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M31CXOHRC2 - M 31 Chandra HRC X-Ray Source Catalog



The authors have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of ~2.5 yr and contain a mean effective exposure of 17 ks. The authors measured the mean fluxes and long-term light curves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well fitted by a power law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. The authors note several sources in the southwestern disk with LX > 1037 ergs s-1. They cross-correlate all of their sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is 1.

The data for this project were originally part of a survey program to look for X-ray transients in M31. Nearly every month from 1999 November to 2001 February, Chandra took HRC-I images of five fields covering most of M31. Observations were then made every few months until 2002 June. The details of the 81 Chandra observations are summarized in Table 1 of the reference paper, which is available in a machine-readable form at The authors combined all the data into three data sets using the task merge_all. One set contained the data for the northern half of the galaxy, another contained the southern half, and the last contained the center. The authors searched for sources in the three data sets using the CIAO task wavdetect. They ran this task searching for sources on four size scales: 1, 2, 4, and 8 pixels. The pixels in the merged images were 1 arcsecond in the central 18 arcmin by 18 arcmin and 2 arcsec outside of this region. By searching on several scales, wavdetect is able to overcome the large changes in the size of the Chandra PSF from about 0.5 arcsec near the center of the field to over 10 arcsec in the outer regions of the field. A total of 166 sources were detected above their 3.5-sigma detection threshold.

Catalog Bibcode



A synoptic X-ray study of M31 with the Chandra high resolution camera.
    Williams B.F., Garcia M.R., Kong A.K.H., Primini F.A., King A.R.,
    Di Stefano R., Murray S.S.
   <Astrophys. J., 609, 735-754 (2004)>


This table was created by the HEASARC in September 2015 based on the CDS Catalog J/ApJ/609, 735 file table2.dat .


The IAU-sanctioned name for the source. This was formulated by the HEASARC by applying the prefix 'CXOM31' to the J2000.0 Right Ascension and Declination. For example, the proper name of the source with alt_name = [KGP2002] s1-74' is 'CXOM31 J003956.3+404100'.

The alternative name for the source using the '[KGP2002] ' prefix recommended by the Dictionary of Nomenclature of Celestial Objects. Identical sources have the same names in Kong et al. (2002, ApJ, 577, 738 = [KGP2002], available at the HEASARC as the M31CXOXRAY table) and Di Stefano et al. (2004, ApJ, 610, 247). The source prefixes are:

     - r1 designates objects in a square region 2' x 2' centered on the nucleus;
     - r2 designates objects outside of r1 but within a square region 8' x 8'
            centered on the nucleus;
     - r3 designates objects outside of r2 but within a square region 23' x 23'
            centered on the nucleus;
     - n1 designates objects outside of r3, north of the nucleus but south
            of Dec = 42:01:00 (J2000);
     - n2 designates objects north of Dec = 42:01:00 (J2000);
     - s1 designates objects outside of r3, south of the nucleus but north
            of Dec = 40:31:22 (J2000);
     - s2 designates objects south of Dec = 40:31:22 (J2000).
New objects within these regions are numbered consecutively starting from the highest published number in either Kong et al. (2002) or Di Stefano et al. (2004).

The Right Ascension of the Chandra X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 0.01 seconds of time in the original table.

The Declination of the Chandra X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 0.1 arcseconds in the original table.

The Galactic Longitude of the Chandra X-ray source.

The Galactic Latitude of the Chandra X-ray source.

The total net counts in the Chandra HRC source detection

The wavdetect-measured signal-to-noise ratio of the Chandra HRC detection.

The mean luminosity of the source in the 0.1 - 10 keV band over all epochs, in erg s-1. To convert their measured mean X-ray fluxes to luminosities, the authors assumed a distance to M31 of 780 kpc, a photon index of 1.7, and an absorption NH of 1021 cm-2. These assumptions provided a constant conversion factor of 2.5 x 1041 ergs cm-2 count-1 from HRC flux in counts cm-2 s-1 to luminosity in ergs s-1 (0.1 - 10 keV).

