4.1.22. Line: transmission model for a single spectral line¶

This model calculates the transmission for a single spectral emission or absorption model. It is used as a multiplicative component, and can be used both for absorption and emission lines. The line profile is a Voigt profile (the convolution between a Gaussian and a Lorentzian profile): with the optical depth given by: where is the Voigt profile given by with and where and are the Full Width at Half Maximum of the Gaussian and Lorentzian components, respectively. Further, is the line centroid.

The program also calculates the equivalent width of the line.

The model can be used both in energy (keV) mode or wavelength (Å) mode. In each case, the quantities for the other scaling (centroid, equivalent width, and FWHMs of the Gaussian and Lorentzian components) are calculated. In addition, like all SPEX absorption models, also the covering factor can be varied.

Warning

The equivalent width is calculated at the energy grid in use for SPEX. Line flux outside this range is not taken into account. That may be important in the case of strong Lorentzian wings.

Warning

The parameter only represents the optical depth at the line centre for a pure Gaussian case. When the Lorentzian component becomes important, the “true” depth at line center is smaller.

Warning

When changing from energy to wavelength units, take care about the frozen/thawn status of the line centroid and FWHM.

Warning

You need to do a calc or fit command to get an update of the wavelength (for type=0) or energy (type=1).

The parameters of the model are:

tau0 : Optical depth at line center. Default value: 1. Positive values correspond to absorption lines, negative values to emission lines.
el : Line center energy (keV). Default value: 6.4 keV.
fwhg : FWHM of the Gaussian component in keV. Default value: 0.1 keV.
fwhl : FWHM of the Lorentzian component in keV. Default value: 0.1 keV.
type : Type of calculation. Type=0: energy units; type=1: wavelength units.
wl : Line center wavelength (Å). Default value: 20 Å.
awhg : FWHM of the Gaussian component in Å. Default value: 0.1 Å.
awhl : FWHM of the Lorentzian component in Å. Default value: 0.1 Å.
ewk : Equivalent width of the line in keV. Calculated by the program, not an input variable.
ewa : Equivalent width of the line in Å. Calculated by the program, not an input variable.
fcov : The covering factor of the absorption/emission line. Default value: 1 (full covering).