GCN Circular 18076

S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), J. L. Racusin (NASA/GSFC) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 19:02:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150727A (trigger=650530).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 203.990, -18.355, which is 
   RA(J2000)  =  13h 35m 58s
   Dec(J2000) = -18d 21' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a large FRED-like peak
structure with a duration of about 60 sec.  The peak count rate
was ~2795 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 19:03:19.5 UT, 77.2 seconds after
the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located
at RA, Dec 203.9705, -18.3259 which is equivalent to:
   RA(J2000)  = +13h 35m 52.92s
   Dec(J2000) = -18d 19' 33.2"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 124 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 8.93e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 86 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:35:52.52 = 203.96883
  DEC(J2000) = -18:19:31.4  = -18.32539
with a 90%-confidence error radius of about 0.62 arc sec. This position is 6.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.31 with a 1-sigma error of about  0.15. No correction has been made for
extinction. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT 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.)