equil, vequil: collisional plasma, ionization equilibrium

Ionization equilibrium collisional plasma model. This is the equilibrium version of Kazik Borkowski's NEI models. Several versions are available. To switch between them use the xset neivers command. The versions available are:

1.0	the version from xspec v11.1
1.1	as 1.0 but with updated ionization fractions using dielectronic recombination rates from Mazzotta et al (1998)
2.0	same ionization fractions as 1.1 but uses AtomDB v2 to calculate the resulting spectrum
3.0	ionization fractions and spectrum calculation uses AtomDB v3

Note that versions 1.x have no emission from Ar. The default is version 3.0. The vequil variant allows the user to set the abundances for the model. For versions 3 and above the abundances of the trace elements (ie Li, Be, B, F, Na, P, Cl, K, Sc, Ti, V, Cr, Mn, Co, Cu, Zn) can be set using xset APEC_TRACE_ABUND. These trace element abundances can be set either to the abundance of one of the main elements or to a numerical value (relative to Solar). For instance,

XSPEC12> xset APEC_TRACE_ABUND Fe

sets trace element abundances to that of iron while

XSPEC12> xset APEC_TRACE_ABUND 1.0

sets them to Solar. The default value for APEC_TRACE_ABUND is 1.0.

For the equil model the parameters are:

par1	plasma temperature (keV)
par2	Metal abundances (He fixed at cosmic). The elements included are C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni. Abundances are defined by the abund command
par3	redshift
norm	$(10^{-14}/(4\pi[D_A(1+z)]^2))\int n_en_H dV$, where $D_A$ is the angular diameter distance to the source (cm), $n_e$ is the electron density (cm$^{-3}$), and $n_H$ is the hydrogen density (cm$^{-3}$).

For the vequil model, the parameters are:

par1	plasma temperature (keV)
par2-par13	Abundances for He, C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni wrt Solar (defined by the abund command)
par14	Redshift, z
norm	$(10^{-14}/(4\pi[D_A(1+z)]^2))\int n_en_H dV$, where $D_A$ is the angular diameter distance to the source (cm), $n_e$ is the electron density (cm$^{-3}$), and $n_H$ is the hydrogen density (cm$^{-3}$).

The references for this model are as follows:
Borkowski, Lyerly & Reynolds, 2001, ApJ, 548, 820
Hamilton, A.J.S., Sarazin, C. L. & Chevalier, R. A. , 1983,ApJS, 51,115
Borkowski, K.J., Sarazin, C.L. & Blondin, J.M. 1994, ApJ, 429, 710
Liedahl, D.A., Osterheld, A.L. Goldstein, W.H. 1995, ApJ, 438, L11