TITLE: GCN CIRCULAR NUMBER: 30820 SUBJECT: GRB 210912A: Swift detection of a burst DATE: 21/09/12 11:18:18 GMT FROM: David Palmer at LANL <palmer@lanl.gov> E. Troja (NASA/GSFC/UMCP), A. P. Beardmore (U Leicester), R. Caputo (NASA/GSFC), N. J. Klingler (GSFC/UMBC/CRESSTII), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 11:01:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210912A (trigger=1072990). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 129.769, +79.194 which is RA(J2000) = 08h 39m 04s Dec(J2000) = +79d 11' 38" 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 40 sec. The peak count rate was ~2800 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 11:02:04.1 UT, 52.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 129.88358, 79.22292 which is equivalent to: RA(J2000) = 08h 39m 32.06s Dec(J2000) = +79d 13' 22.5" with an uncertainty of 7.5 arcseconds (radius, 90% containment). This location is 129 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.40 x 10^20 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 6.40e-10 erg cm^-2 s^-1 (0.2-10 keV). There are no UVOT data available. Burst Advocate for this burst is E. Troja (eleonora.troja 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/)