-2 s^-1. \nThe counts to observed flux conversion factor at the time of\nthis spectrum is 4.9e-11 erg cm^-2 count^-1.\n\nThe PC mode data from the second snapshot (T+5.1ks to T+7.3ks) show a \nsofter spectrum, with a photon index of 1.9 +/- 0.2 and column density \nconsistent with the value determined above. The observed 0.3-10keV flux at \nthis time is (1.1 +/- 0.1) e-11 erg cm^-2 s^-1, and corresponds to a flux \nconversion factor of 4.7e-11 erg cm^-2 count^-1.\n\nIf the underlying powerlaw decay continues as is, we predict an XRT count \nrate of 0.05 count s^-1 at T+24hr, which corresponds to an observed \n0.3-10keV flux of 2.5e-12 erg cm^-2 s^-1.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 7399,
  "createdOn": 1205174994000,
  "email": "apb@star.le.ac.uk",
  "subject": "GRB 080310: Swift-XRT refined analysis",
  "submitter": "Andy Beardmore at U Leicester  <apb@star.le.ac.uk>",
  "eventId": "GRB 080310"
}