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LALACETCXO - LALA Cetus Field Chandra X-Ray Point Source Catalog

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Overview

The 174 ks Chandra Advanced CCD Imaging Spectrometer (ACIS) exposure of the Large Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep Chandra images on LALA fields. In their paper, the authors present the Chandra X-ray sources detected in the Cetus field, along with an analysis of X-ray source counts, the stacked X-ray spectrum, and the optical identifications. A total of 188 X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0 keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived and compared with the 172 ks exposure LALA Bootes field (available as the LALABOOCXO table in Browse). Interestingly, the authors find consistent hard-band X-ray source density, but a (36 +/- 12)% higher soft-band X-ray source density in the Cetus field. The weighted stacked spectrum of the detected X-ray sources can be fitted by a power law with photon index Gamma = 1.55. Based on the weighted stacked spectrum, the authors find that the resolved fraction of the X-ray background drops from (72 +/- 1)% at 0.5-1.0 keV to (63 +/- 4)% at 6.0-8.0 keV. The unresolved spectrum can be fitted by a power law over the range 0.5-7 keV, with a photon index Gamma = 1.22. Optical counterparts are also presented for 154 of the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a deep r'-band image obtained with the MMT.

Catalog Bibcode

2007ApJ...669..765W

References

Chandra X-ray sources in the LALA Cetus field.
    Wang J.X., Zheng Z.Y., Malhotra S., Finkelstein S.L., Rhoads J.E.,
    Norman C.A., Heckman T.M.
   <Astrophys. J., 669, 765-775 (2007)>
   =2007ApJ...669..765W

Provenance

This table was created by the HEASARC in October 2010 based on the CDS table J/ApJ/669/765 file table1.dat

Description

A total of 176 ks of Chandra ACIS exposure on the LALA Cetus field was obtained in very faint (VFAINT) mode, composed of two individual observations. The first observation, with a 160.4 ks exposure, was taken on UT 2002 June 13-14 (Chandra ObsID 4129). The second observation, with a 15.4 ks exposure, was taken 1 day later (UT 2002 June 15, ObsID 4402). Only the data from the ACIS-I chips has been considered in this study. The telescope aim point was (RA, Dec) = 2 04 44.081, -5 05 17.36 (J2000.0), being centered on the ACIS-I3 chip for each exposure.

To detect X-ray sources, the authors used the wavdetect software with a probablility threshold of 1 x 10-7, corresponding to 0.5 false sources expected per image. 188 total X-ray sources were detected: 174 in the total band (0.5 - 7 keV), 154 in the soft band (0.5 - 2.0 keV), and 113 in the hard band (2 - 7 keV).

Deep broadband images of the LALA Cetus field were obtained in four Sloan Digital Sky Survey (SDSS) filters (g', r', i' and z') using the Megacam instrument (Mcleod et al.1998, Proc. SPIE, 3355, 477) at the MMT. The details of the observations and data reduction were presented in Finkelstein et al. (2007, ApJ, 660, 1023), who used the deep broadband images to study the ages and masses of Ly-alpha emitters at z ~ 4.5. The 5-sigma limiting magnitudes (2.32 arcseconds diameter aperture) obtained are 26.38, 25.64, 25.13, and 24.1 (Vega), respectively, in the four bands.

The authors used the MMT r'-band to identify their X-ray sources. They found the astrometry of the X-ray image and the MMT images were well matched, with an average shift less than 0.1 arcseconds. They used circles with radii equal to the root sum square of the 3-sigma positional uncertainties from wavdetect for X-ray sources (the error_radius parameter in this table) and 1 arcsecond, the uncertainty in the absolute X-ray astrometry. Optical counterparts in the r' band were found for 158 X-ray sources. The authors found two possible counterparts each for three of these X-ray sources. In this table 1 only the one which is closer to the center of the search circle is presented. Two of the X-ray sources are located outside the MMT image. There are also eight sources which are too close to nearby bright sources, making it impossible to identify them in the optical band image and to provide reliable upper limits to their r'-band magnitudes. For the remaining 20 X-ray sources without r'-band counterparts, the 5-sigma r'-band upper limits, i.e., 25.86, are tabulated.


