TITLE: GCN CIRCULAR NUMBER: 29699 SUBJECT: GRB 210323A: Swift detection of a short burst DATE: 21/03/23 22:20:56 GMT FROM: David Palmer at LANL J.D. Gropp (PSU), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), D. M. Palmer (LANL) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 22:02:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210323A (trigger=1038247). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 317.952, +25.378 which is RA(J2000) = 21h 11m 49s Dec(J2000) = +25d 22' 40" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single spike structure with a duration of about 1 sec. The peak count rate was ~10,000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 22:03:43.6 UT, 85.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 317.94609, 25.36990 which is equivalent to: RA(J2000) = 21h 11m 47.06s Dec(J2000) = +25d 22' 11.6" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 34 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 in excess of the Galactic value (1.17 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.9 (+3.84/-3.14) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 88 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 19.6 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 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.132. Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.edu). 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/)