The uncertainty in the mean luminosity of the source in the 0.1 - 10 keV band over all epochs, in erg s-1.

The Chi2nu value resulting from a fit of the measured counting rate to a constant source at the mean rate. A value greater than 1.47 indicates that the source has a 90% chance of being variable.

The name of the counterpart to the Chandra HRC source found at another wavelength or in a previous X-ray survey. If the object was detected in previous X-ray surveys as well as in other wavelengths, the optical (or, in one case, radio) counterpart is listed. If the object has no known counterpart outside of the X-ray band, the earliest detection is listed as the X-ray counterpart. With the exception of the "mita" prefix, counterpart names are IAU names taken from the Simbad database ( X-ray sources with no previous detections and no optical counterparts are labeled as "new".

This flag contains either a note concerning the nature of the Chandra HRC source, or, if numerical, the reference corresponding to the origin of the ctrpart_name value, as follows:

  Value   Reference or Source Type

  1       Supper et al. (2001, A&A, 373, 63);
  2       Primini, Forman, & Jones (1993, ApJ, 410, 615);
  3       Trinchieri & Fabbiano (1991, ApJ, 382, 82);
  4       Walterbos & Braun (1992, A&AS, 92, 625);
  5       Ford & Jacoby (1978, ApJ, 219, 437);
  6       Battistini et al. (1987, A&AS, 67..447);
  7       Brown et al. (2000, ApJ, 532, 308);
  8       Brown et al. (1998, ApJ, 504, 113);
  9       Braun (1990, ApJS, 72, 755),
  11      Dodorico, Dopita, & Benvenuti (1980, A&AS, 40, 67);
  12      Williams, Schmitt, & Winkler (1995, BAAS, 27, 883);
  13      Mochejska et al. (2001, AJ, 122, 1383);
  14      Magnier (1993, Ph.D. thesis, MIT);
  15      Kong et al. (2002, ApJ, 577, 738);
  16      Voges et al. (1999, CDS Cat. <IX, 10>);
  17      Meyssonnier, Lequeux, & Azzopardi (1993, A&AS, 102, 251);
  18      de Vaucouleurs & Corwin (1985, ApJ, 295, 287);
  19      Osborne et al. (2001, A&A, 378, 800);
  20      Wirth, Smarr, & Bruno (1985, ApJ, 290, 140);
  21      Kaaret (2002, ApJ, 578, 114);
  22      Garcia et al. (2000, IAUC 7498, 1);
  23      Perlman et al. (1996, ApJS, 104, 251);
  SSS     These are supersoft sources in the catalog of Di Stefano et al. (2004,
          ApJ, 610, 247);
  QSS     These are quasisoft sources in the catalog of Di Stefano et al. (2004,
          ApJ, 610, 247);
  new     This work. New X-ray sources (labeled with an 'X') which were seen
          for the first time in this data set. New optical counterparts (labeled
          with a 'Star') were found by searching the Local Group Survey data on
          the M31 disk Massey et al (2001, BAAS, 199, 130.05), or they are new
          X-ray sources that have no optical counterparts.

The classification of the source, using the following scheme:

      Star  = The source has a stellar counterpart.
      X     = The source has no optical counterpart, but a previous X-ray
              detection is listed.
      GC    = The X-ray source lies in a known M31 globular cluster.
      SNR   = The X-ray source position is coincident with a known M31
              supernova remnant.
      PN    = The X-ray source position is coincident with a known, unconfirmed
              M31 planetary nebula.  These may be misidentified SNRs.
      BLL   = The X-ray source position is coincident with a BL Lac candidate.
      Radio = The source has also been detected in the radio.
      ?     = The source has a counterpart of unknown nature.

The HEASARC Browse object classification, based on the value of the source_type parameter.

Contact Person

Questions regarding the M31CXOHRC2 database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 10-Sep-2015