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CHANSEXOID - Chandra Serendipitous Extragalactic X-Ray Source ID (SEXSI) Optical Follow-Up

HEASARC
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Overview

The Serendipitous Extragalactic X-ray Source Identification (SEXSI) Program is designed to expand significantly the sample of identified extragalactic hard X-ray sources at intermediate fluxes, 10^-15 ergs/cm^2/s < 2-10 keV Flux <~ 10^-13 ergs/cm^2/s. SEXSI, which includes sources derived from more than 2 square degrees of Chandra images, provides the largest hard X-ray-selected sample yet studied, offering an essential complement to the Chandra Deep Fields (total area of 0.2 square degrees). In Eckart et al. (2005, Paper II) R-band optical imaging of the SEXSI fields from the Palomar P60 and P200, the MDM 2.4m and 1.3m, and the Keck I telescopes is described. The authors have identified counterparts or derived flux limits for nearly 1000 hard X-ray sources. Using the optical images, they have derived accurate source positions. They have investigated correlations between optical and X-ray flux, and optical flux and X-ray hardness ratio. They have also studied the density of optical sources surrounding X-ray counterparts, as well as the properties of optically faint, hard X-ray sources. In Eckart et al. (2006, Paper III) optical spectra of 477 counterparts are presented. These spectra reach to R-band magnitudes of <~24 and have produced idnetifications and redshifts for 438 hard X-ray sources. Typical completeness levels in the 27 Chandra fields studied are 40-70%. The vast majority of the 2-10 keV selected sample are AGNs with redshifts between 0.1 and 3; the highest redshift source lies at z = 4.33.

This table which combines data presented in Eckart et al. (2005, 2006) has links to the list of SEXSI X-ray sources (the HEASARC Browse table CHANSEXSI: see Paper I = Harrison et al. 2003, ApJ, 596, 944).


Catalog Bibcodes

2005ApJS..156...35E
2006ApJS..165...19E

References

The serendipitous extragalactic X-ray source identification (SEXSI) program.
II. Optical imaging.
    Eckart M.E., Laird E.S., Stern D., Mao P.H., Helfand D.J., Harrison F.A.
   <Astrophys. J. Suppl. Ser., 156, 35-45 (2005)>
   =2005ApJS..156...35E    (SIMBAD/NED BibCode)

III.Optical spectroscopy.
    Eckart M.E., Stern D., Helfand D.J., Harrison F.A., Mao P.H., Yost S.A.
   <Astrophys. J. Suppl. Ser., 165, 19-56 (2006)>
   =2006ApJS..165...19E    (SIMBAD/NED BibCode)

Provenance

This table was originally created by the HEASARC in June 2005 based on the CDS version of Table 3 from Eckart et al. (2005: CDS table J/ApJS/156/35/table3.dat). It was updated in August 2006 to include information from Table 2 of Eckart et al. (2006: the electronic version available at the electronic ApJ web site).

Parameters

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

RA
The Right Ascension of the X-ray source in the selected equinox. The RA was given in J2000 equatorial coordinates and to a precision of 0.01 seconds of time in the originating table. The X-ray source positions have been corrected for the mean optical to X-ray offsets. Note that since the source names, which are identical to those in Paper I and the HEASARC's CHANSEXSI table, are derived from the hard-band X-ray images, these refined positions will not exactly match the positions encoded in the names, although the mean offsets are typically less than 1 arcsecond.

Dec
The Declination of the X-ray source in the selected equinox. The Dec was given in J2000 equatorial coordinates and to a precision of 0.1 arcseconds in the originating table. The X-ray source positions have been corrected for the mean optical to X-ray offsets. Note that since the source names, which are identical to those in Paper I and the HEASARC's CHANSEXSI table, are derived from the hard-band X-ray images, these refined positions will not exactly match the positions encoded in the names, although the mean offsets are typically less than 1 arcsecond.

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 of the X-ray source, in arcminutes, i.e., the angular distance of the source position from the telescope aim point.

HB_Flux
The unabsorbed hard-band (2.0 - 10 keV) flux of the X-ray source, in units of erg/cm^2/s, converted from counts assuming an intrinsic power-law spectrum with a photon index Gamma of 1.5, and corrected for Galactic absorption.

HB_SNR
The signal-to-noise ratio of the hard-band detection.

Hardness_Ratio
The hardness ratio of the X-ray source, HR=(H-S)/(H+S), where H is the corrected counts in the 2.0-10 keV band and S is the corrected counts in the 0.5-2.0 keV band extracted at the position of the hard-band detection. Note that this last feature is different from the procedure adopted in Paper I, where if there was not a significant soft-band source detected, the hardness ratio was set to 1.0.

