GCN Circular 23928

S. D. Barthelmy (GSFC), A. Y. Lien (GSFC/UMBC) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:12:49 UT, the Swift Burst Alert Telescope (BAT) triggered and
located MAXI J1631-479 (trigger=890732).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 247.821, -47.792 which is 
   RA(J2000) = 16h 31m 17s
   Dec(J2000) = -47d 47' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the available
BAT light curve shows no significant structure. 

The XRT began observing the field at 06:21:16.4 UT, 507.0 seconds after
the BAT trigger. XRT found a bright X-ray source. However, the image was 
contaminated by a cosmic ray, so the localisation was skewed, and no 
accurate position can be given. No event data are yet available to 
determine the column density using X-ray spectroscopy. 

The initial flux in the 0.1 s image was 4.57e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 515 seconds after the BAT trigger. No credible source has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any source in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any source in the
region. No correction has been made for the large, but uncertain, extinction
expected.