TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3976
SUBJECT: GRB050911: Upper limit on X-ray afterglow
DATE:    05/09/15 07:13:54 GMT
FROM:    Kim Page at U.of Leicester  <kpa@star.le.ac.uk>

K. Page (U. Leicester), D. Burrows (PSU), N. Gehrels (GSFC), P. O'Brien
(U. Leicester), F. Marshall (GSFC) and P. Boyd (GSFC-UMBC) report on
behalf of the Swift-XRT team:

Upon analysis of 29 ks of data, starting 4.6 hours after the BAT detected
GRB050911 (trigger number 154630; GCN 3961), no fading X-ray source is
found within the BAT refined error circle (GCN 3964; Tueller et al.) The
source suggested in GCN 3967 (Page et al.) is found to constant over time,
so is not believed to be the afterglow.

We place a 3 sigma upper limit of 6.7e-4 count s^-1 on the count-rate,
which corresponds to an unabsorbed flux of 2.8e-14 erg cm^-2 s^-1 (over
0.3-10 keV), assuming a Crab-like spectrum. Since this GRB looks like a
real burst in the BAT data, we conclude that it must have had a
rapidly-decaying X-ray afterglow to have faded below the XRT sensitivity
in 4.6 hours.