{
  "bibcode": "2009GCN..8937....1B",
  "body": "A.P. Beardmore, K. L. Page, P.A. Evans (U. Leicester), C. Pagani, J. Kennea,\nD.N. Burrows (PSU) report on behalf of the Swift-XRT team :\n\nThe Swift-XRT began observing the field of the short GRB 090305\n(Beardmore et al. GCN Circ. 8932) in photon counting mode at\n2009-03-05T05:21:37, 103.4 s after the trigger.  The first orbit of\ndata lasted only 90 s before the spacecraft slewed to another target\nbecause of an observing constraint.\n\nThe default data processing does not reveal any X-ray source during\nthe first orbit (103.4 to 193.5 s post trigger), either within the BAT\nrefined error circle (Krimm et al. GCN Circ. 8936) or at the location\nof the optical afterglow (Cenko et al. GCN Circ. 8933) - the latter\noccurring close to a bad-column on the XRT CCD.  However, by relaxing\nthe default screening criteria we find 8 counts in a 10 pixel radius\ncircle extraction region at the location of the optical afterglow,\ncompared with a predicted background level of 1.0 count (where an\nannulus with radii 15 to 100 pixels was used to sample the local\nbackground level), implying a greater than 99.99 percent confidence\ndetection (Kraft, Burrows, and Nousek 1991, Ap. J., 374, 344). The\nclustering of the counts, their proximity to the location of the\noptical afterglow and their detection significance allow us to\nconclude the afterglow was detected by the XRT at this time. The\nestimated count rate is 0.112 +0.051 -0.041 count/s (where a\nsystematic error of 5 percent has been included in the 1 sigma error\nestimate to account for the PSF correction uncertainty).  Applying a\ncanonical GRB count to observed flux conversion factor of 5e-11 erg\ncm^-2 count^-1 (for a photon index of 1.98, Evans et al. 2009, MNRAS,\nsubmitted. arXiv:0812.3662, http://www.swift.ac.uk/xrt_spectra), we\nestimate the observed 0.3-10 keV flux was (5.5 +2.6 -2.0)e-12 \nerg cm^-2 s^-1.\n\nA further 2.05 ks of data was taken in orbit two, from 3.92 to 5.97 ks\nafter the trigger, where the optical afterglow position was located\naway from the XRT CCD bad column. This showed the source was no longer\ndetected and a 3 sigma upper limit to the count rate of 7.0e-3 count/s\nwas obtained.  Using all of the available data after the first orbit\n(i.e. presently an exposure of 12.64 ks from 3.92 to 46.1 ks post\ntrigger) reduces the 3 sigma upper limit to 1.7e-3 count/s.  The X-ray\ndata suggest the source faded with a decay slope of at least ~0.8.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 8937,
  "createdOn": 1236302275000,
  "email": "apb@star.le.ac.uk",
  "subject": "GRB 090305: Swift-XRT refined analysis",
  "submitter": "Andy Beardmore at U Leicester  <apb@star.le.ac.uk>",
  "eventId": "GRB 090305"
}