TITLE:   GCN CIRCULAR
NUMBER:  19926
SUBJECT: GRB 160917A: Swift detection of a burst
DATE:    16/09/17 11:46:10 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

J. L. Racusin (NASA/GSFC), S. D. Barthelmy (GSFC),
J. R. Cummings (NASA/UMBC), J. A. Kennea (PSU),
D. Malesani (DARK/NBI and DTU Space), F. E. Marshall (NASA/GSFC),
B. Mingo (U Leicester), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 11:30:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160917A (trigger=712505).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 295.671, +46.399 which is 
   RA(J2000)  =  19h 42m 41s
   Dec(J2000) = +46d 23' 56"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several overlapping peaks
with a total duration of about 25 sec.  The peak count rate
was ~2300 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 11:31:52.9 UT, 93.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 295.66805, 46.40321 which
is equivalent to:
   RA(J2000)  = 19h 42m 40.33s
   Dec(J2000) = +46d 24' 11.6"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 16 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.  We
cannot determine whether the source is fading at the present time. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 7.72e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 125 seconds with the White
filter  starting 99 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. The typical
3-sigma upper limit has been about 19.6 mag.  Results from the list of
sources  generated on-board are not available at this time. No
correction has been made  for the expected extinction corresponding to
E(B-V) of 0.16. 

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.)