# 4.1.14. 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
.

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)