A modification of a spectrum due to scattering off dust on the line-of-sight.
The model assumes that the scattered flux goes into a uniform disk whose
size has a 1/E dependence and whose total flux has a 1/E
dependence.
| par1 | scattering fraction at 1 keV |
| par2 | size of halo at 1 keV in units of the detector beamsize |