TITLE:   GCN CIRCULAR
NUMBER:  20819
SUBJECT: GRB 170306A: Swift detection of a burst
DATE:    17/03/06 07:26:32 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@swift.psu.edu>

C. B. Markwardt (NASA/GSFC), V. D'Elia (ASDC), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 07:06:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170306A (trigger=741220).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 263.082, -44.747 which is 
   RA(J2000) = 17h 32m 20s
   Dec(J2000) = -44d 44' 50"
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 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 07:08:26.0 UT, 133.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 263.06892, -44.74808 which
is equivalent to:
   RA(J2000)  = 17h 32m 16.54s
   Dec(J2000) = -44d 44' 53.1"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 33 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 3.82
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 139 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 0.41. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt 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.)