Spectral Properties
The spectral properties of a source observed by ACIS include a set of hardness ratios determined from the aperture source photon 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_xy, hard_xy_hilim, and hard_xy_lolim.
The hardness ratios that appear in the Master
Sources Table are determined from the best
estimates of the
aperture source photon 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 photon flux (field photflux_aper in MSC) in band x, and F(s), F(m), and F(h) are the aperture source photon fluxes in the soft, medium, and hard bands. For example, hard_ms is the mediumtosoft band hardness ratio, defined as
As the aperture photon fluxes are evaluated assuming a monochromatic energy exposure map in each energy band, the sum of the photon fluxes from individual bands can be somewhat different than the value of the photon flux calculated for the broad band, F(b).
The catalog includes only the ratios hard_ms, hard_hs and hard_hm. Hardness ratios using the broad, ultrasoft, and HRC bands are not included in the catalog. In the Table of Individual Source Observations, the twosided confidence limits associated with the ACIS hardness ratios are computed from the aperture total counts of the observation. In the Master Sources Table, the twosided 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 offaxis angles, where background levels can be high. A backgroundfitting approach will be considered for future releases of the catalog.
Master Sources Table:
If there are at least 150 net (backgroundsubtracted)
counts in the energy range 0.57.0 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 (backgroundsubtracted)
counts in the energy range 0.57.0 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 0.57.0 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_lolimThe power law model spectral fit photon index and associated twosided confidence limits represent the bestfit power law photon index, α, defined as F_{E} ~ E^{  α}.

Power Law Model Spectral Fit Flux
flux_powlaw, flux_powlaw_hilim, flux_powlaw_lolimThe power law model spectral fit flux and associated twosided confidence limits represent the integrated 0.57 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_lolimThe power law model spectral fit N_{H} and associated twosided confidence limits represent the bestfit total neutral Hydrogen absorbing column from the power law model spectral fit, in units of 10^{20} 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 χ^{2} (data variance) statistic per degree of freedom for the bestfitting power law model spectral fit.


Black Body Model Spectral Fit
The black body model spectral fit is performed over the energy range 0.57.0 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_lolimThe black body model spectral fit temperature and associated twosided confidence limits represent the bestfit 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_lolimThe black body spectral fit flux and associated twosided confidence limits represent the integrated 0.57 keV flux derived from the bestfitting black body model spectral fit.

Black Body Spectral Fit N_{H}
nh_bb, nh_bb_hilim, nh_bb_lolimThe black body spectral fit N_{H} and associated twosided confidence limits represent the bestfit total neutral Hydrogen absorbing column from the black body model spectral fit, in units of 10^{20} 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 χ^{2} (data variance) statistic per degree of freedom for the bestfitting black body model spectral fit.
