TITLE:   GCN CIRCULAR
NUMBER:  10489
SUBJECT: GRB 100316B: Swift detection of a burst with optical counterpart
DATE:    10/03/16 08:15:37 GMT
FROM:    Wayne Baumgartner at GSFC  <wayne@milkyway.gsfc.nasa.gov>

W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
S. Campana (INAF-OAB), P. A. Evans (U Leicester), J. M. Gelbord (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
O. M. Littlejohns (U Leicester), V. Mangano (INAF-IASFPA),
C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester),
C. Pagani (U Leicester), K. L. Page (U Leicester),
P. Romano (INAF-IASFPA), A. Rowlinson (U Leicester),
M. H. Siegel (PSU), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 08:01:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100316B trigger=416103).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA, Dec 163.498, -45.480 which is 
   RA(J2000) = 10h 54m 00s
   Dec(J2000) = -45d 28' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 08:02:41.1 UT, 64.1 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 163.48846, -45.47220 which
is equivalent to:
   RA(J2000)  = 10h 53m 57.23s
   Dec(J2000) = -45d 28' 19.9"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 36 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
9.92e+20 cm-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 66 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	10:53:57.15 = 163.48812
  DEC(J2000) = -45:28:21.6  = -45.47267
with a 90%-confidence error radius of about 0.62 arc sec. This position is 2.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.36 with a 1-sigma error of about 0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.14. 

Burst Advocate for this burst is W. H. Baumgartner (wayne AT milkyway.gsfc.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.)