Photoionization cross-sections from exited ionic levels: analytical fits

Analytical fits of photoionisation (bound-free) cross-sections from exited levels of ions with more than two electrons. The fits are based on the numerical tables computed by the Opacity Project (OP) and presented in the TOPBase database. The cross-section for each considered level was fitted using the following expression:

 \( \sigma(E) = \sigma_{\rm th} \left(A \bar E ^{p/2}+(1-A)\bar E^{1+p/2}\right) \)

where \(\bar E =E_{\rm th}/E \) and  \(E \) is the photon energy. Only a few energy levels were considered with the lowest excitation energies, typically not exceeding a quarter of the ionization energy. These fits were implemented in the code, that calculates hot LTE atmospheres of white dwarfs in the Super-Soft X-ray Sources, see the referenced paper for the details.The file 'bf_cross_hot.dat' contains, for each ion, the name of ion, the level's energy and statistical weight and the fitting parameters  \(E_{\rm th}, \sigma_{\rm th}, A, p \). The file 'bf_cross_hot.py' contains simple Python3 function for calculation the fitting cross-section for given ion and frequency or frequency array. In the latter case it also make a plot, if 'matplotlib' package is installed. The following ions were considered:

CII, CIII, CIV, NII, NIII, NIV, NV, OII, OIII, OIV, OV, OVI, NeIII, NeIV, NeV, NeVI, NeVII, NeVIII, NaIV, NaV, NaVI, NaVII, NaVIII, NaIX, MgVI, MgVII, MgVIII, MgIX, MgX, AlVI, AlVII, AlVIII, AlIX, AlX, AlXI, SiVII, SiVIII, SiIX, SiX, SiXI, SiXII, SIX, SX, SXI, SXII, SXIII, SXIV, ArV, ArVI, ArVII, ArVIII, ArXI, ArXII, ArXIII, ArXIV, ArXV, ArXVI, CaV, CaVI, CaVII, CaIX, CaXIII, CaXIV, CaXV, CaXVI, CaXVII, CaXVIII, FeVI, FeVII, FeXI, FeXII, FeXIII, FeXIV, FeXV, FeXVI, FeXVIII, FeXIX, FeXX, FeXXI, FeXXII, FeXXIII, FeXXIV, FeXXV.