TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3714
SUBJECT: GRB 050730: Swift XRT afterglow observations
DATE:    05/07/31 05:31:24 GMT
FROM:    David Burrows at PSU/Swift  <dxb15@psu.edu>

D. Grupe, J. A. Kennea, and D. N. Burrows (PSU) report on behalf of the 
Swift-XRT
team:

We have analyzed the Swift XRT data from the first three orbits of 
observations of GRB 050730 (Holland et al., GCN 3704). The satellite slewed 
to this burst immediately and the XRT began collecting data at T+130 s, but 
was unable to determine an on-board position for the afterglow.  In 
ground-processed data we find a bright, fading, uncataloged X-ray source 
that we identify as the afterglow of this burst.  The XRT ground-calculated 
coordinates are:

RA(J2000) =  14h 08m 17.7s
Dec(J2000) = -03 46 09.7.

This position is 42 arcseconds from the BAT position given in GCN 3704, and 
13.4 arcseconds from the UVOT position given in GCN 3704.  However, we note 
that there is accumulating evidence of a time-dependent systematic shift in 
XRT positions derived from ground-processed data compared with the optical 
counterparts. This effect is being investigated but is not yet 
understood.  Extrapolation of earlier positional errors suggests that the 
correct position could have a systematic error of at least 7 arcseconds in 
addition to the standard error circle of about 6 arcseconds radius.

A preliminary spectral fit (simple absorbed power-law) to the WT data 
yields a photon index of 1.6+/-0.2. The derived NH is (4.4+/-0.6)E20 cm^-2, 
which is roughly consistent with the Galactic value (3.1E20 cm^-2; Dickey & 
Lockman 1990).

The light curve is complex, with at least 3 X-ray flares in the first orbit 
of data (T+130 to T+ 1000 s), making it difficult to estimate a decay 
slope.  A rough approximation of the average decay slope over the first 3 
orbits of data is about -0.8.

Further observations are in progress.