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Continuum Opacity and Thomson Scattering

Continuum photons can be absorbed by photoionization. This is regarded as a photon destruction process since the converse, radiative recombination, produces an 'RRC' spectrum which has a different energy dependnce than the absorption, and so the two are treated separately. Xstar includes the photoabsorption associated with every bound-free transition in the atomic database; there are $\sim 10^5$ of them.

Photons can scatter via Thomson scattering or resonance scattering in lines. Xstar currently includes resonance scattering in lines in the final spectrum seen by a distant observer (as described in the following section). Thomson scattering does not strongly affect the shape of the spectrum unless the photon energy or temperature are a significant fraction of $m_e c^2$. Furthermore, similar arguments apply to Thomson scattering as apply to the radiative excitation in section 9.D.2. In the case of a spherical stationary cloud where all the photons from the cloud are observed, Thomson scattering will have no net effect. For this reason, version of xstar prior to 2.54a omitted Thomson scattering. Beginning with verion 2.54a, Thomson scattering is included with the same multiplicative factor 1-cfrac as is used for radiative excitation. Thus, when cfrac=1 the classical results without Thomson scattering are recovered, and when cfrac=0 Thomson scattering is included in the continuum opacity. Of cours, Thomson scattering is unimportant when the cloud column is less than $\sim 10^{23}$ cm$^{-2}$. And, when the cloud is optically thick to Thomson scattering, xstar does not adequately treat the effects of multiple scatterings.


next up previous contents
Next: Continuum Transfer Up: Recombination Continuum Emission and Previous: Continuum Emission   Contents
Tim Kallman 2021-04-08