Hardness_Ratio_Flag
For a subset of cases where the soft-band counts recorded at the hard-band source position were less than twice the soft-band background counts, the hardness ratio is considered a lower limit and (i) this flag parameter has been set to 1 and (ii) the hardness ratio has been set to HR = (H-S_limit)/(H+S_limit), where S_limit is 2 x soft-band backround counts. For cases where the hardness ratio is solid, i.e., not a limit, this flag parameter is set to 0.

Cluster_Flag
This parameter is a flag which indicates whether or not the source is a potential member of a target cluster, and is defined as follows:

  1 means that the source falls within 1 comoving Mpc of the known target
    cluster center and thus is a potential cluster member
  0 means that the source does not fall within 1 comoving Mpc of the target
    cluster center
  
Notice that the redshifts measured in Paper III demonstrate that many sources with cluster_flag=1 are actually not target cluster members: the 10 sources which do have concordant redshifts are indicated by having a value of 'h' for their spectral_notes parameter.

Optical_Id_Flag
This parameter contains the optical identification flag as given in Table 2 of Paper III, which is defined as follows:

  6 = no optical counterpart, but the location is contaminated by nearby bright
      source (limiting mag will be incorrect)
  5 = more than one optical source in X-ray error circle, the source with
      the smallest positional offset is recorded in the table
  4 = solid ID, but the R-band magnitude is affected by a nearby bright source
  3 = source saturated in the SEXIS optical image, the R-band magnitude is
      taken from the Guide Star Catalog II or a secondary source in Vizier
  2 = no optical counterpart, the listed magnitude is a lower limit
  1 = solid optical identification
  0 = no optical coverage
  

Rc_Mag
The Cousins R-band magnitude of the optical counterpart. Note that special attention must be paid to the optical_id_flag parameter value when interpreting the rc_mag parameter value: for example, if the former has a value of 2 (optical counterpart not detected), then the latter is a limit to the actual rc_mag value.

Rc_Mag_Error
The error in the R-band magnitude.

Image_Limit_Rc_Mag
The limiting R-band magnitude for the image from which the optical counterpart, or a lower limit thereto, was derived.

Rc_Mag_Flag
This flag parameter is set to 1 if Rc_mag > (fainter than) the limiting R-band magnitude for the image, else it is set to 0.

Offset_RA
The difference between the RA of the X-ray source and that of the optical counterpart, RA_X - RA_opt, in arcseconds, where RA_X is the astrometrically-corrected RA of the X-ray source.

Offset_Dec
The difference between the Declination of the X-ray source and that of the optical counterpart, Dec_X - Dec_opt, in arcseconds, where Dec_X is the astrometrically-corrected Declination of the X-ray source.

Offset_Total
The total positional difference between the optical counterpart and the astrometrically-corrected X-ray source position, in arcseconds.

Log_Fx_Fopt
The logarithmic X-ray to optical flux ratio as given by the relation log (f_x/f_opt) = log f_x + (Rc_mag/2.5) + 5.50, derived from the Kron-Cousins R-band filter transmission function. Note that special attention must be paid to the optical_id_flag parameter value when interpreting the log_fx_fopt parameter value: for example, if the former has a value of 2 (optical counterpart not detected), then the latter is a limit to the actual log_fx_fopt value.

Stellarity
The SExtractor CLASS_STAR parameter value for basic star-galaxy separation, i.e., the stellarity fraction, with possible values ranging from 0.0 for significantly extended sources to 1.0 for sources consistent with perfectly stellar PSFs. The stellarity value of the 8 bright sources with values of optical_id_flag = 3 is set to 1.00. The stellarity data are presented with the caveat that the values should only be used for broad separation, for example of sources near 0 versus 1, but not for detailed quantitative analysis.

Redshift
The redshift of the optical counterpart to the X-ray source, determined as described in Section 4 of Eckart et al. (2006, Paper III).

Object_Type
The optical counterpart classification (see Section 4 of Paper III for details on the class definitions):

     ALG = absorption-line galaxy
     ELG = emission-line galaxy
   BLAGN = broad-lined AGN
   NLAGN = narrow-line AGN
    star = any type of star
  

Spectral_Notes
This parameter contains flags for notes on the spectra of individual sources which have the following values:

