4.1.7. CX: model for charge exchange plasmas

This model calculates the spectrum emitted from a hot plasma when it recombines with cold neutral materials. This model is based on three key assumptions: (1) it considers only single electron capture in a ion-neutral collision; (2) all cross section data are obtained only with a atomic hydrogen target, (3) electronic collisional excitation and recombination are ignored in the spectral calculation. More information can be found in Gu et al. (2016). Charge exchange cross sections

The CX cross section data used in the model are partly taken from literature, including quantum molecular-orbital close-coupling calculations for \mathrm C^{5+} and \mathrm O^{6+} by Wu et al. (2012) and Nolte et al. (2012), multi-channel Laudau-Zener results for \mathrm Fe^{25+} and \mathrm Fe^{26+} by Mullen et al. (2016), other data compilations for \mathrm C^{6+} and \mathrm O^{8+} by Janev et al. (1993), and the NIFS Charge Transfer Database (CHART) 1 for \mathrm Be^{4+}, \mathrm B^{5+}, \mathrm N^{7+}, and \mathrm Ne^{10+}. For CHART database, we extracted all the data, from both theoretical calculations and experiments (see a full list in Table 1 of Gu et al. (2016)), and fitted them with Eq.2 of Gu et al. (2016) in the energy range of interests. In typical astrophysical velocity range (\sim 100-5000 km \mathrm{s}^{-1}), the useful CHART data are usually from molecular-orbital and atomic-orbital close-coupling methods, and a few classical trajectory Monte Carlo calculations. All the above data are dependent on collision energy, and resolved to levels described by quantum number n and l.

For ions not available in public sources, we developed a new method to interpolate by analyzing the known ions. First we used a scaling law to determine total cross section for each ion, and applied another scaling law to represent the n- selectivity. The l- dependence is approximated by one of the five empirical weighting functions presented in Eqs.4-8 of Gu et al. (2016).


The CX model only works with the updated atomic database set through the command var calc new.


All Beryllium-like sequence ions are not included in the current version; will be available later.


We will keep updating the CX model when new data (especially for molecular targets) from theoretical calculations and experiments become available. Parameter description

The parameters of the CX model are:

norm : the normalisation, which is the emission measure Y \equiv n_{\mathrm H} n_{\mathrm nh} V in units of 10^{64} \mathrm{m}^{-3}, where n_{\mathrm H} and n_{\mathrm nh} are the Hydrogen densities of the ionized and neutral materials, respectively, and V is the effective interaction volume. Default value: 1.
hden : Hydrogen density of the neutral materials in units of 10^{20} \mathrm{m}^{-3} (or 10^{14} \mathrm{m}^{-3}). Default value: 10^{-14}.
mode : Switch between a hot-cold interaction driven by thermal motion of hot plasma, and the one dominated by flow velocity. Default value: 2 (kinematic).
t : the ionization temperature of hot matter in keV. It is also used to approximate the thermal motion when mode is set to 1. Default value: 1.
sig : the width \sigma_T of the gaussian emission measure profile. Default value: 0. (no temperature distribution i.e. isothermal)
sup : the width \sigma_T of the high-temperature part of the gaussian emission measure profile. If larger than 10^{-5} keV, the sig parameter becomes the sigma value for the low-temperature end. Default value: 0
logt : Switch between linear and logarithmic temperature scale for the gaussian emission measure profile. Default value: 1 (logarithmic)
zv : Collision velocity in unit of km \mathrm{s}^{-1}, used when mode is set to 2. Default value: 100
op : Switch between single and multiple collisions for each ion. In multiple collision case, one ion would continuously undergo CX and produce various emission lines, until it becomes neutral. Default: 1 (single)
wt : Weighting functions for subshell l- population. When wt is set to 1, the l- population is approximated by a series of empirical functions that switchs from one to another as a function of collision velocity. See Gu et al. (2016) for details. These empirical functions are defined in Eqs. 4-8 of Gu et al. (2016), and will be selected when wt is set to 2-6, respectively. Default: 1
vrms : RMS Velocity broadening in km/s (see Definition of the micro-turbulent velocity in SPEX)
ref : reference element. Default value 1 (hydrogen). See above for more details. The value corresponds to the atomic number of the reference element.
01 : Abundance of hydrogen (H, Z=1) in Solar units. Default 1.
02 : Abundance of helium (He, Z=2) in Solar units. Default 1.
\ldots 30 : Abundance of zinc (Zn, Z=30) in Solar units. Default 1.
file : Filename for the nonthermal distribution. If not present, nonthermal effects are not taken into account (default).

Recommended citation: Gu et al. (2016).