GCN Circular 3082

S. Barthelmy (GSFC), D.Burrows (PSU), L. Barbier (GSFC), J. Cummings 
(GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD), 
J. Kennea (PSU), H. Krimm (GSFC/USRA), D. Malesani (SISSA), C. Markwardt 
(GSFC/UMD), A. Moretti (INAF-OAB), K. Page (U. Leicester), D. Palmer 
(LANL), A. Parsons (GSFC), P. Romano (INAF-OAB), T. Sakamoto (GSFC), G. 
Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC)
on behalf of the Swift-BAT/-XRT teams:

Further analysis of Swift data on BAT trigger #107873 has led to an 
ambiguous situation regarding the reality of this marginal BAT trigger.

The Swift-BAT triggered (#107873) at 10:43:21 UT.  The trigger occurred on 
the rising edge of the SAA.  Ground analysis has confirmed the very low 
image-domain detection significance of this trigger.  A visual inspection 
of the mask-tagged light curve also shows very marginal emission.
As a result, a retraction GCN Notice was issued earlier today.  We are now
ammending that Notice.

The BAT position was within the Swift Earth limb constraint at the time of 
the burst, so Swift did not execute a prompt automated slew.  The spacecraft 
executed a delayed slew to the BAT location at about 11:33 UT and XRT 
observations began at 11:35:54 UT.  Two orbits of data were collected by 
the XRT, for a total of 4157s of exposure.  Preliminary analysis indicates 
that there are four X-ray sources in the XRT field of view, two of which 
fall inside the BAT error circle.  These are:

Source #1: RA(J2000) = 12 10 29.1, Dec(J2000) = +77 37 04.8. This is a very 
weak source (about 0.006 cps).  It appears to be roughly constant in 
intensity, but the source is so faint that we cannot determine with 
confidence whether or not it is varying in flux.

Source #2: RA(J2000) = 12 09 37.2, Dec(J2000) = +77 35 56.5.  This source
is brighter (about 0.01 cps) and may be fading.  Again, we cannot be certain 
with the data currently in hand.

The other two sources are a bright star and a very faint object well 
outside the BAT error circle.  The estimated uncertainty in the XRT 
positions is 6 arcseconds radius.

A 20 ks Target of Opportunity observation has been initiated to provide a 
second set of data beginning at about 19:45 UT.  This should help us 
determine the likelihood that either XRT source is related to the BAT trigger.

In light of these XRT observations, the BAT team cannot rule out the 
possibility that this trigger represents a threshold-level GRB.