# 4.1.27. 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.