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Last modified: 27 August 2013

URL: http://cxc.harvard.edu/csc/columns/fluxes.html

Source Fluxes


Source fluxes are determined by aperture photometry in each science energy band. Aperture photometry measurements are obtained in the source region, and in an elliptical aperture that includes the 90% Encircled Counts Fraction of the PSF at the source location, with corrections applied for the PSF aperture fraction. The correction factor is computed from the ray-trace local PSF under the assumption that the source spatial distribution matches the local PSF.


Energy Bands

Aperture Photometry Fluxes

Aperture photometry quantities are derived from the binned image-equivalents of the Level 3 event files (while spectral fit fluxes are derived directly from event data). The image files, recorded in units of photons s-1 cm-2, are produced by subtracting the full-field background image in the appropriate science energy band from the filtered and blocked full-field event data, and dividing the result by the full-field exposure map. See the complete description of the full-field image file in the Data Products section.

  • Aperture Total Counts

    Source Observations Table:
    cnts_aper, cnts_aperbkg, cnts_aper90, cnts_aper90bkg

    The aperture total counts represent the total number of source plus background counts measured in the modified source and background regions (cnts_aper, cnts_aperbkg), and in the modified elliptical aperture and modified elliptical background aperture (cnts_aper90, cnts_aper90bkg), uncorrected by the PSF aperture fraction.

  • Aperture Source Counts

    Source Observations Table:
    src_cnts_aper, src_cnts_aper90

    The aperture source counts represent the net number of background-subtracted source counts in the modified source region (src_cnts_aper) and in the modified elliptical aperture (src_cnts_aper90), corrected by the appropriate PSF aperture fractions.

  • Elliptical Aperture

    Source Observations Table:
    mjr_axis_aper90, mnr_axis_aper90, pos_angle_aper90, mjr_axis1_aper90bkg, mnr_axis1_aper90bkg, mjr_axis2_aper90bkg, mnr_axis2_aper90bkg, pos_angle_aper90bkg

    The elliptical apertures for each source are defined as the ellipses that include the 90% encircled counts fraction of the PSF in each science energy band at the source location, which are used to extract the aperture counts, count rates, and photon and energy fluxes. The elliptical apertures are co-located with the source region. The elliptical background apertures for each science energy band are scaled, annular ellipses co-located with the background region for that source. The parameter values that define the elliptical aperture and the elliptical background aperture for each source are the semi-major axis, semi-minor axis, and position angle of the major axis of each, in addition to the inner and outer annuli of the elliptical background aperture.

    Note: In the first catalog release, the elliptical aperture is defined on a tangent plane projection. The 0 deg position angle reference is defined on that tangent plane to be parallel to the true North direction at the location of the tangent plane reference (refer to the tangent plane reference right ascension (ra_nom), declination (dec_nom), and roll angle (roll_nom)).

  • Modified Elliptical Aperture

    Source Observations Table:
    area_aper90, area_aper90bkg

    The modified elliptical aperture and modified elliptical background aperture for each source and science energy band are defined as the areas of intersection of the elliptical aperture and elliptical background aperture for that source with the field of view, excluding any overlapping source regions.

  • Aperture Source Count Rates

    Source Observations Table:
    src_rate_aper, src_rate_aper90, src_rate_aper_hilim, src_rate_aper_lolim, src_rate_aper90_hilim, src_rate_aper90_lolim

    The aperture source count rates and associated two-sided confidence limits are defined as the background-subtracted source count rates in the modified source region (src_rate_aper) and in the modified elliptical aperture (src_rate_aper90), corrected by the appropriate PSF aperture fractions and livetime.

  • Aperture Source Energy and Photon Fluxes

    Master Sources Table:
    photflux_aper, photflux_aper90, photflux_aper_hilim, photflux_aper_lolim, photflux_aper90_hilim, photflux_aper90_lolim, flux_aper, flux_aper90, flux_aper_hilim, flux_aper_lolim, flux_aper90_hilim, flux_aper90_lolim

    The aperture source energy fluxes (flux_aper) and associated two-sided confidence limits represent the best estimates of the background-subtracted fluxes in the source region (photflux_aper) and in an elliptical aperture that includes the 90% encircled counts fraction of the PSF at the source location (photflux_aper90), corrected by the PSF aperture fraction, livetime, and exposure. The conversion from photons s-1 cm-2 to ergs s-1 cm-2 is performed by summing the photon energies for each incident source photon and scaling by the local value of the ARF at the location of the incident photon.