  a = Optical counterpart falls just outside the strict search area
      (see Section 5 and Table 3 of Paper III).
  b = Identification from the Guide Star Catalog II, McLean et al. (2000)
      or VIZIER database.
  c = Identification from the Einstein Observatory Extended
      Medium-Sensitivity Survey, Stocke et al.(1991), object MS1214.3+3811.
  d = Cataclysmic variable, spectrum shows 274 km/s blueshift.
  e = Member of (non-target) nearby galaxy, NGC 5879, which happened to be
      in the Chandra pointing of SEXSI field QSO 1508. The 3 2-10 keV sources
      spectroscopically confirmed to be associated with NGC 5879 are flagged.
  f = This object was identified using its 2-D spectrum as it could not be
      easily extracted to form a 1-D spectrum; the observable wavelength
      range is thus omitted.
  g = In Paper II this source had optical_id_flag=5 (which means >1 optical
      source in the search area).  The spectrum shows a BLAGN, thus the authors
      changed the source to optical_id_flag=1, meaning that there is a secure
      optical counterpart identification.
  h = Confirmed target cluster member (spectroscopically confiremd within 1 Mpc
      of target cluster center). In Papers I and II sources were flagged as
      being potentially within 1 Mpc of the target cluster center as determined
      by their position in the image (and ignored for the log N - log S
      calculation). Now that redshift information is available, only sources at
      the target cluster redshift remain flagged. See Section 11 of Paper III
      for details on these sources.
  i = Confirmed (non-target) group/cluster detected in Holden et al. 2002
      (see Section 11 of Paper III).
  j = Possible BALQSO.
  k = Line identification or redshift tentative.
  l = This source spectrum shows two ELGs, one at z=0.426, one at z=1.432.
      The nearer source matches the R=22.25 in the photometry, but the
      fainter z=1.432 source, undetected in the photometry, also appears in
      the error circle. This source is thus eliminated from the analysis.
  m = J145215.6+430448: a low-ionization, broad absorption line quasar.
  n = Spectrum has gap of > 100 Angstrom between red and blue side.
  

Log_Lx
The log of the absorbed rest-frame hard-band (2.0 - 10 keV) luminosity, in erg/s. calculated from the hard-band flux and the redshift, assuming that the source has an X-ray photon index of 1.5. The authors use the standard cosmology Omega_m=0.3, Omega_lambda=0.7, and a Hubble Constant of 65 km/s/Mpc to calculate the luminosity distance.

Log_NH_Flag
This parameter contains a flag for the corresponding log_nh value, as follows:

  S = Source detected on off-axis ACIS-S chip (2-3) and thus nH is calculated
      from the hardness ratio, not a spectral fit. See Section 5 of Paper III.
  < = When a nH measurement is zero, the authors report '<' in the
      corresponding flag column.
  > = nH is a lower limit (occurs when the hardness ratio is a lower limit and
      nH is derived from the hardness ratio).
  H = Bad XSPEC fit. nH is calculated from hardness ratio, not a spectral fit.
  

Log_NH
The log of the neutral hydrogen column density, log nH, in cm^-2 as determined by X-ray spectral fitting of each source using XSPEC. Since many sources have a low number of counts, the fits were performed assuming an intrinsic power-law spectrum with a fixed photon index Gamma = 1.9 typical of AGN continua, and the Galactic column density at z = 0.0, and only the intrinsic column density at the source redshift was allowed to vary. When an nH measurement is zero, the authors report '<' for the corresponding flag value.

Log_NH_Lower_Flag
This parameter contains a flag for the corresponding log_nh_lower value, as follows:

  S = Source detected on off-axis ACIS-S chip (2-3) and thus nH is calculated
      from the hardness ratio, not a spectral fit. See Section 5 of Paper III.
  < = When a nH measurement is zero, the authors report '<' in the
      corresponding flag column.
  > = nH is a lower limit (occurs when the hardness ratio is a lower limit and
      nH is derived from the hardness ratio).
  H = Bad XSPEC fit. nH is calculated from hardness ratio, not a spectral fit.
  

Log_NH_Lower
The 1-sigma lower limit to the value of log nH, as derived from the spectral fit. When an nH measurement is zero, the authors report '<' for the corresponding flag value.

Log_NH_Upper_Flag
This parameter contains a flag for the corresponding log_nh_upper value, as follows:

  S = Source detected on off-axis ACIS-S chip (2-3) and thus nH is calculated
      from the hardness ratio, not a spectral fit. See Section 5 of Paper III.
  < = When a nH measurement is zero, the authors report '<' in the
      corresponding flag column.
  > = nH is a lower limit (occurs when the hardness ratio is a lower limit and
      nH is derived from the hardness ratio).
  H = Bad XSPEC fit. nH is calculated from hardness ratio, not a spectral fit.
  

Log_NH_Upper
The 1-sigma upper limit to the value of log nH, as derived from the spectral fit. When an nH measurement is zero, the authors report '<' for the corresponding flag value.

Lower_Rf_Wavelength
The lower limit of the optical spectral wavelength range, in Angstroms, shifted to the rest frame of the object if the redshift is known; when a a source spectrum shows continuum only, and no redshift has been determined, the wavelength range limits are reported in the observed frame.

Upper_Rf_Wavelength
The higher limit of the optical spectral wavelength range, in Angstroms, shifted to the rest frame of the object if the redshift is known; when a a source spectrum shows continuum only, and no redshift has been determined, the wavelength range limits are reported in the observed frame.


Contact Person

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

Page Author: Browse Software Development Team
Last Modified: 30-Aug-2006