TITLE: GCN CIRCULAR NUMBER: 40613 SUBJECT: GRB 250603A: Swift detection of a burst or new Galactic transient Swift J0839.7-3916 DATE: 25/06/03 01:51:05 GMT FROM: David Palmer at LANL <palmer@lanl.gov> T. M. Parsotan (GSFC), R. Brivio (INAF-OAB), R. Gupta (NASA GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL) and C. Salvaggio (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:10:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250603A (trigger=1320335). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 129.918, -39.268 which is RA(J2000) = 08h 39m 40s Dec(J2000) = -39d 16' 05" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 18 sec with a possible short spike at T0+16 sec. The peak count rate was ~1,300 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 01:12:04.7 UT, 101.7 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. Although the initial 2.5s of XRT data do not show a clear X-ray afterglow, the high significance of the BAT image (7.82 sigma) gives us confidence that this is an astrophysical source. Further analysis awaits the full downlinked dataset. We note the presence of Fermi-GBM event at ~T+10 minutes (GBM trigger: 770606412) with a position consistent (to its 25° error radius) with this source location. Given the location of the BAT detection at 1.42° from the Galactic plane, persistent flux over 10 minutes would suggest a Galactic transient. If so, we would name it Swift J0839.7-3916 . Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan AT nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) View this GCN Circular online at https://gcn.nasa.gov/circulars/40613. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...