This is very simple tutorial about how to construct a warm absorption table with code XSTAR.

Reference

  • The XSTAR manual: here

What does XSTAR calculate?

Quote from the manual:

“A spherical gas shell surrounding a central source of ionizing radiation absorbs some of this radiation and reradiates it in other portions of the spectrum; XSTAR computes the effects on the gas of absorbing this energy, and the spectrum of reradiated light.”

To make XSTAR work, we have to specify two things: the incident radiation (spectral continuum shape and luminosity) and properties of the gas (column density, number density, temperature, pressure, turbulent velocity, element abundance…).

How to use XSTAR

I usually use the task XSTAR2XSPEC to calculate the absorption table. I control properties of the source-gas system by editing the XSTAR parameter file.

The spectrum of the incident radiation can be user defined and is controlled by keywords “spectrum”, “spectrum_file” and “spectun” in the parameter file. See here for a more concrete example. The source luminosity is defined by keyword “rlrad38”. The normalization of the spectral continuum in “spectrum_file” does not matter since XSTAR will renormalize the spectrum according to the source luminosity.

Note that the parameter locates at this directory if XSTAR package is installed along with heasoft6.26:

/opt/tools/heasoft/heasoft-6.26/x86_64-pc-linux-gnu-libc2.19/syspfiles/xstinitable.par

By editing this file, we are able to control every parameter concerning the calculation (For script XSTAR2XSPEC). Please refer to the file “xstar.par’ in the same directory for the meaning of each column in the parameter file. For example, if you want the iron abundance to be a variable parameter, edit the line feabundtyp,i,h,0,0,2,"iron abundance variation type" to feabundtyp,i,h,2,0,2,"iron abundance variation type". The minimal and maximum parameter value, interpolation type (linear or logarithmic), number of steps can be defined in the same way. So editing the parameter file can help to define both the incident radiation and properties of the intervening gas.

There are some parameters controlling the calculation process (number of iterations, step size…). For these parameters, I usually take the default values, but modification may be needed in some extreme cases (there are some examples in the manual).

Some important parameters

  • The covering fraction.
  • The temperature.
  • The pressure.
  • The gas number density.
  • The source luminosity (from 1Ry to 1000 Ry, 1Ry=13.6).
  • The column density range and number of steps.
  • The ionization parameter range and number of steps.
  • The input spectrum file.
  • The turbulent velocity (in km/s).
  • The model name.

Output files from XSTAR2XSPEC

There are four output files, I will quote from the XSTAR manual.

  1. xstar2xspec.log is a concatenation of all the xout_step.log files from all the xstar runs called by xstar2xspec.
  2. xout_ain.fits is a fits file containing the atable with the reflected emission spectrum produced by xstar. This file is ready for use by xspec using the `model atable xout_ain.fits …’ command.
  3. xout_aout.fits is fits file containing the atable with the emission spectrum in the forward (transmitted) direction produced by xstar. This file is ready for use by xspec using the `model atable xout_aout.fits …’ command.
  4. xout_mtable.fits is a fits file containing the mtable with the absorption spectrum in the forward (transmitted) direction produced by xstar. This file is ready for use by xspec using the `model mtable xout_mtable.fits pow …’ command.