TITLE:   GCN CIRCULAR
NUMBER:  10034
SUBJECT: GRB 091018: Swift detection of a burst with optical afterglow
DATE:    09/10/18 21:08:34 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

M. Stamatikos (OSU/NASA/GSFC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC),
C. Gronwall (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
W.B Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), M. Perri (ASDC), J. L. Racusin (NASA/GSFC/ORAU)
and G. Stratta (ASDC) report on behalf of the Swift Team:

At 20:48:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091018 (trigger=373172).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 32.211, -57.526 which is 
   RA(J2000) = 02h 08m 51s
   Dec(J2000) = -57d 31` 34``
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single smooth
FRED peak a duration of about 10 sec.  The peak count rate
was ~16000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 20:49:20.5 UT, 60.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 32.18587, -57.54749 which
is equivalent to:
   RA(J2000)  = 02h 08m 44.61s
   Dec(J2000) = -57d 32' 51.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 91 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 in excess of the Galactic value (2.81e+20
cm^-2, Kalberla et al. 2005), with an excess column of 1.4
(+1.00/-0.93) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 4.40e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 68 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7`x2.7` sub-image at
  RA(J2000)  =	02:08:44.61 = 32.18588
  DEC(J2000) = -57:32:53.8  = -57.54827
with a 90%-confidence error radius of about 0.61 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.60 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 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.)