TITLE: GCN CIRCULAR NUMBER: 23194 SUBJECT: GRB 180904A: Swift detection of a burst DATE: 18/09/04 21:42:57 GMT FROM: David Palmer at LANL A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU), J. A. Kennea (PSU), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC), M. J. Moss (George Washington University), D. M. Palmer (LANL), B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 21:28:32 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180904A (trigger=859282). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 274.250, +46.623 which is RA(J2000) = 18h 17m 00s Dec(J2000) = +46d 37' 24" 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 ~5 sec after the trigger. The XRT began observing the field at 21:30:35.3 UT, 123.0 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 274.2495, 46.6298 which is equivalent to: RA(J2000) = 18h 16m 59.87s Dec(J2000) = +46d 37' 47.4" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 24 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (3.37 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.3 (+3.26/-2.80) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 4.47e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 131 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 0.04. Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.inaf.it). 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.)