TITLE:   GCN CIRCULAR
NUMBER:  18478
SUBJECT: GRB 151027A: Swift detection of a burst with a bright optical afterglow
DATE:    15/10/27 04:16:31 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. Maselli (INAF-IASFPA), A. D'Ai (INAF-IASFPA),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC) and T. N. Ukwatta (LANL) report on
behalf of the Swift Team:

At 03:58:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151027A (trigger=661775).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 272.491, +61.381 which is 
   RA(J2000) = 18h 09m 58s
   Dec(J2000) = +61d 22' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
peaked structure lasting at least 150 seconds.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~110 sec after the trigger. 

The XRT began observing the field at 03:59:51.1 UT, 87.0 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 272.4886, 61.3515 which is equivalent to:
   RA(J2000)  = +18h 09m 57.26s
   Dec(J2000) = +61d 21' 05.4"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 106 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. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 95 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
RA(J2000) =	18:09:56.68 = 272.48615
DEC(J2000) = +61:21:13.4 = 61.35373
with a 90%-confidence error radius of about 0.61 arc sec. This position is 9.06
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.46 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.05. 

Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 
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.)