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GCN Circular 21821

Subject
GRB 170906A: Swift detection of a burst with an optical afterglow
Date
2017-09-06T00:55:00Z (6 years ago)
From
David Burrows at PSU/Swift <burrows@astro.psu.edu>
M. H. Siegel (PSU), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), B. Sbarufatti (PSU)
and A. Tohuvavohu (PSU) report on behalf of the Swift Team:

At 00:43:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170906A (trigger=770957).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 203.992, -47.125 which is 
   RA(J2000) = 13h 35m 58s
   Dec(J2000) = -47d 07' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multiply-peaked
structure with a duration of about 120 sec.  The peak count rate
was ~16000 counts/sec (15-350 keV), at ~30 sec after the trigger. 

The XRT began observing the field at 00:44:18.9 UT, 67.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 203.9546, -47.1007 which is equivalent to:
   RA(J2000)  = +13h 35m 49.10s
   Dec(J2000) = -47d 06' 02.5"
with an uncertainty of 5.2 arcseconds (radius, 90% containment). This
location is 126 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 0.1 s image was 4.12e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 77 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:35:49.19 = 203.95497
  DEC(J2000) = -47:06:01.9  = -47.10054
with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.59 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.14. 

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu). 
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.)
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