    Source Observations Table:
    photflux_aper, photflux_aper90, photflux_aper_hilim, photflux_aper_lolim, photflux_aper90_hilim, photflux_aper90_lolim, flux_aper, flux_aper90, flux_aper_hilim, flux_aper_lolim, flux_aper90_hilim, flux_aper90_lolim

    The aperture source energy fluxes (flux_aper) and associated two-sided confidence limits represent the background-subtracted fluxes in the modified source region (photflux_aper) and in the modified elliptical aperture (photflux_aper90), corrected by the appropriate PSF aperture fractions, livetime, and exposure. The conversion from photons s-1 cm-2 to ergs s-1 cm-2 is performed by summing the photon energies for each incident source photon and scaling by the local value of the ARF at the location of the incident photon.

  • Aperture Model Energy Fluxes

    Master Sources Table:
    flux_powlaw_aper, flux_powlaw_aper90, flux_bb_aper, flux_bb_aper90, flux_powlaw_aper_hilim, flux_powlaw_aper_lolim, flux_powlaw_aper90_hilim, flux_powlaw_aper90_lolim, flux_bb_aper_hilim, flux_bb_aper_lolim, flux_bb_aper90_hilim, flux_bb_aper90_lolim

    The aperture model energy fluxes and associated two-sided confidence limits represent the best estimates of the power law and black body aperture model energy fluxes in the source region (flux_powlaw_aper, flux_bb_aper) and in an elliptical aperture that includes the 90% encircled counts fraction of the PSF at the source location (flux_powlaw_aper90, flux_bb_aper90), corrected by the PSF aperture fraction, livetime, and exposure. (Note that the power law and black body aperture model energy fluxes are distinct from the spectral model fit fluxes in that they are derived from the binned images, whereas the latter are derived directly from the event data.) The conversion from aperture source count rates to aperture model energy fluxes is performed by scaling from a model spectrum folded through the calibrated response as follows:

    • For a source model F(E) whose integral over the science band is F(band), calculate the corresponding band count rate C'(band) in counts s-1, given the effective area calibration A(E) [and, if available, the RMF,
      R(E, band)] appropriate to the observation; this is the integral of F(E)A(E)R(E, band) over all energies, or if a diagonal RMF is assumed, the integral of F(E)A(E) over the band.
    • Infer the aperture model energy flux from the measured aperture source count rate C(band) as
      F(band) = F(band)C(band)/C'(band). The power law spectral model shall have a fixed photon index, α, defined as FE~E- α, equal to 1.7, and a fixed total neutral Hydrogen absorbing column NH = NH(Gal) cm-2. The black body spectral model shall have a fixed temperature kT = 1.0 keV, and a fixed total neutral Hydrogen absorbing column NH = 3 x 1020 cm-2.

    Source Observations Table:
    flux_powlaw_aper, flux_powlaw_aper90, flux_bb_aper, flux_bb_aper90, flux_powlaw_aper_hilim, flux_powlaw_aper_lolim, flux_powlaw_aper90_hilim, flux_powlaw_aper90_lolim, flux_bb_aper_hilim, flux_bb_aper_lolim, flux_bb_aper90_hilim, flux_bb_aper90_lolim

    The aperture model energy fluxes and associated two-sided confidence limits represent the power law and black body aperture model energy fluxes in the modified source region (flux_powlaw_aper, flux_bb_aper) and in the modified elliptical aperture (flux_powlaw_aper90, flux_bb_aper90), corrected by the appropriate PSF aperture fractions, livetime, and exposure. (Note that the power law and black body aperture model energy fluxes are distinct from the spectral model fit fluxes in that they are derived from the binned images, whereas the latter are derived directly from the event data.) The conversion from aperture source count rates to aperture model energy fluxes is performed by scaling from a model spectrum folded through the calibrated response as follows:

    • For a source model F(E) whose integral over the science band is F(band), calculate the corresponding band count rate C'(band) in counts s-1, given the effective area calibration A(E) [and, if available, the RMF,
      R(E, band)] appropriate to the observation; this is the integral of F(E)A(E)R(E, band) over all energies, or if a diagonal RMF is assumed, the integral of F(E)A(E) over the band.
    • Infer the aperture model energy flux from the measured aperture source count rate C(band) as
      F(band) = F(band)C(band)/C'(band). The power law spectral model shall have a fixed photon index, α, defined as FE~E- α, equal to 1.7, and a fixed total neutral Hydrogen absorbing column NH = NH(Gal) cm-2. The black body spectral model shall have a fixed temperature kT = 1.0 keV, and a fixed total neutral Hydrogen absorbing column NH = 3 x 1020 cm-2.
  • PSF Aperture Fractions

    Source Observations Table:
    psf_frac_aper, psf_frac_aperbkg, psf_frac_aper90, psf_frac_aper90bkg

    The PSF aperture fraction represents the fraction of the PSF that is included in the modified source and background regions (psf_frac_aper, psf_frac_bkgaper), and the modified elliptical aperture and modified elliptical background aperture (psf_frac_aper90, psf_frac_aper90bkg).


Last modified: 27 August 2013
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