TITLE: GCN CIRCULAR NUMBER: 23928 SUBJECT: Trigger 890732: Swift detection of MAXI J1631-479 DATE: 19/02/26 06:38:46 GMT FROM: Kim Page at U.of Leicester 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.