GCN Circular 10941

R.L.C. Starling, P.A. Evans (U. Leicester) and S. Immler (CRESST/GSFC/UMD) 
report on behalf of the Swift XRT and UVOT teams,

We have analysed 65.4 ks of Swift XRT data for GRB 100628A (Immler et al. GCN 
Circ. 10895
Not found
 
 
). The X-ray source reported in Starling et al. (GCN Circ. 10907
Not found
 
 
) 
shows no evidence for fading. We conclude that this source is not the 
X-ray afterglow of GRB 100628A.

Further inspection of these data reveal another possible X-ray source close
to the persistant source, detected in the first two orbits of data only 
(totalling 3.8 ks from T0+92 s to T0+7200 s). 
This source comprises just 7 counts, however the predicted background 
level is only 0.7 counts in this time interval. Using the Bayesian method 
described in Kraft, Burrows & Nousek (1991) we find a detection 
significance >99.999%. The 0.3-10 keV count rate during this interval is 
0.0017 (+0.0008, -0.0006) counts/s. The source position is
RA, Dec (J2000) = 225.96837, -31.65839, which is equivalent to:
   RA (J2000.0)  = 15 03 52.41
   Dec (J2000.0) = -31 39 30.2
with an uncertainty of 7 arcseconds (radius, 90% containment).

In the subsequent XRT data (>T0+7200 s) the source is not detected; a 
3-sigma upper limit on the 0.3-10 keV count rate is 5.7E-5 count/s. 
Therefore, if this source was real it has faded and can be considered a 
candidate X-ray afterglow.
We caution that with so few total counts a background fluctuation or other
spurious detection cannot be completely ruled out.

No UVOT afterglow candidate is seen in any of the UVOT images at the XRT
position down to the limits reported in GCN 10901
Not found
 
 
.

This circular is an official product of the Swift team.