TITLE: GCN CIRCULAR NUMBER: 30120 SUBJECT: Swift Detection of probable new Swift SGR J1555.2-5402 DATE: 21/06/03 10:25:32 GMT FROM: David Palmer at LANL D.M. Palmer (LANL), P.A. Evans (U. Leicester), N.P.M. Kuin (MSSL/UCL), and K.L. Page (U. Leicester) report on behalf of the Swift Team: At 09:45:46 UT on 2021-06-03, the Swift Burst Alert Telescope (BAT) triggered and located an unknown burst source (trigger=1053220). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 238.819, -54.046 which is RA(J2000) = 15h 55m 17s Dec(J2000) = -54d 02' 46" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT lightcurve shows a single-bin (64 ms) spike with a nominal peak of 2000 counts per second. However, the BAT trigger timescale was very short (8 ms) and moderately intense (29 sigma in count rate), indicating a very short spike emission. The XRT began observing the field at 09:47:23.3 UT, 96.8 seconds after the BAT trigger. No source was detected in the 2.5-s promptly available image. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 61 seconds with the White filter starting 100 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 none of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the large expected extinction. The fast lightcurve shape, combined with the source's location near the Galactic plane (lon, lat = 327.90, -0.34) leads us to the conclusion that this is a new SGR, which we name Swift SGR J1555.2-5402 based on the BAT position. More information on this source will be provided once the ground pass data has been analyzed.