4.1.47. Xabs: photoionised absorption model

The xabs model calculates the transmission of a slab of material, where all ionic column densities are linked through a photoionization model. The relevant parameter is the ionization parameter \xi = L/nr^2, with L the source luminosity, n the hydrogen density and r the distance from the ionizing source. The advantage of the xabs model over the slab model is that all relevant ions are taken into account, also those which would be detected only with marginal significance using the slab model. In some circumstances, the combined effect of many weak absorption features still can be significant. A disadvantage of the xabs model happens of course when the ionization balance of the source is different from the ionization balance that was used to produce the set of runs with the photo ionization code. In that case the xabs model may fail to give an acceptable fit, while the slab model may perform better.

The xabs model needs an ascii-file as input. The user can provide such a file (see parameter “col” in the parameter list), but there is also a default file in SPEX that is read if the user does not provide a separate file. The default is based on a run with Cloudy, using the spectral energy distribution of NGC 5548 as used in Steenbrugge et al. (2005). Such an ascii-files contains a pre-calculated list of ionic column densities versus ionisation parameter, as well as electron temperatures versus ionisation parameter (needed for the thermal line broadening). If you want to produce such a file, you can use the auxiliary program xabsinput, that will run Cloudy for you and make the file to be used in SPEX. See Xabsinput for more details how to use that program.

For more information on this model, the atomic data and parameters we refer to Different types of absorption models.

The parameters of the model are:

nh : Hydrogen column density in 10^{28} \mathrm{m}^{-2}. Default value: 10^{-4} (corresponding to 10^{24} \mathrm{m}^{-2}, a typical value at low Galactic latitudes).
xi : the { }^{10}\log of the ionisation parameter \log\xi in units of 10^{-9} Wm. 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 \sigma_{\mathrm v}
rms : Rms velocity \sigma_{\mathrm b} of line blend components
dv : Velocity distance \Delta v between different blend components
zv : Average systematic velocity v of the absorber (using relativistic Doppler shift).

The following parameters are the same as for the cie-model (see there for a description):

ref : Reference element
01...28 : Abundances of H to Ni; only here we take H, He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, Ni.

The following parameter is unique for the xabs-model and the warm model:

col : File name for the photoionisation balance parameters

Recommended citation: Steenbrugge et al. (2003).