GCN Circular 13299

S. D. Barthelmy (GSFC), D. N. Burrows (PSU), V. D'Elia (ASDC),
N. Gehrels (NASA/GSFC), B. Gendre (ASDC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASFPA),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), C. J. Saxton (UCL-MSSL),
B. Sbarufatti (INAF-OAB/PSU), M. C. Stroh (PSU) and B.-B. Zhang (PSU)
report on behalf of the Swift Team:

As part of a campaign to detect faint nearby GRBs, BAT has lowered its
threshold to events near to known sources and nearby Galaxies. 

At 16:53:59 UT, the Swift Burst Alert Telescope (BAT) triggered at this lower
threshold in the vicinity of the known source IGR J17091-3624.  Swift slewed
immediately to the location.  The BAT on-board calculated location is 
RA, Dec 257.313, -36.282 which is 
   RA(J2000)  =  17h 09m 15s
   Dec(J2000) = -36d 16' 56"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for image triggers, the immediately
available lightcurve show no obvious variation. 

The XRT began observing the field at 16:56:02.5 UT, 122.7 seconds after
the BAT trigger. No source was detected in central 8x8 arcminute window 
in 641 s of promptly downlinked data.  IGR J17091-3624 is outside of this
promptly downlinked window. 

UVOT took a finding chart exposure of 150 seconds with the White filter starting
127 seconds after the BAT trigger. No credible new source has been found in
the initial data products. The 8'x8' region for the list of sources generated
on board covers 100% of the BAT error circle, but not the IGR J17091-3624
source. The list of sources is typically complete to about 18 mag. 

Given the marginal significance of the BAT detection (5.84 sigma)
the distance of the BAT detection from the known source location 
(7.5 arcmin) and the lack of detection by the narrow field instruments,
we believe that this is not a true detection of an astrophysical source.