GCN Circular 8522
Subject
Further Swift-XRT observations of the probable GRB 081105.
Event
Date
2008-11-16T01:15:46Z (18 years ago)
From
Andy Beardmore at U Leicester <apb@star.le.ac.uk>
A.P. Beardmore (U. Leicester), D.N. Burrows (PSU), J.R. Cummings (NASA/UMBC)
report on behalf of the Swift-XRT team.
The Swift-XRT reobserved the field of the Swift-BAT and IPN localisation
GRB 081105 (Cummings et al., GCN 8484) for a further 9.8 ks starting at
2008-11-12T04:15:51(UT), 6.62 days after the trigger.
Using a 10 pixel radius extraction region we detect 9 counts at the
position of the XRT source reported in GCN 8487 (Beardmore & Cummings)
compared with 1.28+/-0.20 estimated background counts. The Bayesian method
of Kraft et al. (1991 ApJ 374 344) produces a 3 sigma confidence level on
the expected source counts of S_min = 1.40 to S_max = 20.08, suggesting
the source is detected. (The lower limit on the expected source counts,
S_min, approaches zero when the confidence level reaches 4.4 sigma.) The
estimated source count rate is (1.07+/-0.40)e-3 count/s (where the quoted
error is 1 sigma), compared with a value of 0.0065 +/-0.0015 count/s from
the earlier observation.
By fitting the low counts per bin data using Cash statistics, we find
the source has faded with a decay slope of 1.01 +0.38-0.32. Both the
source intensity and decay is comparable to those seen in other long GRBs
at this stage of their outburst (e.g. see
http://www.swift.ac.uk/xrt_curves/allcurves.php).
We suggest the X-ray source reported in GCN 8487 was the counterpart to
the BAT/IPN localisation and its decaying nature gives further weight to a
GRB interpretation.
This circular is an official product of the Swift-XRT team.