TITLE: GCN CIRCULAR NUMBER: 22416 SUBJECT: Swift Trigger 810300 is a probable new Galactic source: Swift J1658.2-4242 DATE: 18/02/16 06:28:49 GMT FROM: David Palmer at LANL S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), A. Deich (PSU), C. Gronwall (PSU), A. Melandri (INAF-OAB), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 05:37:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located a source in the Galactic plane (trigger=810300). Swift could not immediately slew to the location due to an observing constraint. The BAT on-board calculated location is RA, Dec 254.545, -42.709 which is RA(J2000) = 16h 58m 11s Dec(J2000) = -42d 42' 31" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Due to the duration of the image that led to the alert (27 minutes), no lightcurve is immediately available. Due to an observing constraint, Swift will not slew until T0+46.4 minutes. There will be no XRT or UVOT data until this time. Based on the duration of the discovery image (27 minutes) and the source's location on the Galactic plane (lat = 0.05, lon = 343.24) we believe that this is a previously-unknown Galactic transient, which we name Swift J1658.2-4242 .