[vsnet-grb-info 14888] Trigger 597570: Swift detection of SGR 1806-20

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16183
SUBJECT: Trigger 597570: Swift detection of SGR 1806-20
DATE:    14/04/29 13:32:17 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), J. K. Cannizzo (NASA/UMBC), V. D'Elia (ASDC),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), M. H. Siegel (PSU) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team:

At 13:13:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located SGR 1806-20 (trigger=597570).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 272.159, -20.404 which is 
   RA(J2000)  =  18h 08m 38s
   Dec(J2000) = -20d 24' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single spike
with a duration of about 0.128sec.  The peak count rate
was ~11,000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 13:14:29.8 UT, 61.7 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 272.1632, -20.4116 which is
equivalent to:
   RA(J2000)  = 18h 08m 39.16s
   Dec(J2000) = -20d 24' 41.7"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 30 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. This
position is 3.1 arcsec from the known position of SGR 1806-20. No spectrum
from the promptly downlinked event data is yet available to determine
the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 63 seconds after the BAT trigger. No credible optical counterpart has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
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 large, but uncertain extinction expected. 

The last previous detection of this Soft Gamma Repeater by 
BAT was on April 5, 2014.