The spectral analysis package allows to reduce a spectrum of single counted photons to a handful parameters like temperature, spectral index, photon or energy flux, luminosity etc., which describe the energy state of its radiating object. It is designed in such a way that all instrument features (e.g.mirror, detector, filter) are well separated in standardized and extentable modules to make it instrument-independent.
This EXSAS subsystem follows general rules defined for the EXSAS architecture. Especially all relevant MIDAS commands can be applied to the resulting files created by this package. The system can be run interactively and in batch mode.
The usual way analyzing a pulse height spectrum
is to start with data projection commands like
SELECT AMPLITUDE and BIN AMPLITUDE.
All necessary corrections
(mirror and detector efficiencies, dead time,
filter inhomogeneities, etc.)
are either applied directly to the raw data
or evaluated with CORRECT/SPECTRUM resulting
in a correction vector to adjust
appropriate models for comparing with observations.
The observed count rate spectrum can be produced
by using the command PREPARE/SPECTRUM
and plotted with PLOT/SPECTRUM.
Simple arithmetic operations between spectra can
be performed with OPERATE/SPECTRUM.
The command BIN/DETECTOR_RESPONSE allows
to create a special detector response matrix binned
according to the observed spectrum.
For analyzing an observed spectrum the main command of this
package -- MODEL/SPECTRUM -- is a powerful tool.
Its derivate FIT/SPECTRUM allows to fit standard
models in an easy way.
By CREATE/PARFIL FIT a normal ASCII parameter file
can be created for these commands.
A standard plot of a fitted spectrum can be obtained
with PLOT/FIT_SPECTRUM.
Command PLOT/ERROR_ELLIPSE allows to draw
error ellipses by evaluating the covariance matrix.
If the output of MODEL/SPECTRUM delivers a 1-D
or 2-D grid of spectral parameters with 
values around best fit values PLOT/CHI2_CONTOUR
gives the adequate graphical output.
Finally the command INTEGRATE/FLUX_DENSITY can be
used to calculate a photon and energy flux, respectively.
In addition, for red-shifted
sources luminosities can be
obtained by specifying the cosmological model.
If an object is observed with different instruments
a joint fit to the merged spectrum is possible.
The utility command MERGE/SPECTRA merges all individual
input files into files which can be used directly in
connection with FIT/SPECTRUM and MODEL/SPECTRUM.
This chapter starts with some typical examples of EXSAS sessions to introduce the user to the spectral commands (section 6.1). General remarks about methods and performances should lead to a deeper understanding of this package (section 6.2). In the following two sections specific and detailed information is given about commands, the parameter file, and the input/output files with their headers in connection with the spectral analysis.
The impulse for developing EXSAS was given by the ROSAT mission. The main instrument of the ROSAT X-ray telescope (XRT) for taking spectra is its position sensitive proportional counter (PSPC). Section 6.5 describes some of its properties, which are helpful in analyzing its spectra. The chapter closes with a section about spectral results, which are produced by standard data processing during the ROSAT mission.