TITLE:   GCN CIRCULAR
NUMBER:  8522
SUBJECT: Further Swift-XRT observations of the probable GRB 081105.
DATE:    08/11/16 01:15:46 GMT
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.