Parameters

Source_Number
A running number for the X-ray source which uniquely identifies the source.

Alt_Name
An alternative name for the X-ray source recommended by the CDS Dictionary of Nomenclature of Celestial Objects using the prefix of '[WZM2007]' where WZM2007 stands for Wang, Zheng, Malhotra, 2004), together with the source number.

Name
The IAU Name of the X-ray source using the 'CXOLALA2' prefix registered with the CDS Dictionary of Nomenclature of Celestial Objects and the truncated J2000.0 coordinates of the source (JHHMMSS.s-DDMMSS.s). When the arcseconds of declination are less than 10, the leading zero is omitted, e.g., the source 'CXOLALA2 J020457.9-05057.4' is situated at J2000.0 equatorial coordinates 02 04 57.9, -05 05 07.4.

Source_Flag
This flag parameter is set to 'X' as a multiplicity index.

RA
The Right Ascension of the X-ray source centrois in the selected equinox. The RA was given in J2000.0 coordinates to a precision of 0.1 seconds of time in the original table. The positions were determined by wavdetect. Whenever possible, the authors used positions derived in the soft band, which has the best spatial resolution among the three bands. For sources which were not detected in the soft band, they used total band positions instead, if available, and hard-band positions were quoted for those sources detected only in the hard band.

Dec
The Declination of the X-ray source centrois in the selected equinox. The RA was given in J2000.0 coordinates to a precision of 0.1 arcseconds in the original table. The positions were determined by wavdetect. Whenever possible, the authors used positions derived in the soft band, which has the best spatial resolution among the three bands. For sources which were not detected in the soft band, they used total band positions instead, if available, and hard-band positions were quoted for those sources detected only in the hard band.

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

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

Error_Radius
The 3-sigma uncertainty of the centroid position of the X-ray source, as directly given by wavdetect, in arcseconds.

FB_Counts
The derived net counts in the total band (0.5 - 7 keV). The counts were calculated using circular aperture photometry. For each source, the authors defined a source region that was a circle centered at the source centroid position, with a radius Rs set to the 95% encircled-energy radius of the Chandra ACIS PSF. The value of Rs varied in the range of 2 - 15 arcseconds from the center to the edge of the field. Source photons were then extracted from the region, and the local background was extracted from an annulus with an outer radius of 2.4 times Rs and an inner radius of 1.2 times Rs, after masking out nearby sources. The aperture correction (x 1/0.95) was applied to the source counts.

FB_Counts_Pos_Err
The upper 1-sigma Poisson uncertainty in the total-band net counts, calculated according to the Bayesian method.

FB_Counts_Neg_Err
The lower 1-sigma Poisson uncertainty in the total-band net counts, calculated according to the Bayesian method.

SB_Counts
The derived net counts in the soft band (0.5 - 2. keV). The counts were calculated using circular aperture photometry. For each source, the authors defined a source region that was a circle centered at the source centroid position, with a radius Rs set to the 95% encircled-energy radius of the Chandra ACIS PSF. The value of Rs varied in the range of 2 - 15 arcseconds from the center to the edge of the field. Source photons were then extracted from the region, and the local background was extracted from an annulus with an outer radius of 2.4 times Rs and an inner radius of 1.2 times Rs, after masking out nearby sources. The aperture correction (x 1/0.95) was applied to the source counts.

SB_Counts_Pos_Err
The upper 1-sigma Poisson uncertainty in the soft-band net counts, calculated according to the Bayesian method.

SB_Counts_Neg_Err
The lower 1-sigma Poisson uncertainty in the soft-band net counts, calculated according to the Bayesian method.

