TITLE: GCN CIRCULAR NUMBER: 15749 SUBJECT: Swift Trigger 584155 is MAXI J1421-613 DATE: 14/01/18 08:59:28 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT W. H. Baumgartner (GSFC/UMBC), V. D'Elia (ASDC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. Pagani (U Leicester), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 08:39:19 UT, the Swift Burst Alert Telescope (BAT) triggered (trigger=584155) and located an outburst from a recently discovered transient, MAXI J1421-613 (Morooka et al., ATel #5750). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 215.411, -61.608 which is RA(J2000) = 14h 21m 39s Dec(J2000) = -61d 36' 29" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 10 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~7 sec after the trigger. The XRT began observing the field at 08:40:43.9 UT, 84.2 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 215.4049, -61.6073 which is equivalent to: RA(J2000) = 14h 21m 37.17s Dec(J2000) = -61d 36' 26.1" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 10 arcseconds from the BAT onboard position, within the BAT error circle and consistent with the position of the MAXI J1421-613 transient. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.71 x 10^22 cm^-2, Kalberla et al. 2005), with an excess column of 2 (+2.01/-1.51) x 10^22 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 88 seconds after the BAT trigger. No credible optical counterpart has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain extinction expected.