TITLE: GCN CIRCULAR NUMBER: 23763 SUBJECT: Trigger 884906: Swift detection of MAXI J1631-479 DATE: 19/01/21 10:51:01 GMT FROM: Kim Page at U.of Leicester P. A. Evans (U Leicester), N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 10:06:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located MAXI J1631-479 (trigger=884906). Swift slewed immediately to the target. The BAT on-board calculated location is RA, Dec 247.749, -47.773 which is RA(J2000) = 16h 31m 00s Dec(J2000) = -47d 46' 20" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for an image trigger, there is nothing visible in the real-time light curve. The XRT began observing the field at 10:09:01.2 UT, 123.6 seconds after the BAT trigger. XRT found a bright X-ray source located at RA, Dec 247.8088, -47.8065 which is equivalent to: RA(J2000) = 16h 31m 14.11s Dec(J2000) = -47d 48' 23.4" with an uncertainty of 5.2 arcseconds (radius, 90% containment). 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 4.51e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 133 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. 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 counterpart in the region. No correction has been made for the large, but uncertain, extinction expected. This source is currently active, as shown at https://swift.gsfc.nasa.gov/results/transients/AXJ1631.9-4752/. BAT has previously triggered on this source (trigger 878548 and 882561; GCNs 23550 and 23639). The trigger was initially mis-identified as AX 1631.9-4752, given the close proximity of the two sources. The XRT position is coincident with the radio localisation given in ATel 12396.