HB_Counts
The derived net counts in the hard band (2 - 7 keV). The counts were calculated using circular aperture photometry. For each source, the authors defined a source region that was a circle centered at the source centroid position, with a radius Rs set to the 95% encircled-energy radius of the Chandra ACIS PSF. The value of Rs varied in the range of 2 - 15 arcseconds from the center to the edge of the field. Source photons were then extracted from the region, and the local background was extracted from an annulus with an outer radius of 2.4 times Rs and an inner radius of 1.2 times Rs, after masking out nearby sources. The aperture correction (x 1/0.95) was applied to the source counts.

HB_Counts_Pos_Err
The upper 1-sigma Poisson uncertainty in the hard-band net counts, calculated according to the Bayesian method.

HB_Counts_Neg_Err
The lower 1-sigma Poisson uncertainty in the hard-band net counts, calculated according to the Bayesian method.

Detection_Bands
An indication of the band(s) of the source detection. "T," "S," and "H" values mean sources detected in the total, soft, and hard bands, respectively. Multiple letters are used for sources detected in more than one band. For example, "TS" means detections in both the total band and the soft band, but a nondetection in the hard band.

Hardness_Ratio
The hardness ratio defined as HR = (H - S)/(H + S), where H and S are the vignetting-corrected net counts in the hard and soft bands, respectively. The hardness ratios were calculated with a Bayesian approach which models the detected counts as a Poisson distribution and can give reliable error bars for both low and high count sources. The hardness ratios versus 0.5-10.0 keV band X-ray fluxes for the detected X-ray sources are plotted in Figure 1 of the reference paper. Assuming a power-law spectrum with the Galactic H I column density (2.2 x 1020 cm-2), the observed hardness ratio can be converted to the photon index Gamma of the spectrum, which is also presented in the figure. An increasing proportion of hard sources is seen at the fainter flux levels (an effect seen inearlier surveys).Most of these are believed to be obscured AGNs.

Hardness_Ratio_Pos_Err
The upper 1-sigma Poisson uncertainty in the hardness ratio.

Hardness_Ratio_Neg_Err
The lower 1-sigma Poisson uncertainty in the hardness ratio.

FB_Flux
The X-ray flux (corrected for Galactic absorption, NH = 2.2 x 1020 cm-2) in the 0.5 - 10.0 keV band, in units of ergs cm-2 s-1. A power-law spectrum with the Galactic column density was assumed in order to calculate the conversion factor from net counts to X-ray flux. The photon index of the power-law was chosen at Gamma = 1.4. Three-band net count rates were calculated by dividing the net counts on the total, soft and hard bands by the effective exposure time at each source position in each band, and then converted into X-ray fluxes in the 0.5-10.0, 0.5-2.0, and 2.0-10.0 keV bands, respectively. The conversion factors used were 1.35 x 10-11 ergs cm-2 count-1 from the 0.5-7.0 keV band observed count rate to the Galactic absorption-corrected X-ray flux in the 0.5-10.0 keV band, 5.66 x 10-12 ergs cm-2 count-1 from the 0.5-2.0 keV band count rate to the 0.5-2.0 keV band flux, and 2.65 x 10-11 ergs cm-2 count-1 from the 2.0-7.0 keV band count rate to the 2.0-10.0 keV band flux. Note that the total band (0.5-10.0 keV) flux is not equal to the sum of the soft- and hard-band fluxes if the actual photon index differs from 1.4.

SB_Flux_Limit
If this parameter has a value of '<', the corresponding flux is an upper limit.

