ISMabs: A high resolution ISM absorption model with variable columns for individual ions.

NOTE: This model is now in the Xspec release and this page is superceded by the Xspec manual

ISMabs is an X-ray photoabsorption model for the interstellar medium that takes into account both neutral and ionized species from H, He, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni and Zn. This model has been developed by Efrain Gatuzz in collaboration with Javier Garcia, Tim Kallman, Claudio Mendoza, and Tom Gorczyca. A complete description of the science behind the model is described in Gatuzz et al. (2014b).

Required Files

File Description Size Version
ismabs.v1.2.tgz Complete Model 78.2 Mb 1.2

The contents of the tarfile include:

atomic_data/AtomicData.fits -- atomic database binary .fits file. This must reside in the directory atomic_data inside the folder where the model is located.

ismabs.f90 -- source code for ISMabs

lmodel_ismabs.dat -- local model definition file needed by xspec. -- installation script written on bash.



You can use the file to install the model by doing
Or you can setting up and using this model as described in the xspec manual:

0) You need to have the heasoft package installed on your machine, but it must be built from source. Local models cannot be installed from a binary installation.

1) Untar the tarfile somewhere in your user area.

2) Setup your headas environment (eg. 'setenv HEADAS /path/to/architecture', and 'source \$HEADAS/headas-init.csh').
'initpackage ismabs lmodel.dat path-to-model-directory',
where path-to-model-directory is the full path of the model directory.

After the build is complete type
'lmod ismabs path-to-current-directory'
In subsequent sessions you don't need to do the initpackage step again, just the lmod.


The input parameters include the column densities for the neutral elements as well as the single and double ionized ions. The redshift is also a model parameter.

Note that the He I column density is not included as a free parameter in the model, due to an inherent degeneracy between the relative columns of H, He I, He II. Instead, we impose the following constraint"

He I = 0.1 * H

Atomic Data

With the default set up - that is, if you have run, the model will look for the cross-section data file in atomic_data/AtomicData.fits, relative to the directory in which the module is located.

The XSPEC xset command can be used to set the ISMABSROOT variable; if this is set then it is used instead of the path to the module. So after
xset ISMABSROOT /data/ismabs/
then the model will use the file /data/ismabs/atomic_data/AtomicData.fits (the ISMABSROOT refers to the directory containing the atomic_data/ directory). Note that ISMABSROOT over-rides any changes made by running when building the model.

The location of the file can be found by setting the XSPEC chatter level to 20 or higher - e.g.

chatter 20

before evaluating the model.

We include in our model the following cross sections:
  • Neutral states of Si, S, Ar and Ca from Verner et al. (1995)
  • Singly and doubly ionized states of Si, S, Ar and Ca from Witthoeft et al. (2009) and Witthoeft et al. (2011)
  • Neutral, singly and doubly ionized states of N from Garcia et al. (2009)
  • Neutral states of O from Gorczyca et al. (2013)
  • Singly and doubly ionized states of O from Garcia et al. (2005), including corrections applied by Gatuzz et al. (2013)
  • Neutral state of Ne from Gorczyca et al. (2000)
  • Singly and doubly ionized states of Ne from Gorczyca et al. (2005)
  • For the Fe-L edge region we use the measurement of metallic iron by Kortright & Kim (2000)
  • Neutral, singly and doubly ionized states of Mg from Hasoglu et al. (2014).

Compared with previous photoabsorption models, which solely rely on neutral species, the inclusion of ions leads to improvements of the spectral fits. Fit parameters comprise the column densities of abundant contributors that allow direct estimates of ionization states. The figure below shows the theoretical photoabsorption cross sections for neutral, single, and double ionized Neon. The vertical lines indicate where the absorption features are observed in the astrophysical observations. The left panel displays the original cross sections, while the right panel shows them after wavelength corrections have been applied (for more details see Gatuzz et al. 2014b).


The ISMabs model has been tested in a variety of observations from bright X-ray sources. Below is an example of the model fit to the Chandra HEGTS spectrum of the X-ray binary XTE J1817-330, showing the performance of our model in the Ne K, Fe L, and O K regions.



  • New in version 1.2 (May 2015):
    • the parameter names do not have mathematical operators.
    • the subroutine names in the fortran code have been changed.
  • New in version 1.1 (December 2014):
    • the startup now does not use the ifl parameter.
    • the model now prints a message to STDOUT if it is unable to read from the AtomicData.fits file.
    • the ISMABSROOT xset variable has been added to allow the location of the AtomicData.fits file to be changed from within XSPEC.

The ISMabs will be continuously expanded and improved. For comments or questions about its implementation please contact Efrain Gatuzz or Javier Garcia.

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Last modified: Tuesday, 14-Feb-2023 16:41:52 EST