TITLE: GCN CIRCULAR NUMBER: 23279 SUBJECT: GRB 180930A: Swift detection of a burst DATE: 18/09/30 12:57:20 GMT FROM: David Burrows at PSU/Swift A. Tohuvavohu (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 12:43:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180930A (trigger=864584). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 76.870, -25.138 which is RA(J2000) = 05h 07m 29s Dec(J2000) = -25d 08' 17" 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 20 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 12:47:17.6 UT, 208.3 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 76.86228, -25.11720 which is equivalent to: RA(J2000) = 05h 07m 26.95s Dec(J2000) = -25d 07' 01.9" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 78 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 http://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 (2.09 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.9 (+1.93/-1.74) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 211 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 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 0.02. Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.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/too.html.)