[vsnet-grb-info 19764] GRB 170902A: Swift detection of a burst or possible Galactic transient

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21788
SUBJECT: GRB 170902A: Swift detection of a burst or possible Galactic transient
DATE:    17/09/02 20:41:05 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

C. B. Markwardt (NASA/GSFC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 20:00:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170902A (trigger=770431).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 233.895, -57.213, which is 
   RA(J2000)  =  15h 35m 35s
   Dec(J2000) = -57d 12' 44"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, there is nothing
significant in the real-time data. 

The XRT began observing the field at 20:07:20.4 UT, 404.1 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 233.8330, -57.2313 which is equivalent to:
   RA(J2000)  = +15h 35m 19.92s
   Dec(J2000) = -57d 13' 52.7"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). This
location is 137 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. 

UVOT took a finding chart exposure of 128 seconds with the White filter
starting 413 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 large, but uncertain extinction expected. 

Because this is a long event (336 second image trigger) that is
close to the Galactic Plane (lat=-1.13 deg) there is a possibility
that this is a Galactic transient rather than a cosmic GRB. 
Determination of the nature of this object will require the full 
downlinked dataset and further observations. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt 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.)