Overview: What is chspec?

The chspec package provides a set of local models for XSPEC , which are used to simulate X-ray emission spectra from hot astrophysical plasmas. These models use pre-calculated spectral data generated using the CHIANTI atomic database , which contains detailed information about atomic transitions, ionization equilibria, and radiative processes relevant to optically thin plasmas.

Currently, chspec includes two models:

  1. chisoth : For isothermal plasmas (plasma at a single temperature).
  2. chgausdem : For plasmas with a Gaussian distribution of temperatures , described by its peak temperature and width.

These models are "local" in XSPEC terminology, meaning they are user-defined models that need to be loaded separately into XSPEC.

Model 1: chisoth – Isothermal Plasma Emission

This model assumes that the emitting plasma has a single temperature .

Parameters:

Parameter Description
LogT Logarithm (base 10) of the plasma temperature in Kelvin (e.g., LogT = 7 corresponds to T = 10⁷ K)
Ax (Z=2 to Z=30) Abundances of elements with atomic number Z from 2 (He) to 30 (Zn), in logarithmic units:
Emission Measure Volume Emission Measure (EM), defined as EM = ∫ nₑ n_H dV (in cm⁻³). The normalization parameter is in units of10⁴⁶ cm⁻³.

Example:

If you specify LogT = 7.0, Ax(Fe) = 6.5 (assuming solar abundance is ~7.5), and Norm = 1.0, then:

Temperature = 10⁷ K

This model computes the spectrum corresponding to this temperature and abundances, scaled by the emission measure.

Model 2: chgausdem – Gaussian Differential Emission Measure

This model represents a distribution of plasma temperatures , assuming a Gaussian shape in log(T).