4.1.15. Etau: simple transmission model¶
This model calculates the transmission between energies and for a simple (often unphysical!) case:
with the optical depth given by:
In addition, we put here for and , where and are adjustable parameters. This allows the user for example to mimick an edge. Note however, that in most circumstances there are more physical models present in SPEX that contain realistic transmissions of edges! If you do not want or need edges, simply keep and at their default values.
Note that should be non-negative. For the spectrum has a high-energy cut-off, for it has a low-energy cut-off, and for the transmission is flat. The larger the value of , the sharper the cut-off is.
The model can be used as a basis for more complicated continuum absorption models. For example, if the optical depth is given as a polynomial in the photon energy , say for example , one may define three etau components, with values of 2, 3, and 4, and indices of 0, 1 and 2. This is because of the mathematical identity .
As of version 3.08.00 a new option has been added to use an
input transmission spectrum instead of an analytical description.
The file has a similar structure as the file model for emission spectra.
The first line must contain the number of energies in the file.
The next lines contain two numbers for each entry: the energy in keV,
and the scaled optical depth (dimensionless).
The entries must be in increasing energy order.
The transmission T(E) is then computed as
where is the first parameter of the model tau0
.
The parameters of the model are:
tau0
: Optical depth at keV. Default
value: 1.a
: The index defined above. Default value: 1.e1
: Lower energy (keV). Default value:
.e2
: Upper energy (keV). Default value:
.f
: The covering factor of the absorber. Default value: 1 (full
covering)mode
: mode=0
: Default etau model (analytical description).
mode=1
: Input of the optical depth of the absorber as a file model.file
: The name of the file that provides the transmission.