GCN Circular 18037

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.)