TITLE:   GCN CIRCULAR
NUMBER:  21718
SUBJECT: GRB 170827A: Swift detection of a burst
DATE:    17/08/27 05:25:35 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

F. E. Marshall (NASA/GSFC), A. D'Ai (INAF-IASFPA),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 05:12:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170827A (trigger=769567).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 276.180, -40.220 which is 
   RA(J2000) = 18h 24m 43s
   Dec(J2000) = -40d 13' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for an image trigger, the
immediately-available BAT lightcurve shows no obvious variation. 

The XRT began observing the field at 05:14:32.5 UT, 124.1 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 276.1508, -40.2016 which is equivalent to:
   RA(J2000)  = +18h 24m 36.19s
   Dec(J2000) = -40d 12' 05.8"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 104 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 4.23e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 133 seconds after the BAT trigger. Only partial
images are available and neither the 2.7'x2.7' sub-image nor the 8'x8'
region for the list of sources generated on-board covers the XRT error
circle. Analysis of any potential afterglow will have to await further
data. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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.)