TITLE: GCN CIRCULAR NUMBER: 22909 SUBJECT: GRB 180706A: Swift detection of a burst with optical counterpart DATE: 18/07/06 08:43:15 GMT FROM: Scott Barthelmy at NASA/GSFC M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), P. A. Evans (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU) and K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift Observatory Team: At 08:24:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180706A (trigger=846395). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 181.665, +66.035, which is RA(J2000) = 12h 06m 40s Dec(J2000) = +66d 02' 05" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two main peaks with a total duration of about 40 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~28 sec after the trigger. The XRT began observing the field at 08:26:08.0 UT, 87.7 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 181.6428, 66.0366 which is equivalent to: RA(J2000) = 12h 06m 34.28s Dec(J2000) = +66d 02' 11.6" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 32 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.44 x 10^20 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 7.91e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 307 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 12:06:34.34 = 181.64308 DEC(J2000) = +66:02:13.6 = 66.03712 with a 90%-confidence error radius of about 0.77 arc sec. This position is 2.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.74 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 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.)