SB_Flux
The X-ray flux (corrected for Galactic absorption, NH = 2.2 x 1020 cm-2) in the 0.5 - 2.0 keV band, in units of ergs cm-2 s-1. A power-law spectrum with the Galactic column density was assumed in order to calculate the conversion factor from net counts to X-ray flux. The photon index of the power-law was chosen at Gamma = 1.4. Three-band net count rates were calculated by dividing the net counts on the total, soft and hard bands by the effective exposure time at each source position in each band, and then converted into X-ray fluxes in the 0.5-10.0, 0.5-2.0, and 2.0-10.0 keV bands, respectively. The conversion factors used were 1.35 x 10-11 ergs cm-2 count-1 from the 0.5-7.0 keV band observed count rate to the Galactic absorption-corrected X-ray flux in the 0.5-10.0 keV band, 5.66 x 10-12 ergs cm-2 count-1 from the 0.5-2.0 keV band count rate to the 0.5-2.0 keV band flux, and 2.65 x 10-11 ergs cm-2 count-1 from the 2.0-7.0 keV band count rate to the 2.0-10.0 keV band flux. Note that the total band (0.5-10.0 keV) flux is not equal to the sum of the soft- and hard-band fluxes if the actual photon index differs from 1.4.

HB_Flux_Limit
If this parameter has a value of '<', the corresponding flux is an upper limit.

HB_Flux
The X-ray flux (corrected for Galactic absorption, NH = 2.2 x 1020 cm-2) in the 2.0 - 10.0 keV band, in units of ergs cm-2 s-1. A power-law spectrum with the Galactic column density was assumed in order to calculate the conversion factor from net counts to X-ray flux. The photon index of the power-law was chosen at Gamma = 1.4. Three-band net count rates were calculated by dividing the net counts on the total, soft and hard bands by the effective exposure time at each source position in each band, and then converted into X-ray fluxes in the 0.5-10.0, 0.5-2.0, and 2.0-10.0 keV bands, respectively. The conversion factors used were 1.35 x 10-11 ergs cm-2 count-1 from the 0.5-7.0 keV band observed count rate to the Galactic absorption-corrected X-ray flux in the 0.5-10.0 keV band, 5.66 x 10-12 ergs cm-2 count-1 from the 0.5-2.0 keV band count rate to the 0.5-2.0 keV band flux, and 2.65 x 10-11 ergs cm-2 count-1 from the 2.0-7.0 keV band count rate to the 2.0-10.0 keV band flux. Note that the total band (0.5-10.0 keV) flux is not equal to the sum of the soft- and hard-band fluxes if the actual photon index differs from 1.4.

RA_Offset
The offset of the detected optical counterpart from the X-ray source position in the Right Ascension direction, in the sense of optical - X-ray position, in arcseconds.

Dec_Offset
The offset of the detected optical counterpart from the X-ray source positions in the Declination direction, in the sense of optical - X-ray position, in arcseconds.

Rmag_Limit
This parameter is set to '>' to indicate that no optical counterpart to the X-ray source was detected in the r' band and that the quoted value is the 5-sigma limit to the R-band magnitude.

Rmag
The r' band AUTO magnitude of the detected optical counterpart to the X-ray source from SExtractor (Kron-like elliptical aperture magnitudes, Bertin & Arnouts 1996, A&AS, 117, 393). For sources which are not detected in the r' band, 5-sigma upper limits to the magnitudes are given when available.

Rmag_Error
The 1-sigma uncertainty in the r-prime band magnitude of the detected optical counterpart to the X-ray source, obtained directly from SExtractor, and without including the uncertainty of the r'-band zero point.

Rmag_Flag
This parameter flags the r-prime band magnitude of the counterpart, as follows:

        b = Sources with the searching circles seriously overlapped by nearby
            bright sources and no optical counterparts can be visually
            identified. Reliable upper limits on their r'-band magnitudes
            are thus not available
        c = Sources with optical counterparts visually identified. SExtractor
            failed to detect these counterparts due to dilution of nearby
            bright sources. For these counterparts, their r'-band magnitudes
            are measured within a 1.5" radius circle.
        d = Two Sources which are not covered by the MMT r'-band image
        e = Sources with two counterparts identified within the searching
            circles. Here we only present the one which is closer to the
            center of the circles
  

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Page Author: Browse Software Development Team
Last Modified: 7-Oct-2010