M31CXOXRAY - M31 Central Region Chandra X-Ray Point Source Catalog
This table countains the M 31 Central Region Catalog of
Chandra X-Ray Point Sources. It is based on Chandra observations of the
central region of M 31. By combining eight Chandra ACIS-I observations which
were taken between 1999 and 2001, the authors have identified 204 X-ray
sources within the central ~17'x17' region of M 31, with a detection limit
of ~2x10^35 erg/s. Of these 204 X-ray sources, 22 are identified with
globular clusters, two with supernova remnants, nine with planetary nebulae,
and nine with supersoft sources. By comparing individual images, about 50%
of the sources were found to be variable on timescales of months.
The authors also found 13 transients, with light curves showing a variety
of shapes. They also extracted the energy spectra of the 20 brightest
sources; they can be well fitted by a single power law with a mean photon
index of 1.8. The spectral shapes of 12 sources were variable, suggesting
that they went through state changes.
All sources in the catalog have S/N > 2.5 and only five have S/N < 3.0.
The detection limit for the sources varies across the image due to the
variation of exposure time, background, and instrumental PSF, and is
highest near the edges, where the PSF broadens rapidly and the exposure
time is lowest. Over the inner 4' of the field, the detection limit is
2.1 x 10^-4 ct/s, which is equivalent to an X-ray luminosity of about
2 x 10^35 erg/s.
Additional information about optical identifications and cross-correlated
ROSAT X-ray sources not provided in this HEASARC table is available in the
published paper (Tables 4 and 5) and/or the CDS at
ftp://cdsarc.u-strasbg.fr/pub/cats/J/ApJ/577/738/ (table4.dat & table5.dat).
X-ray point sources in the central region of M31 as seen by Chandra.
Kong A.K.H., Garcia M.R., Primini F.A., Murray S.S., di Stefano R.,
<Astrophys. J. 577, 738 (2002)>
This table was created by the HEASARC in October 2004 based
on CDS Catalog J/ApJ/577/738/table2.dat.
The catalog source number, using a nomenclature scheme
based on optical studies (Morton et al. 1977, ApJ, 212, 13: a similar
classification was also used by Trinchieri and Fabbiano 1991, ApJ, 382, 82).
The source numbers have a prefix of either 'r1', 'r2', or 'r3'. The 'r1'
sources are located in the central 2'x2' or `inner bulge' region. The 'r2'
sources are located in the central 8'x8' region, excluding the inner bulge
region; the authors refer to this annular region as the `outer bulge'.
The 'r3' sources are located outside the central 8'x8' region in the
The IAU-approved name for a Chandra X-ray source in M 31, of the
form CXOM31 JHHMMSS.s+DDMMSS.
The Right Ascension of the X-ray source in the selected equinox. This
was given in J2000 coordinates and to a precision of 0.001 seconds of time
in the original form of the catalog.
The Declination of the X-ray source in the selected equinox. This
was given in J2000 coordinates and to a precision of 0.01 arcseconds
in the original form of the catalog.
The Galactic Longitude of the X-ray source.
The Galactic Latitude of the X-ray source.
The positional error, in arcseconds.
The net counts in the X-ray source.
The count rate of the X-ray source, in ct/s, corrected for
exposure, background variation and instrumental point spread function.
Source count rates were determined via aperture photometry. The radius
of the aperture was varied with average off-axis angle in order to match
the 90% encircled energy function. The average off-axis angle was computed
for each source based on the off-axis angles of the 8 observations weighted
by their exposure times. The extraction radius varied from ~1" near the
aimpoint to ~17" for the sources with the largest off-axis angles.
Background was extracted from an annulus centered on each source. In some
cases, for example in the nuclear region, the extraction region was modified
to avoid nearby sources. It was also necessary to modify the extraction
radius for some faint sources that were close to more liminous sources.
The one-sigma error in the count rate, in ct/s.
The medium-soft hardness ratio HR1 = (M-S)/(M+S) of the
X-ray source, where S is the counts in the 0.3-1.0 keV energy band and M is
the counts in the 1-2 keV energy band.
The error in the hardness ratio HR1.
The hard-soft hardness ratio HR2 = (H-S)/(H+S) of the
X-ray source, where S is the counts in the 0.3-1.0 keV energy band and H is
the counts in the 2-7 keV energy band.
The error in the hardness ratio HR2.
The 0.3-7 keV luminosity of the X-ray source, in erg/s.
The conversion into X-ray luminosity assumed an absorbed power-law spectrum
with a photon index of 1.7 and a hydrogen column density of 10^21 atoms/cm^2,
which is the typical spectrum of a point source in M 31 according to Shirey
et al. (2001, A&A, 365, L195).
A parameter containing notes codes as follows:
e = Extragalactic object
f = Foreground star
g = Globular Cluster
p = Planetary nebula
r = ROSAT HRI source
s = Supernova remnant
v = Variable
sv = Spectral variable
t = Transient
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