TITLE: GCN CIRCULAR NUMBER: 22589 SUBJECT: GRB 180404A: Swift detection of a burst with an optical afterglow DATE: 18/04/04 00:54:52 GMT FROM: David Palmer at LANL J.D. Gropp (PSU), D. N. Burrows (PSU), A. Deich (PSU), C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 00:45:35 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180404A (trigger=821881). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 83.564, -37.170 which is RA(J2000) = 05h 34m 15s Dec(J2000) = -37d 10' 10" 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 50 sec. The peak count rate was ~1900 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 00:47:02.1 UT, 86.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 83.54900, -37.16824 which is equivalent to: RA(J2000) = 05h 34m 11.76s Dec(J2000) = -37d 10' 05.7" with an uncertainty of 4.2 arcseconds (radius, 90% containment). This location is 43 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. No spectrum from the promptly downlinked event data is yet available to determine the column density. UVOT took a finding chart exposure of 150 seconds with the White filter starting 89 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 05:34:11.63 = 83.54846 DEC(J2000) = -37:10:03.9 = -37.16774 with a 90%-confidence error radius of about 0.65 arc sec. This position is 2.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.40 with a 1-sigma error of about 0.16. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. 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/too.html.)