TITLE:   GCN CIRCULAR
NUMBER:  23043
SUBJECT: GRB 180727A: Swift detection of a burst
DATE:    18/07/27 14:58:03 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. P. Beardmore (U Leicester), S. B. Cenko (GSFC),
P. A. Evans (U Leicester), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU),
R. L. C. Starling (U Leicester) and A. Tohuvavohu (PSU) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 14:15:28 UT, the Swift Burst Alert Telescope triggered
and located GRB180727A (Trigger #850231).  Due to a telemetry 
gap, very little BAT information is immediately available. 
The BAT light curve showed a single-peaked structure with 
a duration of about 1.5 sec.  The peak count rate
was ~5500 counts/sec (15-350 keV), at ~0 sec after the trigger. 
The burst does not seem to show significant emission above 100 keV. 
The full BAT information will be available after the 
next ground downlink. 

Using promptly downlinked data we find an uncatalogued X-ray source
located at RA, Dec 346.66536, -63.05052 which is equivalent to:
   RA(J2000)  = 23h 06m 39.69s
   Dec(J2000) = -63d 03' 01.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.10 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.5
(+3.98/-2.45) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the white
filter. 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 19.2 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 18.0 mag. No  correction has been made for the expected
extinction corresponding to E(B-V) of  0.02. 

We note this GRB was also detected by the GBM on Fermi, and their
automated light curve analysis indicates it is likely to be a short GRB. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 
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.)