TITLE: GCN CIRCULAR NUMBER: 30765 SUBJECT: GRB 210905A: Swift detection of a burst DATE: 21/09/05 00:28:29 GMT FROM: David Palmer at LANL <palmer@lanl.gov> E. Sonbas (Adiyaman Univ.), E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR), C. Gronwall (PSU), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (U Tampa), F. E. Marshall (NASA/GSFC) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 00:12:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210905A (trigger=1071993). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 309.062, -44.431 which is RA(J2000) = 20h 36m 15s Dec(J2000) = -44d 25' 50" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of at least 30 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 00:14:13.0 UT, 91.7 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 309.04815, -44.43959 which is equivalent to: RA(J2000) = 20h 36m 11.56s Dec(J2000) = -44d 26' 22.5" with an uncertainty of 7.0 arcseconds (radius, 90% containment). This location is 47 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. No spectrum from the promptly downlinked event data is yet available to determine the column density. The initial flux in the 2.5 s image was 1.23e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 156 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.034. Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.com). 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/)