TITLE: GCN CIRCULAR NUMBER: 23800 SUBJECT: Swift Trigger 886157: Swift detection of a new transient Swift J1728.9-3613 DATE: 19/01/28 01:47:01 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:22:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located a new transient which we name Swift J1728.9-3613 (trigger=886157). The BAT on-board calculated location is RA, Dec 262.232, -36.237 which is RA(J2000) = 17h 28m 56s Dec(J2000) = -36d 14' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is normal for an image trigger, the onboard light-curve showed no significant structure. Due to a Sun observing constraint, Swift cannot slew to the BAT position until 09:11 UT on 2019 January 29. There will thus be no XRT or UVOT data for this trigger before this time. The rise of this transient was seen by the BAT transient monitor (Krimm et al., 2013), with the first detection on 2019 January 25. The BAT light-curve can found at the following link: https://swift.gsfc.nasa.gov/results/transients/weak/SWIFTJ1728.9-3613/SWIFTJ1728.9-3613.html