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CHANSEXSI - Chandra Serendipitous Extragalactic X-Ray Source ID (SEXSI) Catalog

HEASARC
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Overview

The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) program is designed to extend greatly the sample of identified extragalactic hard X-ray (2 - 10 keV) sources at intermediate fluxes (~10^-13 to 10^-15 erg/cm^2/s). SEXSI, which studies sources selected from more than 2 deg^2, provides an essential complement to the Chandra Deep Fields, which reach depths of 5x10^-16 erg/cm^2/s (2 - 10 keV) but over a total area of less than 0.2 deg^2. In their published paper, the authors describe the characteristics of the survey and their X-ray data analysis methodology. They present the cumulative flux distribution for the X-ray sample of 1034 hard sources and discuss the distribution of spectral hardness ratios. Their log N -log S in this intermediate flux range connects to those found in the Deep Fields, and by combining the data sets, they constrain the hard X-ray population over the flux range in which the differential number counts change slope and from which the bulk of the 2 - 10 keV X-ray background arises. They further investigate the log N - log S distribution separately for soft and hard sources in the sample, finding that while a clear change in slope is seen for the softer sample, the hardest sources are well described by a single power law down to the faintest fluxes, consistent with the notion that they lie at lower average redshift.

In the SEXSI program, fields were selected with high Galactic latitude (|b| > 20 degrees) and with declinations accessible to the optical facilities available to the authors (declination > -20 degrees). They used observations taken with Chandra's Advanced Camera for Imaging Spectroscopy (ACIS I- and S-modes; Bautz et al., 1998, Proc. SPIE, 3444, 210) only (for sensitivity in the hard band). All the fields presented in this paper have data that are available in the Chandra public archive.


Catalog Bibcode

2003ApJ...596..944H

References

The Serendipitous Extragalactic X-ray Source Identification program.
I. Characteristics of the hard X-ray sample.
    Harrison F.A., Eckart M.E., Mao P.H., Helfand D.J., Stern D.
   <Astrophys. J., 596, 944-956 (2003)>
   =2003ApJ...596..944H

Provenance

This table was created by the HEASARC in June 2004 based on CDS Catalog J/ApJ/596/944/table4.dat which is a representation of Table 4 from the published version. Note that it does not include the Soft-Band-Only Source Catalog (Table 6 in the published version of the paper).

Parameters

Name
The name of the X-ray source in the standard truncated J2000 equatorial coordinate-based format and the IAU-registered CXOSEXSI prefix: i.e., "CXOSEXSI JHHMMSS.s+DDMMSS".

RA
The Right Ascension of the X-ray source in the selected equinox as derived from the hard-band X-ray image. Only sources detected with a chance coincidence probability of less than 10^-6 in the hard band are included. The RA was given in J2000 equatorial coordinates and to a precision of 0.01 seconds of time in the originating table.

Dec
The Declination of the X-ray source in the selected equinox as derived from the hard-band X-ray image. Only sources detected with a chance coincidence probability of less than 10^-6 in the hard band are included. The Declination was given in J2000 equatorial coordinates and to a precision of 0.1 arcseconds in the originating table.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Off_Axis_Angle
The off-axis angle, in arcminutes. This is the angular distance of the source position from the telescope axis.

HB_Counts
The background-subtracted counts within the specified aperture derived from the hard-band (2.1 - 7.0 keV) image.

HB_Bck_Counts
The estimated background counts in the same aperture used for the source counts in the hard-band (2.1 - 7.0 keV) image.

HB_Counts_SNR
The signal-to-noise (SNR) of the hard-band detection as calculated using the approximate formula for the Poisson distribution with small numbers of counts given in Gehrels (1986, ApJ, 303, 336). These values are not a measure of source significance (for which the chance coincidence probablity is less than 10^-6) but are a measure of the uncertainty in the source flux estimates.

HB_Flux
The unabsorbed hard-band flux, in erg/cm^2/s, corrected for source counts falling outside the aperture and translated to the standard 2 - 10 keV band, assuming a power-law photon spectral index of 1.5 and a Galactic absorbing column density appropriate for the field.

SB_Counts
The background-subtracted counts within the specified aperture derived from the soft-band (0.3 - 2.1 keV) image. Note that there are a large number of soft sources that lack a statistically significant hard counterpart which are not included in this table (see ftp://cdsarc.u-strasbg.fr/pub/cats/J/ApJ/596/944/table6.dat corresponding to Table 6 of the published paper).

SB_Bck_Counts
The estimated background counts in the same aperture used for the source counts in the soft-band (0.3 - 7.0 keV) image.

SB_Counts_SNR
The signal-to-noise (SNR) of the soft-band detection as calculated using the approximate formula for the Poisson distribution with small numbers of counts given in Gehrels (1986, ApJ, 303, 336). These values are not a measure of source significance but are a measure of the uncertainty in the source flux estimates.

SB_Flux
The unabsorbed soft-band flux, in erg/cm^2/s, corrected for source counts falling outside the aperture and translated to the standard 0.5 - 2 keV band, assuming a power-law photon spectral index of 1.5 and a Galactic absorbing column density appropriate for the field. Soft-band fluxes are derived by employing the same procedures on the wavdetect output from the 0.3 - 2.1 keV images and then matching sources in the two bands.

Hardness_Ratio
The hardness ratio HR) defined as (H-S)/(H+S), where H and S are the corrected counts in the 2 - 10 keV and 0.5 - 2 keV bands, respectievely. See Section 3.3 of the published paper for more details.

Cluster_Flag
A flag parameter which is set to 1 if the X-ray source falls within the `excluded area' within 1 Mpc of the center of a galaxy cluster, else is 0. Sources with a value of 1 for this parameter were not used in the authors' log N - log S calculation.


Contact Person

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

Page Author: Browse Software Development Team
Last Modified: 9-Nov-2004