Swift J1913.1+1946 16.6-mag X-ray transient. ATEL #15650 ATEL #15650 Title: Swift J1913.1+1946 a new bright hard X-ray and optical transient Author: S. Dichiara (PSU), J. D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa), F. E. Marshall (NASA/GSFC), A. Tohuvavohu (U Toronto) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team Queries: jak51@psu.edu Posted: 9 Oct 2022; 14:44 UT Subjects:Optical, X-ray, Transient At 2022-10-09 14:10:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located the new transient Swift J1913.1+1946 (triggers=1126853 and 1126854). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 288.263, +19.803 which is RA(J2000) = 19h 13m 03s Dec(J2000) = +19d 48' 09" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). After slewing, the source was still visible (leading to the second trigger). The XRT began observing the field at 14:13:30 UT, 143 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 288.2643, 19.7712 which is equivalent to: RA(J2000) = 19h 13m 3.43s Dec(J2000) = +19d 46' 16.3" with an uncertainty of 5.6 arcseconds (radius, 90% containment). This location is 1.9 arc minutes from the BAT onboard position, within the BAT error circle. UVOT took a finding chart exposure of 150 seconds with the White filter starting 179 seconds after the BAT trigger. There is a candidate counterpart in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 19:13:03.48 = 288.26452 DEC(J2000) = +19:46:24.6 = 19.77350 with a 90%-confidence error radius of about 0.61 arc sec. This position is 8.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.63 with a 1-sigma error of about 0.14. No correction has been made for the large, but uncertain, extinction expected.