4.1.18. Hot: collisional ionisation equilibrium absorption modelΒΆ

This model calculates the transmission of a plasma in collisional ionisation equilibrium with cosmic abundances.

For a given temperature and set of abundances, the model calculates the ionisation balance and then determines all ionic column densities by scaling to the prescribed total hydrogen column density. Using this set of column densities, the transmission of the plasma is calculated by multiplying the transmission of the individual ions.

The transmission includes both continuum and line opacity. For a description of what is currently in the absorption line database, we refer to Absorption model theory. By default, the model mimics the transmission of a neutral plasma by setting the default temperature to 8E-3 eV (8 \times 10^{-6} keV).

Please note that since SPEXACT updates 3.06.00 and 3.06.01, the behaviour of the hot model at low temperatures changed. Since these versions, SPEX includes also charge exchange processes in the hot model and these change the ionisation balance at low temperature. While for previous SPEX versions, setting a temperature of 5 \times 10^{-4} was enough to obtain a neutral gas, now, the temperature needs to be set to 8 \times 10^{-6} keV to obtain the same result.

Since the hot model is mostly used to model neutral gas in the ISM, we decided to set the default temperature to the minimum temperature of 8 \times 10^{-6} keV. This should result in a neutral gas and give the user the result most users expect.

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).
t : the electron temperature T_{\mathrm e} in keV. Default value: 8 \times 10^{-6} keV.
rt : the ratio of ionization balance to electron temperature, R_{\mathrm b} = T_{\mathrm b} / T_{\mathrm e} in keV. Default value: 1.
hden : Hydrogen density in 10^{20} \mathrm{m}^{-3}. Default value: 10^{-14} (corresponding to 10^{6} \mathrm{m}^{-3}, a typical value for the ISM).

The following parameters are common to all our absorption models:

f : 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

The following parameters are the same as for the cie-model (Cie: collisional ionisation equilibrium model):

ref : Reference element
01...30 : Abundances of H to Zn
file : Filename for the nonthermal electron distribution

Recommended citation: de Plaa et al. (2004) and Steenbrugge et al. (2005).