TITLE: GCN CIRCULAR NUMBER: 18037 SUBJECT: Swift Trigger 649706: a possible GRB or bright Galactic transient DATE: 15/07/20 14:29:11 GMT FROM: Scott Barthelmy at NASA/GSFC E. Sonbas (NASA/GSFC/Adiyaman Univ.), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), S. B. Cenko (GSFC), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), S. L. Gibson (U Leicester), L. Izzo (URoma/ICRA), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 14:02:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located possible GRB 150720A (trigger=649706). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 119.582, -28.301, which is RA(J2000) = 07h 58m 20s Dec(J2000) = -28d 18' 02" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image trigger (64-sec), there is nothing significant in the real-time TDRSS light curve. The XRT began observing the field at 14:04:23.7 UT, 134.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 119.5810, -28.2623 which is equivalent to: RA(J2000) = 07h 58m 19.45s Dec(J2000) = -28d 15' 44.2" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 139 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 5.72 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 0.1 s image was 2.93e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 142 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 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 mag. No correction has been made for the large, but uncertain extinction expected. The event is near the galactic plane (l=245, b=0.5) and has a rapid decrease in X-ray afterglow. Also, it was a faint image trigger in BAT but very bright in X-rays. We can not rule out that the source is a galactic transient instead of a gamma-ray burst. Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.com). 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.)