TITLE: GCN CIRCULAR NUMBER: 22556 SUBJECT: Swift Trigger 819301: possible GRB 180329A DATE: 18/03/29 01:17:44 GMT FROM: David Palmer at LANL J. L. Racusin (NASA/GSFC), A. Deich (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 00:58:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located a marginal-significance (6.9 sigma) image peak that may be GRB 180329A (trigger=819301). Swift could not slew to the location due to an observing constraint. The BAT on-board calculated location is RA, Dec 329.298, -15.071 which is RA(J2000) = 21h 57m 11s Dec(J2000) = -15d 04' 14" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows some variation consistent with a ~8 second long GRB, however this variation is also consistent with the presence of the currently-active black hole transient MAXI J1820+070 in the FOV. The peak count rate was ~1400 counts/sec (15-350 keV), at ~5 sec after the trigger. Due to a Sun observing constraint, Swift cannot slew to the BAT position until 23:07 UT on 2018 April 02. There will thus be no XRT or UVOT data for this trigger before this time. Due to the low significance of this detection, the presence of a bright variable X-ray source in the BAT Field of View, and the lack of follow-up observations from the XRT and UVOT, we cannot determine whether this is a GRB at this time. Further analysis of the full downlinked data will be required to decide if this is a GRB or a statistical fluctuation in the image plane. 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.)