This is an analytic model describing Comptonization of soft photons in a
hot plasma, developed by L. Titarchuk (see ApJ, 434, 313). This replaces
the Sunyaev-Titarchuk Comptonization model in the sense that the theory
is extended to include relativistic effects. Also, the approximations used
in the model work well for both the optically thin and thick regimes. The
Comptonized spectrum is determined completely by the plasma temperature and
the so-called 
parameter which is independent of geometry. The
optical depth is then determined as a function of for a given
geometry. Thus par5 switches between spherical and disk geometries so
that is not a direct input here. This parameter MUST be frozen.
If 
, is obtained from the optical depth using analytic
approximation (e.g. Titarchuk 1994). If 
and 
, is
obtained by interpolation from a set of accurately calculated pairs of
and 
from Sunyaev & Titarchuk 1985 (A& 143, 374).
In this incarnation of the model, the soft photon input spectrum is a
Wien law [x**2 exp(-x) photons] because this lens itself to a particularly
simple analytical form of the model. For present X-ray detectors this should
be adequate. Note that in energy flux space the peak of the Wien law occurs at
3kT as opposed to 2.8kT for a blackbody. The spectral fitter is strongly urged
to read the following references before using this model in order to
fully understand and appreciate the phyiscal assumptions made:
1. Titarchuk, L., 1994, ApJ, 434, 313
2. Hua, X-M., Titarchuk, L., 1994, ApJ, xxx, xxx
3. Titarchuk, L., Lyubarskij, Y., 1995, ApJ, submitted.
