[vsnet-grb-info 2324] GRB051012: Swift-XRT analysis

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4162
SUBJECT: GRB051012: Swift-XRT analysis
DATE:    05/10/25 16:29:16 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

A.P. Beardmore, K.L Page (U. Leicester), D.N. Burrows (PSU), O. Godet (U.
Leicester), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), N. Gehrels
(GSFC), M. Chester (PSU) and R. Fink (GSFC) report on behalf of the
Swift-XRT team:

The XRT began observing the field of GRB051012 on 13 October 2005, 102.5
ks after the burst. Because Swift was executing a preplanned slew at the
time of the burst, the burst was not identified on-board, thus causing a
significant delay in follow-up observations (Sato et al., GCN 4093). Using
the first 19.1 ks of data, collected between 102.5 and 206.7 ks after the
burst, a faint uncatalogued source was detected, with a confidence level
of greater than 99.9%, at the following coordinates:

RA(J2000):   18h 02m 11.1s
Dec(J2000): -52d 47' 12.3"

This position uses the latest XRT boresight correction and has an
estimated uncertainty of 6.4" (90% containment); it is 46" from the BAT
ground position reported by Sato et al.

The count-rate of this source is (5.3 +/- 2.5)e-4 ct s^-1, giving an
estimated observed flux of 2.1e-14 erg cm^-2 s^-1 (0.3-10 keV), assuming a
Crab-like spectrum and the Galactic absorbing column of 9e20 cm-2. The
corresponding unabsorbed flux is 2.8e-14 erg cm^-2 s^-1.

Because the source was quite faint, we were not able immediately to
determine whether or not it was variable. Two additional long exposures
were required to determine whether this source was fading. A total of 50.3
ks of data, taken between 206.7 and 797.9 ks after the trigger (between
2005-10-15 and 2005-10-21), revealed no detection of the source, with a
3-sigma upper limit on the count-rate of 2.4e-4 ct s^-1 [corresponding
observed (unabsorbed) flux of 9.7e-15 (1.3e-14) erg cm^-2 s^-1]. Since the
source has almost certainly faded, it is likely to be the X-ray afterglow
of GRB051012.