TITLE: GCN CIRCULAR NUMBER: 22580 SUBJECT: GRB 180402A: Swift detection of a possibly short burst DATE: 18/04/02 10:08:54 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT A. Y. Lien (GSFC/UMBC), A. P. Beardmore (U Leicester), J.D. Gropp (PSU) and K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift Observatory Team: At 09:44:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180402A (trigger=821103). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 251.905, -14.963 which is RA(J2000) = 16h 47m 37s Dec(J2000) = -14d 57' 48" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a short peak with a duration of about 0.3 sec. The peak count rate was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. There are some additional variation in the light curve, however, this variation is also consistent with the presence of the currently-active black hole transient MAXI J1820+070 in the FOV. Further analysis of the full downlinked data will be required to determine the cause of the variation. The XRT began observing the field at 09:46:19.2 UT, 79.9 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 251.9335, -14.9683 which is equivalent to: RA(J2000) = 16h 47m 44.05s Dec(J2000) = -14d 58' 05.8" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 100 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.94 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 250 seconds with the U filter starting 139 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 none of the XRT error circle. 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.77. Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)