TITLE: GCN CIRCULAR NUMBER: 22558 SUBJECT: GRB 180329B: Swift detection of a burst with an optical afterglow DATE: 18/03/29 14:21:48 GMT FROM: David Palmer at LANL J. L. Racusin (NASA/GSFC), A. Deich (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 14:08:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180329B (trigger=819490). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 82.916, -23.708 which is RA(J2000) = 05h 31m 40s Dec(J2000) = -23d 42' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a small complex peak from T-10 to T+20 s (peak of 800 counts/s) with a stronger peak from T+140 to T+160 (the limit of currently available data). The peak count rate was ~1400 counts/sec (15-350 keV), at ~147 sec after the trigger. The XRT began observing the field at 14:10:07.5 UT, 103.6 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 82.9031, -23.6889 which is equivalent to: RA(J2000) = +05h 31m 36.74s Dec(J2000) = -23d 41' 20.0" with an uncertainty of 6.5 arcseconds (radius, 90% containment). This location is 80 arcseconds from the BAT onboard position, within the BAT error circle. The initial flux in the 2.5 s image was 4.57e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 111 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 05:31:36.90 = 82.90376 DEC(J2000) = -23:41:25.9 = -23.69052 with a 90%-confidence error radius of about 0.62 arc sec. This position is 6.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.65 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. The brighter, second, peak of this GRB occurs during the time that XRT and UVOT were taking data, so the early observations sample the prompt burst emission. Burst Advocate for this burst is J. L. Racusin (judith.racusin AT nasa.gov). 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.)