Spectral Properties

The spectral properties of a source observed by ACIS include a set of hardness ratios determined from the aperture source energy fluxes in the source region, as well as flux determination by power law and black body model spectral fits to PI event data extracted from the source region.

Hardness Ratios

Hardness ratios appear in the both the Master Sources Table and the Source Observations Table with the field names:
hard_<x><y>, hard_<x><y>_hilim, hard_<x><y>_lolim

The hardness ratios that appear in the Master Sources Table are determined from the best estimates of the aperture source energy fluxes in the source regions of the contributing individual source observations contained in the Source Observations Table.

Source hardness ratios are an approximate measure of the source spectral shape based on count ratios. The hardness ratio for a pair of bands <x>, <y> is defined as

where F(<x>) is the aperture source energy flux (field flux_aper in MSC) in band <x>, and F(<b>) is the aperture source energy flux in band b (the broad energy band). For example, hard_ms is the medium-to-soft band hardness ratio, defined as

The catalog includes only the ratios hard_ms, hard_hs and hard_hm. Hardness ratios using the broad, ultra-soft, and HRC bands are not included in the catalog. In the Table of Individual Source Observations, the two-sided confidence limits associated with the ACIS hardness ratios are computed from the aperture total counts of the observation. In the Master Sources Table, the two-sided confidence limits associated with the ACIS hardness ratios are computed from the aperture total counts of the set of individual source observations contributing to the corresponding master source observation.

Spectral Model Fits

The descriptions below apply to fields in both the Master Sources Table and Source Observations Table, unless noted otherwise. However, the properties of a master source observation represent the "best estimates" of the actual source properties derived from the set of individual source observations contributing to the master source observation.

Note: Spectral fit parameters may be unreliable for sources at large off-axis angles, where background levels can be high. A background-fitting approach will be considered for future releases of the catalog.

Master Sources Table:
If there are at least 150 net (background-subtracted) counts in the energy range 0.5-7.0 keV present in the source region in at least one of the individual ACIS observations contributing to the Master source observation, then power law and black body model spectra are fitted to PI event data extracted from the source region, with corrections for the PSF aperture fraction, livetime, and ARF applied when fitting models.

Source Observations Table:
If there are at least 150 net (background-subtracted) counts in the energy range 0.5-7.0 keV present in the source region of an ACIS observation, then power law and black body model spectra are fitted to PI event data extracted from the source region, with corrections for the PSF aperture fraction, livetime, and ARF applied when fitting models.

• Power Law Model Spectral Fit

  The power law model spectral fit is performed over the energy range 0.5-7.0 keV; the free parameters to be fitted are the total integrated flux, total neutral Hydrogen absorbing column density, and power law photon index.

  • Power Law Model Spectral Fit Photon Index
    alpha, alpha_hilim, alpha_lolim

    The power law model spectral fit photon index and associated two-sided confidence limits represent the best-fit power law photon index, α, defined as F_E ~ E ^{- α}.

  • Power Law Model Spectral Fit Flux
    flux_powlaw, flux_powlaw_hilim, flux_powlaw_lolim

    The power law model spectral fit flux and associated two-sided confidence limits represent the integrated 0.5-7 keV flux derived from the best- fitting power law model spectral fit.

  • Power Law Model Spectral Fit N_H
    nh_powlaw, nh_powlaw_hilim, nh_powlaw_lolim

    The power law model spectral fit N_H and associated two-sided confidence limits represent the best-fit total neutral Hydrogen absorbing column from the power law model spectral fit, in units of 10²⁰ cm^-2.

  • Power Law Model Spectral Fit Statistic
    powlaw_stat (appears only in the Table of Individual Source Observations)
    

    The power law model spectral fit statistic is defined as the value of the χ² (data variance) statistic per degree of freedom for the best-fitting power law model spectral fit.

• Black Body Model Spectral Fit

  The black body model spectral fit is performed over the energy range 0.5-7.0 keV; the free parameters to be fitted are the total integrated flux, total neutral Hydrogen absorbing column density, and black body temperature.

  • Black Body Spectral Fit Temperature
    kt, kt_hilim, kt_lolim

    The black body model spectral fit temperature and associated two-sided confidence limits represent the best-fit temperature (kT) in units of keV from the black body model spectral fit.

  • Black Body Spectral Fit Flux
    flux_bb, flux_bb_hilim, flux_bb_lolim

    The black body spectral fit flux and associated two-sided confidence limits represent the integrated 0.5-7 keV flux derived from the best-fitting black body model spectral fit.

  • Black Body Spectral Fit N_H
    nh_bb, nh_bb_hilim, nh_bb_lolim

    The black body spectral fit N_H and associated two-sided confidence limits represent the best-fit total neutral Hydrogen absorbing column from the black body model spectral fit, in units of 10²⁰ cm^-2.

  • Black Body Model Spectral Fit Statistic
    bb_stat
    (appears only in the Table of Individual Source Observations)
    

    The black body model spectral fit statistic is defined as the value of the χ² (data variance) statistic per degree of freedom for the best-fitting black body model spectral fit.