GCN Circular 21171

P.A. Evans (U Leicester), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), A. Cholden-Brown (PSU), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of
the Swift Team:

At 22:02:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170531B (trigger=755354).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 286.895, -16.426 which is 
   RA(J2000) = 19h 07m 35s
   Dec(J2000) = -16d 25' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 50 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 22:04:30.1 UT, 140.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 286.8835,
-16.4196 which is equivalent to:
   RA(J2000)  = 19h 07m 32.03s
   Dec(J2000) = -16d 25' 10.4"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 45 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 gives a column density consistent with the Galactic value of 1.41
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 2.73e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 152 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 expected extinction corresponding
to E(B-V) of 0.19. 

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.)