Comptonization, Poutanen and Svenson. XSPEC model.
Comptonization spectra computed for different geometries using exact numerical solution of the radiative transfer equation. The computational "iterative scattering method" is similar to the standard Lambda-iteration and is described in Poutanen J., Svensson R., 1996, ApJ, 470, 249 (PS96). The Compton scattering kernel is the exact one as derived by Jones F. C., 1968, Phys. Rev., 167, 1159 (see PS96 for references).
Comptonization spectra depend on the geometry (slab, sphere, hemisphere, cylinder), Thomson optical depth tau, parameters of the electron distribution, spectral distribution of soft seed photons, the way seed soft photons are injected to the electron cloud, and the inclination angle of the observer.
The resulting spectrum is reflected from the cool medium according to the computational method of Magdziarz & Zdziarski (1995) (see xspexrav and xspexriv models). rel_refl is the solid angle of the cold material visible from the Comptonizing source (in units 2 pi), other parameters determine the abundances and ionization state of reflecting material (Fe_ab_re, Me_ab, xi, Tdisk). The reflected spectrum is smeared out by rotation of the disk due to special and general relativistic effects using "diskline"-type kernel (with parameters Betor10, Rin, Rout).
Electron distribution function can be Maxwellian, power-law, cutoff Maxwellian, or hybrid (with low temperature Maxwellian plus a power-law tail).
Possible geometries include plane-parallel slab, cylinder (described by the height-to-radius ratio H/R), sphere, or hemisphere. By default the lower boundary of the "cloud" (not for spherical geometry) is fully absorbive (e.g. cold disk). However, by varying covering factor parameter cov_frac, it may be made transparent for radiation. In that case, photons from the "upper" cloud can also be upscattered in the "lower" cloud below the disk. This geometry is that for an accretion disk with cold cloudlets in the central plane (Zdziarski, Poutanen, et al. 1998, MNRAS, 301, 435). For cylinder and hemisphere geometries, an approximate solution is obtained by averaging specific intensities over horizontal layers (see PS96). For slab and sphere geometries, no approximation is made.
The seed photons can be injected to the electron cloud either isotropically and homogeneously through out the cloud, or at the bottom of the slab, cylinder, hemisphere or center of the sphere (or from the central plane of the slab if cov_frac ne 1). For the sphere, there exist a possibility (IGEOM=-5) for photon injection according to the eigenfunction of the diffusion equation sin (pi*tau'/tau)/(pi*tau'/tau), where tau' is the optical depth measured from the center (see Sunyaev & Titarchuk 1980).
Seed photons can be blackbody (bbodyrad) for Tbb>0 or multicolor disk (diskbb) for Tbb<0. The normalization of the model also follows those models: (1) Tbb>0, K = (RKM)**2 /(D10)**2, where D10 is the distance in units of 10 kpc and RKM is the source radius in km; (2) Tbb<0 K = (RKM)**2 /(D10)**2 cos(theta), where theta is the inclination angle.
Thomson optical depth of the cloud is not always good parameter to fit. Instead the Compton parameter y=4 * tau * Theta (where Theta= Te (keV) / 511 ) can be used. Parameter y is directly related to the spectral index and therefore is much more stable in fitting procedure. The fitting can be done taking 6th parameter negative, and optical depth then can be obtained via tau= y/(4* Te / 511).
The region of parameter space where the numerical method produces reasonable results is constrained as follows : 1) Electron temperature Te > 10 keV; 2) Thomson optical depth tau < 1.5 for slab geometry and tau < 3, for other geometries.
In versions 4.0 and above, the Compton reflection is done by a call to the ireflct model code, and the relativistic blurring by a call to rdblur. This does introduce some changes in the spectrum from earlier versions. For the case of a neutral reflector (i.e. the ionization parameter is zero), more accurate opacities are calculated. For the case of an ionized reflector, the old version assumed that for the purposes of calculating opacities the input spectrum was a power-law (with index based on the 2-10 keV spectrum). The new version uses the actual input spectrum, which is usually not a power law, giving different opacities for a given ionization parameter and disk temperature. The Greens' function integration required for the Compton reflection calculation is performed to an accuracy of 0.01 (i.e. 1%). This can be changed using, e.g.
sherpa> set_xsxset('COMPPS_PRECISION', '0.05')
This is an additive model component.
|1||kTe||electron temperature in keV|
|2||EleIndex||electron power-law index [ N(gamma)=gamma^-p ]|
|3||Gmin||minimum Lorentz factor gamma|
|4||Gmax||maximum Lorentz factor gamma. (a) if any of Gmin or Gmax < 1 then Maxwellian electron distribution with parameter kTe; (b) if kTe=0. then power-law electrons with parameters EleIndex, Gmin, Gmax; (c) if both Gmin,Gmax>=1 but gmax<gmin then cutoff Maxwellian with kTe, EleIndex, Gmin (cutoff Lorentz factor) as parameters; (d) if kTe.ne.0, Gmin, Gmax >=1 then hybrid electron distribution with parameters kTe, EleIndex, Gmin, Gmax.|
|5||kTbb||temperature of soft photons: kTbb>0 blackbody; kTbb<0 multicolor disk with inner disk temperature kTbb|
|6||tauy||if > 0 : tau, vertical optical depth of the corona; if < 0 : y = 4*Theta*tau; limits: for the slab geometry - tau < 1, if say tau~2 increase MAXTAU to 50 for sphere - tau < 3|
|7||geom||0 - approximate treatment of radiative transfer using escape probability for a sphere (very fast method); 1 - slab; 2 - cylinder; 3 - hemisphere; 4,5 - sphere input photons at the bottom of the slab, cylinder, hemisphere or center of the sphere (or from the central plane of the slab if cov_frac not 1). if < 0 then geometry defined by |geom| and sources of incident photons are isotropic and homogeneous. -5 - sphere with the source of photons distributed according to the eigenfunction of the diffusion equation f(tau')=sim(pi*tau'/tau)/(pi*tau'/tau) where tau' varies between 0 and tau.|
|8||HRcyl||H/R for cylinder geometry only|
|9||cosIncl||cosine of inclination angle (if < 0 then only blackbody)|
|10||cov_frac||covering factor of cold clouds; if geom =+/- 4,5 then cov_frac is dummy|
|11||rel_refl||amount of reflection Omega/(2*pi) (if R < 0 then only reflection component)|
|12||Fe_ab_re||iron abundance in units of solar|
|13||Me_ab||abundance of heavy elements in units of solar|
|14||xi||disk ionization parameter L/(nR^2)|
|15||Tdisk||disk temperature for reflection in K|
|16||Betor10||reflection emissivity law (r^beta); if beta=-10 then non-rotating disk; if beta=10 then 1.-sqrt(6./rg))/rg**3|
|17||Rin||inner radius of the disk (Schwarzschild units)|
|18||Rout||outer radius of the disk|
This information is taken from the XSPEC User's Guide. Version 12.9.1n of the XSPEC models is supplied with CIAO 4.10.
For a list of known bugs and issues with the XSPEC models, please visit the XSPEC bugs page.
To check the X-Spec version used by Sherpa, use the get_xsversion routine from the xspec module:
sherpa> from sherpa.astro.xspec import get_xsversion sherpa> get_xsversion() '12.9.1n'
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