4.1.37. 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 (\mathrm{m}^{-2}). So, a H  column of 10^{28} \mathrm{m}^{-2} (10^{24} \mathrm{cm}^{-2} 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 (\mathrm{m}^{-2}) of H . Default value: 1.

h2:

log column density (\mathrm{m}^{-2}) of H . Default value: 1.

he1:

log column density (\mathrm{m}^{-2}) of He . Default value: 1.

he2:

log column density (\mathrm{m}^{-2}) of He . Default value: 1.

he2:

log column density (\mathrm{m}^{-2}) of He . Default value: 1.

c1:

log column density (\mathrm{m}^{-2}) of H . Default value: 1.

\ldots

ni27:

log column density (\mathrm{m}^{-2}) of Ni . Default value: 1.

ni28:

log column density (\mathrm{m}^{-2}) of Ni . Default value: 1.

The following parameters are common to all our absorption models:

icov:

Type of the covering fraction. Default value: 2 (constant, set by fcov). If icov=1, full covering is applied. If icov=3, covering fraction follows a tangent function that increases with energy. If icov=4, covering fraction follows an inverse tangent function that decreases with energy. See description in pion.

fcov:

The covering factor of the absorber if icov=2. Default value: 1 (full covering). If icov=3 or 4, it sets the covering factor at the high energy end.

lcov:

The covering factor of the absorber at the low energy end. Default value: 1. lcov is applied only when icov=3 or 4. See description in pion.

ecov:

The energy when the covering factor changes from lcov to fcov. Only applied if icov=3 or 4.

acov:

The width of the transit on covering factor. Only applied if icov=3 or 4.

v:

Root mean square velocity \sigma_{\mathrm v}

zv:

Average systematic velocity v of the absorber (using relativistic Doppler shift)

Recommended citation: Kaastra et al. (2002).