J. E. Hill, J. A. Kennea, D. C. Morris, D. N. Burrows (PSU), A. A. Wells, J. P.
Osborne (U. Leicester), G. Tagliaferri (OAB), M. Ivanushkina, S. Hunsberger, J.
A. Nousek, P. Roming (PSU), P. Schady (MSSL), S. Barthelmy, F. Marshall and N.
Gehrels (NASA/GSFC) report on behalf of the Swift XRT team:
We have derived a refined Swift XRT position for GRB050117, based on preliminary
alignment corrections to the raw position reported in Burrows et al. (GCN 2951).
The refined XRT position is:
RA(J2000) = 358.47067 = 23h 53m 53.0s
Dec(J2000) = +65.93882 = +65:56:20
The estimated uncertainty in this position is 15 arcseconds (radius).
This is the first time that the Swift observatory has executed a prompt slew to
a GRB, and the XRT position was measured 192 seconds after the burst occurred.
Because the burst was quite long, the first XRT data were collected before the
burst ended, a first for a focussed X-ray GRB/afterglow observation. We note
that the XRT in-flight calibration program has not yet begun because the Swift
observatory is still in its commissioning phase.
The burst occurred at 12:52:36 UT (Sakamoto et al., GCN 2952) just before Swift
entered the South Atlantic Anomaly (SAA) and while the burst location was
within the Swift Earth limb constraint. The Swift spacecraft began slewing to
the burst at 12:54:38 UT, while in the SAA. The XRT imaged the source
immediately after the slew ended, at 12:55:48.64 UT. The source was very
bright, resulting in a successful centroid on the first 0.1s Image Mode
exposure. The XRT then collected a partial Photodiode Mode frame and stopped
data collection because it was in the SAA. Although we have not yet calibrated
fluxes measured in XRT's Image Mode, the X-ray source intensity in the Image
Mode frame appears to be comparable to Cyg X-2, which puts it at roughly 1/2
Crab (based on RXTE ASM data on Cyg X-2 and the Crab).
The source is located just outside the zero-visibility region around the Swift
orbit pole and has only short periods of visibility on each orbit. Most of
these visible periods occur while Swift is in the SAA, which limits the
amount of time when the source can be viewed by the XRT. Automated observations
will proceed for the rest of 2005 Jan 17 when the source is visible. We expect
to obtain a total observing time of about 15 minutes, spread out between 12:55
UT and 19:29 UT. Because of orbital precession, the source will not be visible
again by the Swift XRT until 2005 Jan 21.