[vsnet-grb-info 17780] Swift detects GX 301-2 in very high state

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19221
SUBJECT: Swift detects GX 301-2 in very high state
DATE:    16/03/24 04:02:38 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

K. L. Page (U Leicester) report on behalf of the Swift Team:

At 03:26:39 UT, the Swift Burst Alert Telescope (BAT) detected
the known source GX 301-2 at a level that exceeds the
on-board threshold (trigger=680331).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 186.676, -62.760 which is 
   RA(J2000) = 12h 26m 42s
   Dec(J2000) = -62d 45' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows an increase
of ~1500 counts/s when the source first came into the field of
view, with substantial variability of +/-500 counts/s over
a timescale of 10s of seconds during the observation. 

The XRT began observing the field at 03:28:44.2 UT, 124.3 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
186.6531, -62.7693 which is equivalent to:
   RA(J2000)  = +12h 26m 36.74s
   Dec(J2000) = -62d 46' 09.5"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
position is 3.0 arcseconds from a known X-ray source: GX 301-2, also known as 
1SXPS J122637.1-624610. This source is in the Swift XRT 1SXPS catalogue with
a mean 0.3-10 keV count-rate of 1.73 +/- 0.11 ct/sec; see
http://www.swift.ac.uk/1SXPS/1SXPSJ122637.1-624610 for details of these
previous observations. No event data are yet available to determine the
column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.08e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 140 seconds with the U filter starting
341 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 25% 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 large, but uncertain extinction expected. 

GX 301-2 is currently undergoing one of its periodic (41.5 day)
outbursts, and this is a stronger one than usual. 

We note that this is the second strongest detection of
BAT during any 64 second imaging interval over the lifetime
of the mission. (The strongest was on 2005-11-22 during another
outburst.)