TITLE:   GCN CIRCULAR
NUMBER:  28435
SUBJECT: GRB 200917A: Swift detection of a burst
DATE:    20/09/17 04:05:53 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

E. Sonbas (Adiyaman Univ.), E. Ambrosi (INAF-IASFPA),
S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA), N. J. Klingler (PSU),
D. M. Palmer (LANL), B. Sbarufatti (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 03:52:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200917A (trigger=996184).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 235.753, -4.695 which is 
   RA(J2000)  =  15h 43m 01s
   Dec(J2000) = -04d 41' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
with a duration of about 15 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 03:54:09.0 UT, 100.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 235.7247, -4.7011 which
is equivalent to:
   RA(J2000)  = 15h 42m 53.93s
   Dec(J2000) = -04d 42' 04.0"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 103 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
https://www.swift.ac.uk/sper.  We cannot determine whether the source
is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.49 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.6
(+5.36/-4.35) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 104 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. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.19. 

Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.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/)