GCN Circular 4022

J. A. Kennea, D. N. Burrows (PSU), M. Goad (U. Leicester), J. Norris and
N. Gehrels (GSFC) report on behalf of the Swift/XRT Team:

We report on detailed ground analysis of the first 2 orbits of data from
X-ray afterglow of GRB050922C (GCN 4013
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 ) utilizing downlinked Swift/XRT
data.  We find a refined position for the XRT afterglow of:

RA(J2000):  21:09:33.3,
Dec(J2000): -8:45:27.5,

with an estimated uncertainty of 6 arcseconds radius (90% containment). We note 
that this position is 4.6 arcseconds from the NOT optical afterglow position 
reported in GCN 4015
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 .

We have extracted a lightcurve of combined Window Timing and Photon 
Counting mode data from the first two orbits of XRT observations, covering 
6500ks of time.  The lightcurve of the afterglow is well described by a 
single powerlaw model with a slope of -1.14 +/- 0.02.

The spectrum of the afterglow is well fit (reduced Chi^2 = 0.94 with 44 dof) by 
an absorbed power-law model with the following parameters:

N_H = (6.5 +/- 2.7) x 10^20 cm^-2
Photon Index = 1.89 +/- 0.16

The absorption seen here is consistant with the expected Galactic absorption of 
5.75 x 10^20 cm^-2. Utilizing this spectral model, we estimate the flux of this 
burst (corrected for absorption) to be:

BAT Trigger +1 hour: 1.5 x 10^-11 erg/s/cm^2 (0.5 - 10 keV)
BAT Trigger +1 day:  3.9 x 10^-13 erg/s/cm^2 (0.5 - 10 keV)

Note that the latter flux estimate assumes that the lightcurve does not
break.

Observations of this burst by Swift are on-going.