chspec: Local models in XSPEC for X-ray spectra from Astrophysical plasma with CHIANTI database

Introduction

The chspec package includes multiple models for calculating photon spectrum for different temperature distributions that can be loaded in XSPEC spectral fitting package. At present it includes two models:

• chisoth : Emission from isothermal plasma
• chgausdem : Emission from Gaussian distribution of Differential Emission Measure

These models calculate the synthetic photon spectrum from a spectral library, generated by using the atomic database package CHIANTI. The pre-calculated spectra over grid of temperatures is stored in a FITS file.

The isothermal model takes logarithm of the temperature in K (LogT), abundances (A_x) of the elements with Z=2 to Z=30, and the volume emission measure as input parameters. The volume emission measure is implemented as a normalization factor, which is in the units of 10⁴⁶ cm^-3. For the gaussian DEM model, peak temperature and width of Gaussian (in log) are input parameters in addition to the abundances. Normalization gives peak emission measure in units of 10⁴⁶ cm^-3. The abundances A_x are in logarithmic units such that A_x = 12+log₁₀(N_x) where N_x is abundance of element X relative to Hydrogen. The default abundances are from 'sun_coronal_1992_feldman_ext.abund' file in CHIANTI database.

Details of chisoth model and the tabulated spectra used in the model is given in the appendix of Mondal, Biswajit et al. 2021, ApJ.

Pre-requisites

HEASOFT package along with XSPEC is required for the installation of chspec package.

Installation

• Download the chspec package from this repository by selecting "Code" > "Download ZIP".

• Unzip and place the resulting chspec directory in desired location.

• Go to this chspec directory:

cd chspec

Note: If you have not executed the init script for heasoft in your environment, it needs to be done (usually by executing heainit if that alias is set).

• To install chspec package, run the install.sh script:

./install.sh

This will first download the FITS table file using wget and save it in the directory chspec/tables. Then, the model will be compiled and installed.

Usage

Open XSPEC command prompt and run the following:

xspec > lmod chspec /path to/chspec/

You can also include this in your .xspec/xspec.rc file to load the model on XSPEC startup automatically.

Now the chisoth and chgausdem models will be available for use as as an XSPEC model. They may be defined like:

xspec > model chisoth
xspec > model chgausdem

For an example of the use of chisoth model in PyXSPEC, see this Jupyter notebook.