TITLE: GCN CIRCULAR NUMBER: 27326 SUBJECT: GRB 200306C: Swift detection of a burst with an optical counterpart DATE: 20/03/06 23:02:02 GMT FROM: David Palmer at LANL J.D. Gropp (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), J. A. Kennea (PSU), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 22:50:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200306C (trigger=960102). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 198.582, +11.255 which is RA(J2000) = 13h 14m 20s Dec(J2000) = +11d 15' 18" 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 ~1800 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 22:52:35.9 UT, 116.9 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 198.55562, 11.26981 which is equivalent to: RA(J2000) = 13h 14m 13.35s Dec(J2000) = +11d 16' 11.3" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 107 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 https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.91 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.8 (+2.73/-2.37) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 250 seconds with the U filter starting 117 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 13:14:13.44 = 198.55602 DEC(J2000) = +11:16:11.5 = 11.26985 with a 90%-confidence error radius of about 1.10 arc sec. This position is 1.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.47. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. 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/)