4.1.31. Pdem: DEM models¶
The pdem model is intended to be used for differential emission measure analysis, simultaneous with fitting of abundances etc. of an optically thin plasma.
It works as follows. The user gives a a number of temperature grid
points , a minimum temperature
, a maximum
temperature
, a total emission measure
and relative
contributions
. SPEX assumes that the grid points
between
and
are distributed logarithmically.
The relative contributions represents the values
of
(note the logarithm!) at the grid
points. SPEX then interpolates in the
space
on a finer grid using splines. That temperature grid on which the data
are being evaluated has a fine mesh: step size is about 0.10 in
(finer is not useful because uniqueness is no longer
guaranteed), with the additional constraint that the number of mesh
points is at least
and not larger than 64 (practical limit in
order to avoid excessive cpu-time). The emission measure distribution is
then simply scaled in such a way that its sum over the fine mesh equals
the total emission measure
that went into the model.
Warning
At least one of the values should be kept
frozen during fitting, when
is a free parameter, otherwise no
well defined solution can be obtained! If
is fixed, then all
can be free.
The parameters of the model are:
- norm:
Normalisation, i.e. total integrated emission measure
in units of
.
- t1:
Lower temperature
in keV.
- tn:
Upper temperature
in keV.
- npol:
Number of temperature grid points
; minimum value 2, maximum value 8.
- y1:
Relative contribution
at
.
- y2:
Relative contribution
at
- y8:
Relative contribution
at
; note that the higher contributions
are neglected if
.
The following parameters are the same as for the cie-model:
- hden:
Hydrogen density in
.
- it:
Ion temperature in keV.
- vrms:
RMS Velocity broadening in km/s (see Definition of the micro-turbulent velocity in SPEX)
- ref:
Reference element.
- 01…30:
Abundances of H to Zn.
- file:
Filename for the nonthermal electron distribution
Note that the true emission measure on the finer mesh can be displayed
by using the ascdump term # # dem
command; you will get a list of
temperature (keV) versus emission measure.