Chandra Observations of the X-Ray Point Source Population in Ngc 4636

J Posson-Brown, Somak Raychaudhury, W Forman, RH Donnelly, C Jones

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We present the X-ray point-source population in the nearby Virgo elliptical galaxy NGC 4636 from three Chandra X-ray observations. These observations, totaling similar to 193 ks after time filtering, were taken with the Advanced CCD Imaging Camera (ACIS) over a three-year period. Using a wavelet decomposition detection algorithm, we detect 318 individual point sources. For our analysis, we use a subset of 277 detections with >= net 10 counts (a limiting luminosity of approximately 1.2 x 10(37) erg s(-1) in the 0.5-2 keV band, outside the central 1'.5 bright galaxy core). We present a radial distribution of the point sources. Between 1'.5 and 6' from the center, 25% of our sources are likely to be background sources (active galactic nuclei (AGNs)) and 75% are low-mass X-ray binaries (LMXBs) within the galaxy, while at radial distances greater than 6', background sources (AGN) will dominate the point sources. We explore short and long-term variability (over timescales of 1 day to three years) for X-ray point sources in this elliptical galaxy. 54 sources (24%) in the common ACIS fields of view show significant variability between observations. Of these, 37 are detected with at least 10 net counts in only one observation and thus may be "transient." In addition, similar to 10% of the sources in each observation show significant short-term variability; we present an example light curve for a variable bright source. The cumulative luminosity function (LF) for the point sources in NGC 4636 can be represented as a power law of slope alpha = 1.14 +/- 0.03. We do not detect, but estimate an upper limit of similar to 4.5 x 10(37) erg s(-1) to the current X-ray luminosity of the historical supernova SN1939A. We find 77 matches between X-ray point sources and globular cluster (GC) candidates found in deep optical images of NGC 4636. In the annulus from 1'.5 to 6' of the galaxy center, 48 of the 129 X-ray point sources (37%) with >= 10 net counts are matched with GC candidates. Since we expect 25% of these sources to be background AGN, the percentage matched with GCs could be as high as 50%. Of these matched sources, we find that similar to 70% are associated with the redder GC candidates, those that are thought to have near-solar metal abundance. The fraction of GC candidates with an X-ray point source match decreases with decreasing GC luminosity. We do not find a correlation between the X-ray luminosities of the matched point sources and the luminosity or color of the host GC candidates. The LFs of the X-ray point sources matched with GCs and those that are unmatched have similar slopes over 1.8 x 10(37) erg s(-1)
Original languageEnglish
Pages (from-to)1094-1110
Number of pages17
JournalThe Astrophysical Journal
Issue number2
Early online date7 Apr 2009
Publication statusPublished - 20 Apr 2009


  • X-rays: galaxies
  • galaxies: individual (NGC 4636)
  • X-rays: binaries


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