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ETGALCXO - Early-Type Galaxies Chandra X-Ray Point Source Catalog


This table contains the results of a Chandra survey of low-mass X-ray binaries (LMXBs) in 24 early-type galaxies. Correcting for detection incompleteness, the X-ray luminosity function (XLF) of each galaxy is consistent with a power law with negative logarithmic differential slope, Beta, ~ 2.0. However, Beta strongly correlates with incompleteness, indicating the XLF flattens at low X-ray luminosity (L_X). The composite XLF is well fitted by a power law with a break at (2.21 [+0.65,-0.56]) x 1038 ergs s-1 and Beta = 1.40 [+0.10,-0.13] and = 2.84 [+0.39,-0.30] below and above it, respectively. The break is close to the Eddington limit for a 1.4 solar-mass neutron star, but the XLF shape rules out its representing the division between neutron star and black hole systems. Although the XLFs are similar, the authors find evidence of some variation between galaxies. The high-L_X XLF slope does not correlate with age, but may correlate with [Alpha/Fe]. Considering only LMXBs with L_X > 1037 ergs s-1, matching the LMXBs with globular clusters (GCs) identified in HST observations of 19 of the galaxies, the authors find the probability a GC hosts an LMXB is proportional to L_GCAlpha Z_FeGamma where Alpha = 1.01 +/- 0.19 and Gamma = 0.33+/-0.11. Correcting for GC luminosity and color effects, and detection incompleteness, they find no evidence that the fraction of LMXBs with L_X > 1037 ergs s-1 in GCs (40%), or the fraction of GCs hosting LMXBs (~ 6.5%) varies between galaxies. The spatial distribution of LMXBs resembles that of GCs, and the specific frequency of LMXBs is proportional to the GC specific luminosity, consistent with the hypothesis that all LMXBs form in GCs. If the LMXB lifetime is Tau_L and the duty cycle is F_d, their results imply ~ 1.5(Tau_L/108 yr)-1 F_d-1 LMXBs are formed per gigayear per GC, and they place an upper limit of one active LMXB in the field per 3.4 x 109 solar luminoaities of V-band luminosity.

This table contains 1194 X-ray point sources that were detected within the B-band 25th magnitude ellipse D_25 (as listed in the de Vaucouleurs et al. Catalog of Bright Galaxies) of 24 early-type galaxies observed by Chandra (listed in Table 1 of the reference paper). The D_25 restriction should mitigate against contamination by background AGNs.

Catalog Bibcode



Low-Mass X-Ray Binaries and Globular Clusters in Early-Type Galaxies.
I. Chandra Observations
    Humphrey P. J., Buote D. A.
   <Astrophys. J. 689, 983-1004 (2008)


This table was created by the HEASARC in February 2009 based on the electronic versions of Table 6 from the paper which was obtained from the Astrophysical Journal web site.


The Chandra X-ray Observatory (CXO) X-ray designation, using the standard 'CXOU J' prefix and the J2000.0 equatorial source position.

The name of the host galaxy in which the Chandra X-ray source is located.

The Right Ascension of the Chandra X-ray source in the selected equinox. This was constructed from the positional part of the X-ray source name, and so has a precision of 0.1 seconds of time.

The Declination of the Chandra X-ray source in the selected equinox. This was constructed from the positional part of the X-ray source name, and so has a precision of 1 arcsecond.

The Galactic Longitude of the Chandra X-ray source.

The Galactic Latitude of the Chandra X-ray source.

The background-subtracted total-band X-ray counts in the Chandra source.

The X-ray (0.3 - 7 keV) luminosity of the Chandra point source, in erg/s, calculated from the observed X-ray flux and assuming that it is at the distance of its host galaxy (see Table 1 of the reference paper). The authors adopted a simple power-law model, with Gamma = 1.55 , which has been shown to fit adequately the composite spectra of detected LMXBs (see also Section 3.3 of the reference paper and Irwin et al. 2003, ApJ, 587, 356) and modified by photoelectric absorption due to the Galactic ISM along the line of sight (Dickey & Lockman 1990, ARAA, 28, 215). Such a model is broadly consistent with the source hardness ratios (see Section 3.1 of the reference paper), and models of a similar shape have also been fitted to the spectra of LMXBs within the Milky Way. This model was folded through the response matrices generated at each source position, and used to infer a counts-to-flux conversion ratio in the 0.3 - 7.0 keV band. The 0.3 - 7.0 keV background-subtracted flux of each source was extracted from the same region that was defined from which to extract the spectrum.

The uncertainty in the Chandra X-xay source (0.3 - 7 keV) luminosity, in erg/s.

The X-ray source spectral hardness ratio HR1 = (S - M)/(S + M), where S is the background-subtracted count rate in the band from 0.3 - 1.5 keV, and M is the count rate in the band from 1.5 - 3.0 keV.

The uncertainty in the X-ray source spectral hardness ratio HR1.

The X-ray source spectral hardness ratio HR2 = (M - H)/(M + H), where M is the background-subtracted count rate in the band from 1.5 - 3.0 keV, and H is the count rate in the band from 3.0 - 5.0 keV.

The uncertainty in the X-ray source spectral hardness ratio HR2.

The projected distance of the X-ray source from the galaxy center, in arcminutes.

For those LMXBs within the appropriate WFPC2 field, the authors list here the GC catalog number given in Humphrey (2009, ApJ, 690, 512: Paper II) for any which match GCs. If the source has no match, the authors report "none" for this field, and if the source is not in the WFPC2 field, it is left blank.

Contact Person

Questions regarding the ETGALCXO database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk