GCN Circular 19382

D. Malesani (DARK/NBI), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL), T. G. R. Roegiers (PSU),
B. Sbarufatti (INAF-OAB/PSU) and T. N. Ukwatta (LANL) report on behalf
of the Swift Team:

At 19:30:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160504A (trigger=685124).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 170.043, +55.997 which is 
   RA(J2000) = 11h 20m 10s
   Dec(J2000) = +55d 59' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 10 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 19:32:46.0 UT, 128.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 170.07309, 56.00016 which is equivalent
to:
   RA(J2000)  = 11h 20m 17.54s
   Dec(J2000) = +56d 00' 00.6"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 61 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 8.12
x 10^19 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 98 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 none of
the XRT error circle. 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 0.01. 

Burst Advocate for this burst is D. Malesani (malesani AT dark-cosmology.dk). 
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.)