GCN Circular 6852

R. Starling, J.P. Osborne, K.L. Page (U. Leicester) and F.E. Marshall
(NASA/GSFC) report, on behalf of the Swift XRT team:

Previously, Swift XRT data for the short GRB 070810B (trigger=287409; 
Marshall et al., GCN Circ. 6743
Not found
 
 
) up to 13.8 ks after the BAT trigger 
showed two possible sources within the BAT refined error circle, detected 
at 2-sigma significance in 7.2 ks and termed S1 and S2 (Starling et al., 
GCN Circ. 6754
Not found
 
 
).
 
We have analysed a further 12.6 ks of Swift XRT photon counting mode data
beginning 42 days and ending 54 days after the burst, in order to assess 
the variability of the two possible afterglow candidates.

In this second epoch of data we detect one X-ray source at 4-sigma 
significance within the refined BAT error circle (Sakamoto et al., GCN 
Circ. 6753
Not found
 
 
), corresponding to the source S2 described in GCN Circ. 6754
Not found
 
 
.
Using Bayesian analysis we find that S2 has between 20 and 54 counts at 
the 99% confidence level or 1.6e-3 to 4.3e-3 count/s, consistent with the 
first epoch limits of 5.6e-4 to 3.6e-3 count/s. This suggests that S2 is 
a persistent faint source, and not the afterglow of GRB 070810B.
 
The possible source S1 (GCN Circ. 6754
Not found
 
 
), detected only at the 2-sigma 
level in the intial epoch observations, is not detected in the second 
epoch observations, providing an upper limit on the count rate of 6.7e-4 
count/s at 99% confidence based on the background count rate. This is  
above the 3-sigma lower limit on the source count rate at epoch 1 of 
5.6e-4 count/s, hence we cannot determine whether or not this source has 
faded. 
 
This is an official product of the Swift XRT team.