[vsnet-grb-info 15038] Swift detection of a bright X-ray source in M31

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16332
SUBJECT: Swift detection of a bright X-ray source in M31
DATE:    14/05/27 22:15:08 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
S. B. Cenko (GSFC), V. D'Elia (ASDC), D. Malesani (DARK/NBI),
V. Mangano (PSU), F. E. Marshall (NASA/GSFC) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 21:24:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source in M31 (NGC224) (trigger=600114).  Swift slewed immediately
to the target.  The BAT on-board calculated location is 
RA, Dec 10.402, +41.556, which is 
   RA(J2000)  =  00h 41m 37s
   Dec(J2000) = +41d 33' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is an image trigger of 64 second duration,
with significance 6.18 sigma, which is lower than typical blind detection 
significance.  The BAT light curve does not show obvious structure, which 
is typical for image triggers. 

The XRT began observing the field at 21:27:05.4 UT, 157.5 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 10.4292, 41.5734 which is equivalent to:
   RA(J2000)  = 00h 41m 43.02s
   Dec(J2000) = +41d 34' 24.3"
with an uncertainty of 2.9 arcseconds (radius, 90% containment). This
location is 96 arcseconds from the BAT onboard position, within the BAT
error circle. The XRT position may be improved as
more data are received; the latest position is available at
http://www.swift.ac.uk/sper. This position is 4.0 arcseconds from that
of a known X-ray source: 3XMM J004143.0+413420 in the XMM-NEWTON XMMSSC
catalogue. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.81
x 10^21 cm^-2 (Willingale et al. 2013). 

The associated source in the 3rd XMM catalog has a spectrum and variability
which is consistent with a low-mass X-ray binary (Stiele 2010).  However,
the target is also spatially consistent with a globular cluster FMZ2005-7 of M31. 
There may be multiple X-ray binaries within the same globular cluster. 

The XRT count rate is approximately 30 counts per second.  Assuming a typical
conversion factor to flux, the 0.3-10 keV flux is approximately 1e-9 erg/cm2/s
(uncertain to a factor of 2x).  At the distance of Andromeda, this would be a
luminosity of 7e40 erg/s (uncertain to the same factor). 

This luminosity is clearly super-Eddington for a ~solar mass compact object,
so either the object is of ULX class, or it is significantly beamed. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 162 seconds after the BAT trigger. UVOT detects a 16.9 mag source
at the position
  RA (J2000) = 00:41:43.10 = 10.42957
  Dec(J2000) =+41:34:20.4  = 41.57232
with a 90% confidence error radius of 0.61 arc sec. 
This position is 4 arc seconds from the center of the XRT error circle
and consistent with the position of the globular cluster. 
No correction has been made for the expected extinction corresponding to E(B-V) of 0.06.