TITLE:   GCN CIRCULAR
NUMBER:  21170
SUBJECT: GRB 170531A: Swift detection of a burst
DATE:    17/05/31 20:33:44 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

P.A. Evans (U Leicester), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Swift Team:

At 20:07:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170531A (trigger=755343).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 121.261, -14.033 which is 
   RA(J2000)  =  08h 05m 03s
   Dec(J2000) = -14d 01' 57"
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 light curve.  We note that this trigger occurred
while coming out of the SAA. 

The XRT began observing the field at 20:09:35.5 UT, 122.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 121.25380, -14.02292 which
is equivalent to:
   RA(J2000)  = 08h 05m 00.91s
   Dec(J2000) = -14d 01' 22.5"
with an uncertainty of 4.5 arcseconds (radius, 90% containment). This
location is 44 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 2.92e-10 erg cm^-2 s^-1 (0.2-10
keV). 

No UVOT data are available at this time. 

Burst Advocate for this burst is P.A. Evans (pae9 AT star.le.ac.uk). 
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.)