TITLE:   GCN CIRCULAR
NUMBER:  21097
SUBJECT: GRB 170516A: Swift detection of a burst
DATE:    17/05/16 13:06:41 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA),
P.A. Evans (U Leicester), C. Gronwall (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 12:49:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170516A (trigger=753110).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 41.598, -55.899, which is 
   RA(J2000)  =  02h 46m 24s
   Dec(J2000) = -55d 53' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a couple peaks
with a total duration of about 35 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 12:50:41.1 UT, 80.5 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 41.5663, -55.9092 which is equivalent to:
   RA(J2000)  = +02h 46m 15.91s
   Dec(J2000) = -55d 54' 33.1"
with an uncertainty of 6.4 arcseconds (radius, 90% containment). This
location is 73 arcseconds from the BAT onboard position, within the BAT
error circle. 

The initial flux in the 2.5 s image was 4.96e-10 erg cm^-2 s^-1 (0.2-10
keV). 

Automated UVOT data processing is currently not working, but manual
inspection of the UVOT image shows no GRB afterglow counterpart. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)