{
  "circularId": 2347,
  "version": 2,
  "createdOn": 1060219679000,
  "body": "XRF030723(=H2777): Chandra Fading X-ray Afterglow Confirmed\n\nN. Butler, P. Ford, G. Ricker, R. Vanderspek, J. Villasenor (MIT); D. \nLamb (U. Chicago); G. Garmire (PSU); L. Piro (CNR); and J.G. Jernigan \n(UCB), on behalf of a Chandra GRB ToO Team,\n\nwrite:\n\nOn 4 August 2003, the Chandra Observatory targeted the field of \nXRF030723, an X-ray flash localized by the HETE satellite (=H2777; \nPrigozhin et al, GCN 2313). The duration of the observation was 85 \nksec, and it was a followup to the 25 ksec observation reported by \nButler et al.  in GCN 2328.  The observation spanned the interval 4 \nAugust 22:22 UT to 5 August 22:27 UT, 12.66 to 13.67 days after the \nburst. The SXC error circle from Prigozhin et al. was completely \ncontained within the field-of-view of the Chandra ACIS-S3 chip.\n\nWithin the SXC error circle, the three X-ray sources from GCN 2328 \nare detected.  Only source #1 is observed to vary in flux at a \nsignificance level of >1 sigma.  We detect 74 counts in the 0.5-8.0 \nkeV band, corresponding to a >7-sigma significance decrease (i.e. \nfactor of ~6) in flux since the first epoch observation.  We are thus \nextremely confident that source #1 is the X-ray counterpart to \nXRF030723 and is the counterpart to the optical source discovered by \nFox et al. (GCN 2323).\n\nIf we jointly fit the counts from the first and second Chandra \nepochs, we find that the data are well described by a power-law with \nabsorption fixed at the Galactic value in the source direction (chi^2 \n= 8.9 for 9 [12-3] degrees of freedom).  The best fit photon number \nindex (for both epochs) is gamma = 1.9 +/- 0.3, which is a typical \nvalue for the X-ray afterglows of long duration GRBs.  Using this \nmodel, we find that the first epoch flux is ( 2.2 +/- 0.3 ) x 10-14 \nerg cm-2 s-1 (0.5-8.0 keV band), while the second epoch flux is ( 3.5 \n+/- 0.5 ) x 10-15 erg cm-2 s-1 (0.5-8.0 keV band).  The decrease in \nflux between the two epochs can be described by a power-law with a \ndecay index of alpha= -1.0 +/- 0.1.  This value of alpha is \nconsistent with the power-law decline reported in the optical by \nDullighan et al. (GCN 2336) for <~1.5 day after the GRB; however, the \nindex is considerably flatter than the index at t>1.5 days reported \nby Dullighan et al. This flatter X-ray decay may possibly be related \nto the rebrightening of the optical afterglow reported by Fynbo et \nal. (GCN 2345).\n\nWe gratefully acknowledge the timely assistance of the observatory \nstaff at the Chandra Science Center in arranging for the acquisition \nand preliminary processing of these data.\n\nThis message may be cited.",
  "submitter": "George Ricker at MIT  <grr@space.mit.edu>",
  "editedOn": 1777052931055,
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>",
  "bibcode": "2003GCN..2347....1B",
  "eventId": "XRF 030723",
  "email": "grr@space.mit.edu",
  "subject": "XRF030723(=H2777): Chandra Fading X-ray Afterglow Confirmed"
}