GCN Circular 12308

B.-B. Zhang (PSU) and D. Grupe (PSU) reports on behalf of the Swift-XRT
team:


Swift began a target of opportunity observation of the IPN-detected
burst: GRB110825A (Hurley et al. et al. GCN Circ. 12307
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 ) on August 26,
2011 at 01:05 UT, approximately 22.7 hours after the burst was detected
by IPN. Swift data for these observations utilize Target ID 20183.

We have analysed 2.9 ks of XRT data for this burst, from 81.3 ks to
94.0 ks after the IPN trigger. The data are entirely in Photon Counting
(PC) mode. An X-ray source is detected within the XRT field-of-view.
The refined XRT position is RA, Dec = 44.79846, +15.42733 which is
equivalent to:

RA (J2000): 02 59 11.63
Dec(J2000): +15 25 38.4

with an uncertainty of 3.9 arcsec (radius, 90% confidence). This
position is 346 arcsec from the IPN position and about 180 arcsec from
the edge of IPN error box (see
http://www.personal.psu.edu/buz12/IPN_ERRORBOX_VS_XRT_GRB110825A.png). 
We cannot determine at the present time whether the source is fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.8, -0.4). The
best-fitting absorption column is consistent with the Galactic value of
9.9 x 10^20 cm^-2 (Kalberla et al. 2005). The count rate for the X-ray
source is about 0.01 count/sec, which is relatively bright when
comparing to other GRB's X-ray afterglow at t-T0=10^5 seconds. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.9 x 10^-11 (4.7 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     0 (+1.7, -0) x 10^21 cm^-2
Galactic foreground: 9.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.8 (+0.8, -0.4)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020183.


No source was found from UVOT data within the XRT error circle.
Preliminary 3-sigma up-limits are : 

Filter	     EXPOSURE	    Mag
---------------------------------------------------
    v	 &    579 &   >19.19  
    u	 &    1365 &  >20.79 
white	 &    895.63 &	 >21.40
---------------------------------------------------


We notice that there is no known source at the X-ray source position in
Simbad or the NED. We also notice that the RASS data for this position
were not deep enough to detect a source of this flux (0.01 count/sec).

We have scheduled further Swift TOO for this target. 

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


Burst Advocate for this burst is B.-B. Zhang (bbzhang AT psu.edu).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases,
after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)