TITLE:   GCN CIRCULAR
NUMBER:  21864
SUBJECT: GRB 170912B: Swift detection of a burst
DATE:    17/09/12 06:47:58 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

S. J. LaPorte (PSU), J. A. Kennea (PSU), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of
the Swift Team:

At 06:33:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170912B (trigger=772052).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 215.466, -62.030 which is 
   RA(J2000) = 14h 21m 52s
   Dec(J2000) = -62d 01' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 20 sec.  The peak count rate
was ~9500 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 06:34:36.9 UT, 50.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 215.45921, -61.99674 which
is equivalent to:
   RA(J2000)  = 14h 21m 50.21s
   Dec(J2000) = -61d 59' 48.3"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 120 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 does not constrain the column density. 

The initial flux in the 2.5 s image was 4.76e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 58 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the large, but uncertain extinction
expected. 

Burst Advocate for this burst is S. J. LaPorte (extragsam AT gmail.com). 
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.)