[vsnet-grb-info 17764] Swift detection of a likely new SGR: SGR 0755-2933

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19204
SUBJECT: Swift detection of a likely new SGR: SGR 0755-2933
DATE:    16/03/16 23:13:39 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),
L. Izzo (URoma1), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
D. M. Palmer (LANL), M. H. Siegel (PSU) and T. N. Ukwatta (LANL)
report on behalf of the Swift Team:

At 22:41:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a burst from an apparent new Soft Gamma Repeater,
SGR 0755-2933 (trigger=679432).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 118.884, -29.552 which is 
   RA(J2000)  =  07h 55m 32s
   Dec(J2000) = -29d 33' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a duration of less than 128 msec.  The peak count rate
was ~1900 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:42:39.1 UT, 74.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 118.9270, -29.5637
which is equivalent to:
   RA(J2000)  = 07h 55m 42.48s
   Dec(J2000) = -29d 33' 49.2"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 141 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 in excess of the Galactic value (6.09 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.2
(+3.68/-3.17) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 250 seconds with the U filter
starting  135 seconds after the BAT trigger. No credible afterglow
candidate has been  found in the initial data products. The 2.7'x2.7'
covers none of the XRT error circle. 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 18.0 mag. No correction
has been made for the  large, but uncertain extinction expected. 

Given that this was a short event (<128 msec) and spectrally soft (in BAT) 
and that it is on the Galactic Plane (lat = -0.6 deg), it is likely a 
new SGR, tentatively named SGR 0755-2933.  The XRT source has a count rate of 
several counts per second and is variable but not fading.