TITLE:   GCN CIRCULAR
NUMBER:  10270
SUBJECT: GRB 091208A: Swift-XRT observations
DATE:    09/12/08 22:24:53 GMT
FROM:    Claudio Pagani at PSU/Swift-XRT  <pagani@astro.psu.edu>

C. Pagani, L. Vetere (PSU), P.A. Evans (U. Leicester), J. L. Racusin
(NASA/GSFC) and E. A. Hoversten (PSU) report on behalf of the Swift Team:

Swift/XRT observed the field of GRB 091208A (BAT trigger number 378554;
Hoversten et al. GCN Circ. 10253) from 3.1 ks to 3.3 ks after the BAT
trigger and again from 25 ks to 27 ks after the BAT trigger.  The first
observation was delayed due to an observing constraint.

During the first observation no X-ray afterglow is detected inside the BAT
error circle (Barthelmy et al. GCN Circ. 10264) in 180 seconds of Photon
Counting data.  The corresponding 3-sigma upper limit is 0.04 counts/s. 
For comparison, the median XRT count rate for long GRB afterglows at 3 ks
after the BAT trigger is approximately 0.1 counts/s.

During the second observation no X-ray afterglow is detected inside the BAT
error circle in 694 seconds of PC data.  The corresponding 3-sigma upper
limit is 0.01 counts/s.  For comparison, the median XRT count rate for long
GRB afterglows at 25 ks after the BAT trigger is approximately 0.02
counts/s.  Based on the count rates of the XRT afterglows observed during
the Swift mission the chance of detecting a long GRB with the XRT in a 1 ks
observation 25 ks after the BAT trigger is 61%.

This circular is an official product of the Swift-XRT team.