GCN Circular 22982

S. J. LaPorte (PSU), P. A. Evans (U Leicester), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (George Washington University),
B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 22:23:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB 180720C or Galactic transient (trigger=848932). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 265.623, -26.584 which is 
   RA(J2000) = 17h 42m 30s
   Dec(J2000) = -26d 35' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:25:34.8 UT, 97.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 265.63486, -26.63041 which is
equivalent to:
   RA(J2000)  = 17h 42m 32.37s
   Dec(J2000) = -26d 37' 49.5"
with an uncertainty of 5.7 arcseconds (radius, 90% containment). This
location is 171 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 in excess of the Galactic value (6.76 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.7
(+3.01/-2.29) x 10^22 cm^-2 (90% confidence). 

We note that the detected source is at the edge of the reduced field of view available with the promptly down linked data. Because of that the accuracy of the position could be lower than stated. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 100 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. Because of the density of
catalogued stars, further analysis is required to report an upper limit for any
afterglow in the region. No correction has been made for the large, but
uncertain, extinction expected. 

While the BAT and XRT data are consistent with a GRB, because of the
proximity to the Galactic Center, at present we cannot rule out a
Galactic transient nature for the source. 

Burst Advocate for this burst is S. J. LaPorte (extragsam AT gmail.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.)