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CBFGRXECXO - Chandra Bulge Field X-Ray Point Source Catalog

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Overview

Apparently diffuse X-ray emission has been known to exist along the central quarter of the Galactic Plane since the beginning of X-ray astronomy; this is referred to as the Galactic Ridge X-ray emission (GRXE). Recent deep X-ray observations have shown that numerous X-ray point sources account for a large fraction of the GRXE in the hard band (2 - 8 keV). However, the nature of these sources is poorly understood. Using the deepest X-ray observations made in the Chandra bulge field, the authors present the result of a coherent photometric and spectroscopic analysis of individual X-ray point sources for the purpose of constraining their nature and deriving their fractional contributions to the hard-band continuum and Fe K line emission of the GRXE. Based on the X-ray color-color diagram, they divided the point sources into three groups: A (hard), B (soft and broad spectrum), and C (soft and peaked spectrum). The group A sources are further decomposed spectrally into thermal and non-thermal sources with different fractions in different flux ranges. From their X-ray properties, the authors speculate that the group A non-thermal sources are mostly active galactic nuclei and the thermal sources are mostly white dwarf (WD) binaries such as magnetic and non-magnetic cataclysmic variables (CVs), pre-CVs, and symbiotic stars, whereas the group B and C sources are X-ray active stars in flares and quiescence, respectively. In the log N - log S curve of the 2 - 8 keV band, the group A non-thermal sources are dominant above ~10-14 erg/cm2/s, which is gradually taken over by Galactic sources in the fainter flux ranges. The Fe K-alpha emission is mostly from the group A thermal (WD binaries) and the group B (X-ray active stars) sources.

The authors retrieved 10 archived data sets of the Chandra bulge field (CBF) taken with the Advanced CCD Imaging Spectrometer-I (ACIS-I; 0.5 - 8.0 keV energy band with a spectral resolution of ~280 eV for the full width at half-maximum at 5.9keV) array on board Chandra. The observations were carried out from 2008 May to August with a total exposure time of ~900 ks.

The authors first extracted point-source candidates using the wavdetect algorithm in the CIAO package. They set the significance threshold at 2.5 x 10-5, implying that one false positive detection would be expected at every 4 x 104 trials. As a result, 2596 source candidates were found. The number of their source candidates is nearly the same as that found by Revnivtsev et al.(2009, A&A, 507, 1211) in the same region. To select significant point sources from the candidates, the authors examined their validity based on their photometric significance (PS) and the probability of no source (PB). The PS is defined as the background-subtracted source counts (Cnet) divided by its background counts normalized by the area. PB is the probability that the source is attributable to a background fluctuation, assuming Poisson statistics. The authors recognized a source to be valid if it satisfied both these criteria: PS >= 1.0 and PB <= 1.0 x 10-2. As a result, they obtained 2002 valid point sources.


Catalog Bibcode

2013ApJ...766...14M

References

X-ray point-source populations constituting the Galactic Ridge X-ray emission.
    Morihana K., Tsujimoto M., Yoshida T., Ebisawa K.
   <Astrophys. J., 766, 14 (2013)>
   =2013ApJ...766...14M    (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in December 2014 based on CDS catalog J/ApJ/766/14 file table2.dat.

Parameters

Source_Number
A unique source number for each X-ray source in the catalog in order of increasing J2000.0 Right Ascension..

Name
The name of the Chandra ACIS X-ray point source using the 'CXOU' prefix, for Chandra X-ray Observatory, Unregistered, and the J2000.0 equatorial coordinates truncated to 0.01 seconds of time in RA and 0.1 arcseconds in Dec, e.g., 'CXOU JHHMMSS.ss+DDMMSS.s'. Note that this is one place in precision higher than the convention recommended by the CDS Dictionary of Nomenclature of Celestial Objects for Chandra sources.

RA
The Right Ascension of the Chandra ACIS X-ray point source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10-5 degrees in the original table.

Dec
The Declination of the Chandra ACIS X-ray point source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10-6 degrees in the original table.

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

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

Error_Radius
The estimated standard deviation of the positional error, in arcseconds.

Off_Axis
The off-axis angle of the Chandra ACIS X-ray point source from the pointing direction, in arcminutes.

FB_Counts
The estimated net counts in the Chandra ACIS X-ray point source in the 0.5-8 keV band.

FB_Counts_Error
The uncertainty in the estimated net counts in the Chandra ACIS X-ray point source in the 0.5-8.0 keV band.

Bck_Counts
The number of 0.5-8.0 keV counts in the background region for the source.

HB_Counts
The estimated net counts in the Chandra ACIS X-ray point source in the 2.0-8.0 keV (hard) band.

PSF_Fraction
The fraction of the Chandra point-spread function (PSF) at 1.497 keV which is enclosed in the extraction region. A reduced PSF fraction (significantly below 90%) may indicate that the source is in a crowded region.

Significance_Limit
This limit flag parameter is set to '<' if the quoted photometric significance of the Chandra ACIS X-ray point source is an upper limit.

Significance
The photometric significance, PS, of the Chandra ACIS X-ray point source. The PS is defined as the background-subtracted source counts (Cnet) divided by its background counts normalized by the area.

Log_Bck_Prob
The logarithm of the probability PB that the net counts in the Chandra ACIS X-ray point source are solely from the background. PB is the probability that the source is attributable to a background fluctuation, assuming Poisson statistics. The authors recognized a source to be valid if it satisfied both these criteria: PS >= 1.0 and PB <= 1.0 x 10-2.

Source_Anomaly_Flag
This anomaly flag is set to 'g' to indicates the presence of a source anomaly, namely that the fraction of the exposure that the source was on a detector (FRACEXPO from mkarf) was < 0.9

Var_Flag
This flag parameter has the following possible values indicating source variability characterization based on the K-S statistic in the 0.5-8.0 keV band:

      a = no evidence for variability (0.05 < PKS);
      b = possibly variable (0.005 <PKS < 0.05);
      c = definitely variable (PKS < 0.005).
  
No value is reported for sources in chip gaps or on field edges.

Exposure
The effective exposure time for the Chandra ACIS X-ray point source, in seconds, i.e., the approximate time that the source would have to be observed on-axis in order to obtain the reported number of counts.

Median_Energy
The background-corrected median photon energy over the 0.5-8.0 keV band of the photons detected in the source, in keV.

FB_Flux
The photometric 0.5-8.0 keV photometric flux estimate for the Chandra ACIS X-ray point source, in erg s-1 cm-2.

Q1_Quantile_Value
The q1 quantile value indicating the spectral shape of the source, where q1 = log10(Q50)/(1-Q50), Qx = (Ex - Emin)/(Emax - Emin) and Emin and Emax are 0.5 and 8.0 keV, respectively, and Ex (keV) is the energy below which x% of the photons reside in the energy-sorted event list (see the text in Section 4.1 of the reference paper for full details).

Q2_Quantile_Value
The q2 quantile value indicating the spectral shape of the source, where q2 = 3 x (Q25/Q75), Qx = (Ex - Emin)/(Emax - Emin) and Emin and Emax are 0.5 and 8.0 keV, respectively, and Ex (keV) is the energy below which x% of the photons reside in the energy-sorted event list (see the text in Section 4.1 of the reference paper for full details).


Contact Person

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Page Author: Browse Software Development Team
Last Modified: 15-Dec-2014