[vsnet-grb-info 3559] GRB 060728: Upper limits of the X-ray emission

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5393
SUBJECT: GRB 060728: Upper limits of the X-ray emission
DATE:    06/08/03 20:52:00 GMT
FROM:    Claudio Pagani at PSU/Swift-XRT  <pagani at astro.psu.edu>

C. Pagani (PSU), D. Morris (PSU) and D.N. Burrows (PSU) report on behalf of 
the Swift-XRT team:

The XRT performed 2 observations of the field of GRB 060728 (GCN 5360), 
starting at 2006-07-28-22:26:35 UT (124 seconds after the BAT trigger) to 
2006-07-29-19:06:56 UT and from 2006-08-02-01:41:22 to 2006-08-02-22:57:56 
UT, collecting 17.8 and 13.2ks of data in PC mode respectively.

No new source is detected within the BAT refined error circle (GCN 5363) in 
either of the two exposures.  The 3 sigma upper limit on the count-rate is 
1.0e-3 counts/sec for the first observation, which corresponds to an 
unabsorbed flux of 4.0e-14 erg cm-2 s-1 in the energy range 0.3-10 keV, 
assuming a Crab-like spectrum, and 1.2e-3 counts/sec for the second 
observation (unabsorbed flux of 4.8e-14 erg cm-2 s-1). 
Combining the two observations the 3 sigma upper limit is 9.2e-4 counts/sec, 
corresponding to an unabsorbed flux of 3.7e-14 erg cm-2 s-1.

This is the first case of a long GRB for which Swift performed a prompt slew 
without a decection of the afterglow by the XRT. The only previously 
undetected X-ray afterglow of a long GRB discovered by Swift is GRB 050911 
(Page et al. 2006), for which the XRT observations started 4.6 hours after 
the BAT trigger.