A Walkthrough of XSTAR
Spherical photoionized cloud
In this section we illustrate how to use XSTAR’s interface. Once XSTAR is installed and configured, at the unix prompt, type:
unix> xstar
By invoking XSTAR with this simple command, you will be prompted for a series of physical and control parameters for the simulation.
The input parameters that must always be specified are: The model (initial) temperature, density, spectrum shape, ionizing luminosity, column density, ionization parameter, and elemental abundance table. Definitions for these and the units assumed are described in detail in chapter User Input to XSTAR. All input parameters have default values, selected by pressing return at the prompt, thus it is possible to simply start XSTAR as above to invoke the default model.
In this example we model a spherical, constant density cloud with a
source at its center. The cloud is optically thin. The source
luminosity is \(10^{32}\,\text{erg}\,\text{s}^{-1}\). The
ionization parameter at the inner edge of the cloud is
\(\log(\xi)=5\). The ionizing spectrum is a power law with energy
index \(-1\). We assume all elemental abundances are solar but
exclude Li, Be, B. The total column denstity is
\(10^{19}\,\text{cm}^{-1}\). In this example, we require thermal
equilibrium by expicitely setting the hidden parameter niter to 99
and spherical symmetry by setting the covering fraction cfrac
to 1.
Call XSTAR from the command line:
unix> xstar niter=99 cfrac=1
xstar version 2.59cj
temperature (/10**4K) (0.:1.e4) [400.] 1000.
density (cm**-3) (0.:1.e21) [1.e+4] 1E+6
spectrum type?[pow]
radiation temperature or alpha?[-1.]
luminosity (/10**38 erg/s) (0.:1.e10) [1.e-6]
column density (cm**-2) (0.:1.e25) [1.E17] 1E19
log(ionization parameter) (erg cm/s) (-10.:+10.) [5.]
abundance table[xdef]
model name[XSTAR Default] 'filled sphere'
Alternatively, XSTAR can called directly with command-line parameters to avoid prompting.
xstar cfrac=1 temperature=1000. density=1.E+6 spectrum='pow ' \
trad=-1. rlrad38=1.E-6 column=1.E+19 rlogxi=5. abundtbl='xdef' \
modelname='filled sphere' niter=99
The terminal output reports the progress of the XSTAR run. Each line represents a spatial zone. For each spatial zone, XSTAR prints the radius, stepsize, column density, ionization parameter, electron fraction, temperature, heating/cooling balance, optical depth in forward and reverse direction, and number of iterations required to reach thermal equilibrium.
xstar version 2.59cj
pass number= 1 -1
log(r) delr/r log(N) log(xi) x_e log(n) log(t) h-c(%) h-c(%) log(tau)
fwd rev
10.50 -36.00 -10.00 5.00 1.21 6.00 7.77 -0.00 0.00 -10.00 -10.00 15
10.50 -36.00 -10.00 5.00 1.21 6.00 7.77 -0.00 -0.00 -10.00 -10.00 1
10.62 -0.60 16.02 4.75 1.21 6.00 7.75 0.00 -0.00 -10.00 -10.00 5
[...]
12.98 -0.00 18.98 0.04 1.20 6.00 4.33 0.01 0.12 -2.39 -10.00 14
13.00 -0.00 18.99 0.01 1.20 6.00 4.32 -0.01 0.12 -2.32 -10.00 13
13.00 -0.00 19.00 -0.00 1.20 6.00 4.32 -0.01 0.12 -2.30 -10.00 13
final print: 0
xstar: Prepping to write spectral data
xstar: Done writing spectral data
total time 1663.5743254758418
Upon completion, the following output files are produced:
unix> ls
xout_abund1.fits
xout_cont1.fits
xout_lines1.fits
xout_rrc1.fits
xout_spect1.fits
xout_step.log
A detailed description of the output files is given in XSTAR output. The ASCII file “xout_step.log” contains mostly diagnostic information. Information about ion fractions, line and continuum emission and absorption as well as spectral information is stored in several FITS files.
In this example we are interested in the total optical depth of the
cloud as a function of energy. This information can be obtained from
the spectral file “xout_spect1.fits”. The file structure can for
example be inspected with the FTOOL fstruct:
unix> fstruct xout_spect1.fits
No. Type EXTNAME BITPIX Dimensions(columns) PCOUNT GCOUNT
0 PRIMARY 16 0 0 1
1 BINTABLE PARAMETERS 8 66(5) 56 0 1
Column Name Format Dims Units TLMIN TLMAX
1 index 1I
2 parameter 20A
3 value 1E
4 type 10A
5 comment 30A
2 TABLE XSTAR_SPECTRA 8 69(5) 9999 0 1
Column Name Format Dims Units TLMIN TLMAX
1 energy E13.5 eV
2 incident E13.5 erg/s/erg
3 transmitted E13.5 erg/s/erg
4 emit_inward E13.5 erg/s/erg
5 emit_outward E13.5 erg/s/erg
FITS files can be processed with various programming languages and tools. In this example we are interested in the transmitted spectrum. Here we use Python to plot the transmitted spectrum in \(300\,\text{eV}\text{--}800\,\text{eV}\) range.
>>> import matplotlib.pyplot as plt
>>> from astropy.io import fits
>>> import numpy as np
>>>
>>> hdul = fits.open('xout_spect1.fits')
>>> en = hdul[2].data['energy']
>>> trns = hdul[2].data['transmitted']
>>> hdul.close()
>>>
>>> plt.plot(en, trns)
>>> plt.xscale("log")
>>> plt.yscale("log")
>>> plt.xlim(300,800)
>>> plt.ylim(100, 400)
>>> plt.xlabel("Energy (eV)")
>>> plt.ylabel("Transmitted Flux")
>>> plt.show()
