[vsnet-grb-info 11417] Swift detection of an SGR burst from AXP 4U 0142+61

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12829
SUBJECT: Swift detection of an SGR burst from AXP 4U 0142+61
DATE:    12/01/12 13:30:31 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC),
B. Gendre (ASDC), C. Guidorzi (U Ferrara),
H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (NASA/GSFC/ORAU),
O. M. Littlejohns (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU),
R. L. C. Starling (U Leicester), G. Stratta (ASDC) and
G. Tagliaferri (INAF-OAB) report on behalf of the Swift Team:

At 13:09:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located an outburst from AXP 4U 0142+61 (trigger=511611).  Swift slewed
immediately to the burst.  The BAT on-board calculated location is 
RA, Dec 26.621, +61.763 which is 
   RA(J2000)  =  01h 46m 29s
   Dec(J2000) = +61d 45' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a total duration about 64 millisec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 13:10:56.9 UT, 78.5 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
located at RA, Dec 26.59414, 61.75160 which is equivalent to:
   RA(J2000)  = 01h 46m 22.59s
   Dec(J2000) = +61d 45' 05.8"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). 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 3.0
arcseconds from that of a known X-ray source: AXP 4U 0142+61. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 8.13
x 10^21 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 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. The typical 3-sigma upper limit has been about 19.6 mag. 
The coverage of the XRT error circle by the 8'x8' region for the list of
sources generated on-board is uncertain because the large number of sources
filled the available telemetry. The list of sources is typically complete to
about 18 mag. No correction has been made for the large, but uncertain
extinction expected. 

This source has produced previous SGR bursts (most recently last July,
Oates et al. GCN 12209) and is part of a regular monitoring program
of AXPs (PI: V. Kaspi).