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NGC4636CXO - NGC 4636 Chandra X-Ray Point Source Catalog

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Overview

This catalog lists the X-ray point-source population in the nearby Virgo elliptical galaxy NGC 4636 from three Chandra X-ray observations. These observations, totaling ~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, the authors detected 318 individual point sources. For their analysis, they used a subset of 277 detections with >= net 10 counts (a limiting luminosity of approximately 1.2 x 1037 erg s-1 in the 0.5-2 keV band, outside the central 1.5 arcminutes bright galaxy core). This table contains this subset of 277 X-ray sources. The authors discuss the radial distribution of the point sources. Between 1.5 and 6 arcminutes from the center, 25% of the sources are likely to be background sources (active galactic nuclei (AGNs)) and 75% to be low-mass X-ray binaries (LMXBs) within the galaxy, while at radial distances greater than 6 arcminutes, background sources (AGN) will dominate the point sources.

The authors 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, ~10% of the sources in each observation show significant short-term variability. 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. The authors do not detect, but estimate an upper limit of ~4.5 x 1037 erg s-1 to the current X-ray luminosity of, the historical supernova SN1939A. They 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 arcminutes of the galaxy center, 48 of the 129 X-ray point sources (37%) with >=10 net counts are matched with GC candidates. Since they expect 25% of these sources to be background AGN, the percentage matched with GCs could be as high as 50%. Of these matched sources, the authors find that ~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. The authors 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 1037 erg s-1 <= L_x <= 1 x 1038 erg s-1.


Catalog Bibcode

2009ApJ...695.1094P

References

Chandra Observations of the X-Ray Point Source Population in NGC 4636
    Posson-Brown J., Ray chaudhury, S., Forman, W., Donnelly, R.H., Jones, C.
   <Astrophys. J. 695, 1094 (2009)>
   =2009ApJ...695.1094P

Provenance

This table was created by the HEASARC in July 2009 based on electronic versions of Tables 2 and 3 from the paper obtained from the ApJ web site, but excluding the 7 entries in Table 3 which corresponded to weaker X-ray sources which were not listed in Table 2.

Parameters

Name
The name of the Chandra X-ray source using the 'CXOU J' prefix (Chandra X-ray Observatory, Unregistered) and the J2000 coordinates of the source in the standard JHHMMSS.s+DDMMSS format, e.g., 'CXOU J1242498.8+024116'. This was created by the HEASARC in the absence of any explicit naming convention being given in the reference paper.

RA
The Right Ascension of the Chandra X-ray source in the selected equinox. This was given in J2000 coordinates and to a precision of 0.0001 degrees in the original table.

Dec
The Declination of the Chandra X-ray source in the selected equinox. This was given in J2000 coordinates and to a precision of 0.0001 degrees in the original table.

LII
The Galactic Longitude of the Chandra X-ray source.

BII
The Galactic Latitude of the Chandra X-ray source.

Counts
The net 0.5-2 keV band counts in the X-ray source. The source detection algorithm that was used is discussed in Section 2 of the reference paper

Lx
The 0.5-2 keV band luminosity of the source, in erg/s, assuming a distance of 15 Mpc to NGC 4636 and using a counts-to-flux conversion factor for each source calculated by computing response matrices at each source location on the appropriate ACIS chip and assuming a power-law model with a spectral power-law index of 1.5 and an equivalent (Galactic) hydrogen column density of 1.81 x 1020 cm-2 (Dickey and Lockman 1990, ARAA, 28, 215).

HR_1
The soft color (hardness ratio) of the X-ray source, C_1, defined as C_1 = (M-S)/(H+M+S), where H is the flux in the hard (2-8 keV) band, M is the flux in the medium (1-2 keV) band and S is the flux in the soft (0.5-1.0 keV) band.

HR_2
The hard color (hardness ratio) of the X-ray source, C_2, defined as C_2 = (H-M)/(H+M+S), where H is the flux in the hard (2-8 keV) band, M is the flux in the medium (1-2 keV) band and S is the flux in the soft (0.5-1.0 keV) band.

Center_Offset
The angular offset of the X-ray source from the center of NGC 4636, in arcseconds. (Notice that the HEASARC converted this parameter from the arcminutes units in which it was given in the original table).

Notes
Additional note(s) about the X-ray source, as follows:

  1:  The source was detected by the ACIS-S
  2:  The source was detected by the ACIS-I
  3:  The source was detected by both the ACIS-I and the ACIS-S
  V:  The source is a long-term, i.e., between observations, variable
  SV: The source is a short-term, i.e., during a single observation, either
      in the ACIS-S (SV-S) or co-added ACIS-I (SV-I), variable
  MV: The source flux varies significantly between the sequential ACIS-I
      observations
  GC: The source is matched with a globular cluster counterpart: the number
      following is the globular cluster identification, gc_number
  

GC_Number
A unique running identification number for the globular cluster candidate counterpart to the Chandra X-ray source.

CXO_GC_Offset
The angular offset between the Chandra X-ray source and the globular cluster candidate positions, in arcseconds.

CTIO_Id_Number
The Dirsch et al. (2005, A&A, 433, 43) CTIO identification number for the globular cluster candidate.

RA_Optical
The optical Right Ascension of the globular cluster candidate in the selected equinox. This was given in J2000 coordinates and to a precision of 0.0001 degrees in the original table.

Dec_Optical
The optical Declination of the globular cluster candidate in the selected equinox. This was given in J2000 coordinates and to a precision of 0.0001 degrees in the original table.

CR_Color
The (C-R) color of the globular cluster candidate, where C is the Washington C magnitude (Canterna 1976, AJ, 81, 228) and R is the Kron-Cousins R magnitude, from Dirsch et al. (2005, A&A, 433, 43).

Rmag
The Kron-Cousins R-band magnitude of the globular cluster candidate, from Dirsch et al. (2005, A&A, 433, 43).


Contact Person

Questions regarding the NGC4636CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 7-Jul-2009