Spectral model (in XSPEC atable format) of the X-ray emission
from warm-hot intergalactic medium, calculated in Khabibullin \& Churazov 2019
(https://ui.adsabs.harvard.edu/abs/2019MNRAS.482.4972K/abstract)
as proposed earlier by Churazov et al. 2001
(https://ui.adsabs.harvard.edu/abs/2001MNRAS.323...93C/abstract).

I) Version 1 - https://wwwmpa.mpa-garching.mpg.de/~ildar/igm/

Total emission is given by igmDTz03bct.fits,
contribution of the resonantly scattered CXB is in igmDTz03bcs.fits.
Purely thermal (CIE) emission from gas with the same parameters is presented
for comparison by igmDTz03bcth.fits.

Parameters of the model are:

1) logarithm of hydrogen number density n_{H}, 
2) logarithm of temperature T
3) redshift
4) normalisation defined as slab's hydrogen column density relative to N_{H}=1.5x10^{18} cm^{-2}
(corresponding to \tau_T=10^{-6}).

The model predicts surface brightness of X-ray emission from such a slab (in erg/s/cm^2/sr).
Corresponding gas emissivity \epsilon can be calculated as Model/n_{H},
so that luminosity of the volume element dV equals L=\epsilon*n_{H}^2*dV.

Assumed gas metallicity equals 0.3 in solar units (relative to Feldman 1992).
Incident radiation field is those of Haardt \& Madau 2010 at z~0. 

Examples of usage of these models in XSPEC are given by the corresponding *.xcm files.

II) Version 2 - https://wwwmpa.mpa-garching.mpg.de/~ildar/igm/v2

A set of models which will allow more explicit treatment of the gas metallicities.

The model files are here -
https://wwwmpa.mpa-garching.mpg.de/~ildar/igm/v2/

The set contains versions without resonant scattering included, *v2ph*,
and the resonantly scattered contribution, *v2sc*.

igm_v2ph_nome.fits is the model with zero metallicity,
igm_v2ph_me.fits - the model with metals emission only,
so that the gas with metallity Z will produce igm_v2ph_nome.fits+Z*igm_v2ph_me.fits
(as given in the igm_v2ph_zme.xcm file).

igm_v2ph_ox.fits, igm_v2ph_ne.fits, igm_v2ph_fe.fits give the emission of Oxygen, Neon and Iron,
while igm_v2ph_mx.fits gives emission of all other elements, i.e.
igm_v2ph_ox.fits+igm_v2ph_ne.fits+igm_v2ph_fe.fits+igm_v2ph_mx.fits=igm_v2ph_me.fits
(the xspec implementation of the corresponding combined model is igm_v2ph_zoxnefe.xcm).

Correspondingly,  igm_v2sc_me.fits, igm_v2sc_ox.fits, igm_v2sc_ne.fits, igm_v2sc_fe.fits
and igm_v2sc_mx.fits give scattered emission of all metals, oxygen, neon, iron, or all other elements,
respectively. Their contribution to the total observed spectrum needs to be scaled with the resolved fraction
of the CXB (the combined model for the total emission is given in igm_v2sc_zme.xcm  and igm_v2sc_zoxnefe.xcm).
Normalization is defined as before (cf  igmt_to_apec.xcm  for correspondence with APEC and
https://wwwmpa.mpa-garching.mpg.de/~ildar/igm/readme.txt),
the energy range is from 0.05 to 50 keV, lg(n_H)=-7.9:-1.6:0.1, lg(kT)=4.0:7.1:0.1.

I've made a few basic tests, but will continue now with more systematic testing.
We can probably discuss whether this would be enough for the current implementation
or it's better to produce table models with Z, Z_x, Z_ox, Z_ne, Z_fe being explicit parameters
(I don't think that's efficient, since dependence on them can be just linearized as described above).

Here are also some general points about the model to keep in mind:

1) These versions of the model were calculated with Cloudy
assuming Solar metallicity dataset of Feldman.
We have a version calculated with Eugene's code which has mekal behind it,
the differences were not too big but still noticeable
(see blue and magenta dashed curves ("Thermal") here
https://wwwmpa.mpa-garching.mpg.de/~ildar/igm/speE05to1comparison.pdf).

2) Given all the uncertainties in the abundance tables and inaccuracies of
linear interpolation on quite a coarse logarithmic grid in n and T,
~20% accuracy is typically a very good result.
It should be better in the exact grid points of the original calculated model.

3) At low temperatures, <0.2 keV, APEC might also suffer from the interpolation problems,
since it is a table model as well.

4) For temperatures above 1 keV or so, the impact of photoionization typically becomes less important,
since the majority of the sensitive ion species are already ionized by collisions there.
What we are doing right now is just replacing the IGM model by the APEC model for the particles
with temperatures and densities beyond the current IGM table grid.