# 4.1.33. Slab: thin slab absorption model¶

The *slab* model calculates the transmission of a slab of material,
where all ionic column densities can be chosen independently. This has
the advantage that a spectrum can be fit without any knowledge of the
ionisation balance of the slab. After a spectral fit has been made, one
may try to explain the observed column densities by comparing the with
predictions from any model (as calculated by SPEX, Cloudy, XSTAR, ION or
any other existing (photo)ionisation code).

For more information on this model, the atomic data and parameters we refer to Different types of absorption models. Below we do not list all the parameters of the model, but for each ion of H, He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe and Ni there is a parameter, namely the logarithm (base 10) of the column density in SI units (). So, a H column of ( should be entered as 28.0. The default values of 1 therefore correspond to an almost negligible column.

Warning

We do include here fully stripped ions, as their free electrons do produce Thomson scattering. However, the user is advised not to take more than one column density of a bare nucleus as a free parameter, as the shape of the Thomson scattering contribution is the same for all electrons, regardless from which ion they came from. In the spectral fitting, there would be 100% correlations between the column densities of these bare ions, which is undesirable.

The parameters of the model are:

`h1`

: log column density () of H . Default value: 1.`h2`

: log column density () of H . Default value: 1.`he1`

: log column density () of He . Default value:
1.`he2`

: log column density () of He . Default value:
1.`he2`

: log column density () of He . Default value:
1.`c1`

: log column density () of H . Default value: 1.`ni27`

: log column density () of Ni . Default value:
1.`ni28`

: log column density () of Ni . Default value:
1.The following parameters are common to all our absorption models:

`fcov`

: The covering factor of the absorber. Default value: 1 (full
covering)`v`

: Root mean square velocity `rms`

: Rms velocity of line blend
components`dv`

: Velocity distance between different blend
components`zv`

: Average systematic velocity of the absorber*Recommended citation:* Kaastra et al. (2002).