TITLE: GCN CIRCULAR NUMBER: 22590 SUBJECT: GRB 180404B: Swift detection of a burst with an optical counterpart DATE: 18/04/04 02:23:56 GMT FROM: David Palmer at LANL S. J. LaPorte (PSU), C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:11:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180404B (trigger=821902). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 53.384, -50.195 which is RA(J2000) = 03h 33m 32s Dec(J2000) = -50d 11' 41" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 50 sec. The peak count rate was ~9000 counts/sec (15-350 keV), at ~9 sec after the trigger. The XRT began observing the field at 02:12:53.9 UT, 69.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 53.3967, -50.2151 which is equivalent to: RA(J2000) = +03h 33m 35.21s Dec(J2000) = -50d 12' 54.4" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 78 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. UVOT took a finding chart exposure of 150 seconds with the White filter starting 81 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:33:34.24 = 53.39265 DEC(J2000) = -50:12:54.1 = -50.21504 with a 90%-confidence error radius of about 0.76 arc sec. This position is 9.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.61 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is S. J. LaPorte (extragsam AT gmail.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/too.html.)