TITLE:   GCN CIRCULAR
NUMBER:  18143
SUBJECT: GRB 150817A: Swift detection of a burst
DATE:    15/08/17 02:14:02 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

V. D'Elia (ASDC), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC),
L. Izzo (URoma/ICRA), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 02:05:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150817A (trigger=652334).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 249.643, -12.066 which is 
   RA(J2000) = 16h 38m 34s
   Dec(J2000) = -12d 03' 55"
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 ~17000 counts/sec (15-350 keV), at ~11 sec after the trigger. 

The XRT began observing the field at 02:06:18.2 UT, 64.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 249.63352, -12.05336 which
is equivalent to:
   RA(J2000)  = 16h 38m 32.04s
   Dec(J2000) = -12d 03' 12.1"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 56 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 in excess of the Galactic value (2.71 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.5
(+3.63/-3.05) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.27e-09 erg cm^-2 s^-1 (0.2-10
keV). 

There will be no prompt UVOT data available for this burst. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 
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.)