Last modified: December 2023

AHELP for CIAO 4.16 Sherpa


Context: models


Model the attenuation by the Inter-Stellar Medium (ISM).




This model calculates the transmission of the interstellar medium using the description of the ISM absorption of [1] . It includes neutral He autoionization features. Between 1.2398 and 43.655 Angstroms (i.e. in the 0.28-10 keV range) the model also accounts for metals as described in [2] . It should only be used when the independent axis has units of Angstroms.


>>> create_model_component("atten", "mdl")
>>> print(mdl)

Create a component of the atten model and display its default parameters. The output is:

   Param        Type          Value          Min          Max      Units
   -----        ----          -----          ---          ---      -----
   mdl.hcol     thawed        1e+20        1e+17        1e+24           
   mdl.heiRatio thawed          0.1            0            1           
   mdl.heiiRatio thawed         0.01            0            1           


The attributes for this object are:

Attribute Definition
hcol The column density of HI in atoms cm^-2.
heiRatio The ratio of the HeI to HI column densities.
heiiRatio The ratio of the HeII to HI column densities.


The code uses the best available photoionization cross-sections to date from the atomic data literature and combines them in an arbitrary mixture of the three ionic species: HI, HeI, and HeII.

This model provided courtesy of Pat Jelinsky.

The grid version is evaluated by numerically intgerating the function over each bin using a non-adaptive Gauss-Kronrod scheme suited for smooth functions [3] , falling over to a simple trapezoid scheme if this fails.



See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.