Last modified: December 2023

AHELP for CIAO 4.16 Sherpa


Context: models


A de-reddening model.




De-reddening model applied multiplicatively to a spectrum. The integrate flag of this model should be set to False when used with an integrated grid.


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

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

   Param        Type          Value          Min          Max      Units
   -----        ----          -----          ---          ---      -----
   mdl.rv       thawed           10        1e-10         1000           
   mdl.nhgal    thawed        1e-07        1e-07       100000           


The attributes for this object are:

Attribute Definition
rv The ratio of total to selective extinction.
nhgal The absorbing column density of H_gal in units of 10^20 cm^-2.


This dereddening model uses the analytic formula for the mean extension law described in [1] :

A(lambda) = E(B-V) * (a * rv + b)
          = 1.086 tau(lambda)

where tau(lambda) is the wavelength-dependent optical depth:

I(lambda) = I(0) * exp(-tau(lambda))

and a and b are computed using wavelength-dependent formulae, which are not reproduced here, for the wavelength range 1000 Angstroms to 3.3 microns. The relationship between the color excess and the column density (nhgal) is [2] :

E(B-V) = nhgal / 58.0

where the units of nhgal is 10^20 cm^-2. The value of the ratio of total to selective extinction, rv, is initially set to 3.1, the standard value for the diffuse ISM. The final model form is:

I(lambda) = I(0) exp(-nhgal * (a * ev + b) / (58.0 * 1.086)

This model provided courtesy of Karl Forster.



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