GCN Circular 3714
Subject
GRB 050730: Swift XRT afterglow observations
Event
Date
2005-07-31T05:31:24Z (20 years ago)
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.