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NGC4649CXO - NGC 4649 Chandra X-Ray Discrete Source Catalog

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Overview

The authors performed a Chandra X-ray observation of the X-ray bright E2 elliptical galaxy NGC 4649 (M 60). In addition to bright diffuse emission, they resolved 165 discrete sources, most of which are presumably low-mass X-ray binaries (LMXBs). As found in previous studies, the luminosity function of the resolved sources is well-fitted by a broken power law. In NGC 4697 and NGC 1553, the break luminosity was comparable to the Eddington luminosity of a 1.4 solar mass neutron star. One possible interpretation of this result is that those sources with luminosities above the break are accreting black holes and those below are mainly accreting neutron stars. The total X-ray spectrum of the resolved sources is well fitted by a hard power law.

NGC 4649 was observed on 2000 April 20 on the ACIS-S3 CCD operated at a temperature of -120 C and with a frame time of 3.2 s. In addition to the S3 chip, the ACIS chips I2, I3, S1, S2, and S4 were also turned on for the duration of the observation. Although a number of serendipitous sources were seen on the other chips, the analysis of NGC 4649 in this paper was based on data from the S3 chip alone. The total exposure for the S3 chip was 36,780 s. The discrete X-ray source population on the ACIS S3 image was determined using a wavelet detection algorithm in the 0.3 - 10.0 keV band, and they were confirmed with a local cell detection method. The authors used the CIAO, WAVDETECT, and CELLDETECT programs. The high spatial resolution of Chandra implies that the sensitivity to point sources is not affected very strongly by the background. Thus, the source detection was done using the entire exposure of 36,780 s, including periods with background flares. The wavelet source detection significance threshold was set at 10^-6, which implies that less than 1 false source (due to a statistical fluctuation in the background) would be detected in the entire S3 image. This significance threshold approximately corresponds to requiring that the source flux be determined to better than 3 sigma.


Catalog Bibcode

2004ApJ...600..729R

References

Chandra observation of diffuse gas and low-mass X-ray binaries in the
elliptical galaxy NGC 4649 (M60).
    Randall S.W., Sarazin C.L., Irwin J.A.
   <Astrophys. J., 600, 729-742 (2004)>
   =2004ApJ...600..729R

Provenance

This table was created by the HEASARC in April 2007 based on CDS catalog J/ApJ/600/729 file table1.dat.

Parameters

Source_Number
A running sequential identification number in order of increasing radial distance from the center of NGC 4649, which uniquely identifies each discrete source.

Source_Flag
This parameter contains flags indicating the existence of notes concerning particular sources, coded as follows:

        a = Source 1 is extended, and appears to be a combination of diffuse
            structure with one or more point sources.
        b = Positions and count rates of sources near the center of NGC 4649
            are uncertain due to the bright diffuse emission and/or
            confusion with nearby sources.
        c = Globular cluster is possible optical counterpart.
        d = Possible faint optical counterpart.
        e = Source may be variable.
        f = May be associated with the companion galaxy NGC 4647.
        g = Source is at the edge of the S3 detector, and flux is uncertain
            due to large exposure correction.
        h = Possible USNO-A2.0 (CDS Cat. <I/252>) optical counterpart.
  

Name
The recommended designation for the X-ray source, using the prefix 'CXOU J' (for Chandra X-ray Observatory, Unregistered') and the truncated J2000 coordinates of the source, e.g., 'CXOU JHHMMSS.s+DDMMSS'.

Alt_Name
An alternative name for the X-ray source using the '[RSI2004]' prefix (for Randall, Sarazin, and Irwin, 2004) and the source number, as recommended by the CDS Dictionary of Nomenclature of Celestial Objects.

RA
The Right Ascension of the X-ray source in the selected equinox. This was given in J2000.0 coordinates to a precision of 0.01 seconds of time in the original table. The statistical errors in the positions of most of the sources are quite small (~ 0.2"), and the overall absolute errors are probably ~ 1" near the center of the field, with larger errors near the outside of the field.

Dec
The Declination of the X-ray source in the selected equinox. This was given in J2000.0 coordinates to a precision of 0.1 arcseconds in the original table. The statistical errors in the positions of most of the sources are quite small (~ 0.2"), and the overall absolute errors are probably ~ 1" near the center of the field, with larger errors near the outside of the field.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Radial_Distance
The radial distance d of the X-ray source from the center of the galaxy NGC 4649, in arcseconds. Since the authors did not detect a distinct source at the center of the galaxy, they adopted the central position from 4.86 GHz radio observations of R.A. = 12h43m40.02s, and Dec. = +11d33m10.2s (J2000.0; Condon, Frayer, & Broderick 1991, AJ, 101, 362). The accuracy of this position is <~ 1". Because of the uncertainty in the exact position of the center of the galaxy, the values of d might be off by ~ 2".

Count_Rate
The count rate of the X-ray source, in ct/s, in the 0.3-10 keV energy band.

Count_Rate_Error
The 1-sigma uncertainty in the X-ray count rate, in ct/s.

SNR
The signal-to-noise ratio of the X-ray source.

Lx
The X-ray luminosity of the source in the 0.3 - 10 keV band, in erg/s. The count rates for the sources were converted into unabsorbed luminosities assuming that all of the sources were at the distance of NGC 4649, which the authors took to be 16.8 Mpc. They adopted the best-fit Chandra X-ray spectrum of the resolved sources within the major axis effective radius R_eff = 82". The factor for converting the count rate (0.3 - 10 keV) into the unabsorbed luminosity L_X (0.3 - 10 keV) was 2.25 x 10^41 ergs counts^-1. The resulting X-ray luminosities range roughly from 7 x 10^37 to 8 x 10^39 ergs s^-1.

HR_1
The hardness ratio H21 between the medium (1 - 2 keV) and soft (0.3 - 1 keV) bands. The authors defined this as H21 = (M - S)/(M + S), where S and M are the net counts in the soft and medium bands, respectively.

HR_1_Pos_Err
The upper error for the medium-soft hardness ratio H21. The errors in the hardness ratios are determined from the Poisson errors in the original counts in the bands and are carefully propagated so as to avoid mathematically impossible hardness ratios; that is, the error ranges are limited to -1 to 1.

HR_1_Neg_Err
The lower error for the medium-soft hardness ratio H21. The errors in the hardness ratios are determined from the Poisson errors in the original counts in the bands and are carefully propagated so as to avoid mathematically impossible hardness ratios; that is, the error ranges are limited to -1 to 1.

HR_2
The hardness ratio H31 between the hard (2 - 10keV) and soft (0.3 - 1 keV) bands. The authors defined this as H31 = (H - S)/(H + S), where S and H are the net counts in the soft and hard bands, respectively.

HR_2_Pos_Err
The upper error for the hard-soft hardness ratio H31. The errors in the hardness ratios are determined from the Poisson errors in the original counts in the bands and are carefully propagated so as to avoid mathematically impossible hardness ratios; that is, the error ranges are limited to -1 to 1.

HR_2_Neg_Err
The lower error for the hard-soft hardness ratio H31. The errors in the hardness ratios are determined from the Poisson errors in the original counts in the bands and are carefully propagated so as to avoid mathematically impossible hardness ratios; that is, the error ranges are limited to -1 to 1.


Contact Person

Questions regarding the NGC4649CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 25-Apr-2007