Energy conservation is imposed as a constraint when determining the temperature in xstar when the
input parameter niter is non-zero. If so, the temperature is iteratively improved until
the heating and cooling rates are locally equal. This is implemented by calculating the
integral over the absorbed and emitted continuum energy in a given spatial zone, and also
the sum over the energy emitted in the lines. Compton heating and cooling are added
analytically, since Comptonization of the radiation field is not treated.
The error resulting from this procedure is tabulated in the log file 'xout_step.log' in the 8th column
of the step-by-step output, in units of .
Energy conservation locally should correspond to global energy
conservation, i.e. that the total absorbed energy in the radiation field
equals the total emitted energy in lines plus continuum. This is
tested at each spatial zone in xstar by calculating
.
The error resulting from this procedure is tabulated in the log file 'xout_step.log' in the 9th column
of the step-by-step output, in units of
.
It is important to point out that the specific luminosities in
the file 'xout_spect1.fits' 'xout_cont1.fits' are not expected, in general,
to show energy conservation. This is primarily because the transmitted
spectra in both of these files contain the effects of binned lines.
Line opacity is expected to produce a scattering event, i.e. the photon is
likely to be reemitted near the same energy. This differs qualitatively from
photoelectric absorption, in which an absorbed photon is likely to be reemitted
at a very different energy, with an accompanying net loss or gain of energy
to the electron thermal bath. Line opacity is not included in the radiative
equilibrium integral used to calculate the gas temperature, and so
the total absorbed energy in the radiation field
will in
general not equal the emitted energy in
.
Energy conservation can be checked using the quantities
,
and
from the file xout_step.log.
Energy conservation checked using binned line spectra is also affected by the
errors introduced by binning. This is discussed at length in the section of this
manual on table models for xspec, but we emphasize here that the binned
spectrum cannot be accurately integrated to derive the total
line absorption or emission unless the lines are broad compared
with the energy grid spacing (requiring turbulent velocities 500 km/s currently).