TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2901
SUBJECT: GRB041223: Swift XRT Detection of X-ray Afterglow Emission
DATE:    04/12/24 05:41:36 GMT
FROM:    David Burrows at PSU/Swift  <burrows@astro.psu.edu>

D. N. Burrows, J. E. Hill, J. Racusin, J. Kennea, D. Morris, J. A. Nousek 
(PSU), G. Chincarini, G. Tagliaferri, A. Moretti, P. Romano, S. Campana, D. 
Malesani, C. Pagani (OAB), A. Wells, J. Osborne, A. Beardmore, K. Page (U. 
Leicester), P. Giommi (ASI), M. Chester (PSU), S. Barthelmy, N. Gehrels, N. 
White (GSFC), K. Mason (MSSL), on behalf of the Swift XRT team.

The Swift X-Ray Telescope (XRT) was pointed at GRB041223 (GCN 2898, Tueller 
et al.) on 2004/12/23 at 18:43:59 UT for 1490 s, at 20:16:24.4s for 1600 s, 
and at 21:50:40 for 430 s.  The spacecraft did not autonomously slew to the 
burst since automated slewing is not yet enabled and the XRT is in the 
midst of engineering measurements.  The observation was performed as a 
Target of Opportunity beginning about 4.5 hours after the burst.

We detect a fading X-ray source about 3.5 arcminutes from the BAT 
position.  The ground-calculated positions were checked through two 
independent data processing and analysis techniques, which yielded 
consistent sky positions within 22 arcseconds.  Our best estimate of the 
X-ray afterglow position is 06:40:49.2, -37:04:21.5 (J2000) for the first 
observation.  The position determined independently for the second 
observation was within 4 arcseconds of these coordinates.  The XRT 
alignment is not yet fully calibrated, and we estimate a systematic 
uncertainty of about 15 arcseconds in this position.  Checks against 
SIMBAD, DSS and X-ray catalogs from ROSAT, ASCA, XMM, and Chandra yielded 
no known source at this position on the sky.

We have a total of about 580 counts from this object in the first two 
observations.  A simple power-law fit to the spectrum gives a photon index 
of 1.43 +/- 0.09 and model fluxes of 1.7E-12 ( 0.5-2 keV) and 4.7E-12 (2-10 
keV) ergs/cm**2/s.  We caution that the instrument is not yet fully 
calibrated and that these fluxes may have systematic uncertainties of 
15%.  The light curve based on all three observations is well fit with a 
power-law index of 2.2 +/-  0.3.