TITLE:   GCN CIRCULAR
NUMBER:  10048
SUBJECT: GRB 091020: Swift detection of a burst with an optical afterglow
DATE:    09/10/20 21:47:36 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

J. L. Racusin (NASA/GSFC/ORAU), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (MSSL), W.B Landsman (GSFC), V. Mangano (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), A. Rowlinson (U Leicester),
M. H. Siegel (PSU), E. Troja (NASA/GSFC/ORAU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 21:36:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091020 (trigger=373458).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 175.715, +50.972 which is 
   RA(J2000) = 11h 42m 52s
   Dec(J2000) = +50d 58' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a bright peak
with a second much smaller peak at T+30 s, for a total duration 
of about 45 sec.  The peak count rate was ~5000 counts/sec (15-350 keV),
at ~2 sec after the trigger. 

The XRT began observing the field at 21:38:05.5 UT, 80.8 seconds after
the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located
at RA, Dec 175.7284, +50.9788 which is equivalent to:
   RA(J2000)  = 11h 42m 54.81s
   Dec(J2000) = +50d 58' 43.6"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 39 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 3.01e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 89 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:42:55.18 = 175.72993
  DEC(J2000) = +50:58:41.9  = 50.97831
with a 90%-confidence error radius of about 0.74 arc sec. This position is 3.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
15.73 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin 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.)