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The expected PSPC count rates depend sensitively on the assumed
incident spectrum which is unknown for most sources. In general, the
convolution of effective area and incident spectrum is non-trivial
because of substantial changes in effective area as a function
of energy. In order to
facilitate count rate calculations, the count rate curves are provided
for power law spectra, thermal line spectra and blackbody spectra as a
function of interstellar absorption column density 
.
In Figures 10.2 
- 10.7,
the energy-to-counts conversion factor (ECF) is shown,
i.e., the factor by which the unabsorbed source flux (in units of
ergs cm 
s 
in the 0.1 - 2.4 keV band) has
to be multiplied in order to obtain the expected PSPC on-axis
count rate. Figure 10.2 shows the ECF for power
law spectra with photon index 
for various N 
values
between 
cm and 
cm . Whereas
for small values of N a distinct variation of ECF vs.
occurs, the ECF is basically constant (with respect to ) at
values around 
counts cm 
erg for
larger values of
typically found for AGNs. Figure 10.3 shows the
equivalent curve for thermal line spectra. For such spectra the ECF does
depend sensitively on the assumed temperature (in addition to ) with
the largest ECF values found at temperatures 
K. Lastly,
Figure 10.4 shows the ECF for blackbody spectra;
here the maximum ECF is found at 
.
Figures 10.5 - 10.7
show the ECF for the PSPC used with the boron filter
for power law, thermal line and blackbody spectra for various values
of . Although the general shapes of the curves remain unchanged,
the absolute values change significantly due to the reduced transmission
of the boron filter.
Figures 10.8 - 10.13
show the ECF for the PSPC if only the hard pulse height
channels are used for power law, thermal line and blackbody spectra for
various values of . Because the PSPC background is dominated
by soft events (cf., Chapter 7 ), sources with hard
spectrum can be more easily detected in the higher energy band;
the curves provided in
Figures 10.8 - 10.13
allow to assess (in conjunction with Figure 12.1 )
whether a restriction to the higher energies increases sensitivity or not.
The curves shown in
Figures 10.2 - 10.13
should be used as follows:
(in units of
erg cm s ) in the ROSAT pass band
0.1 - 2.4 keV assuming no interstellar absorption. and power laws and/or temperatures
read from Figures 10.2 - 10.7 the resulting energy-to-count
conversion factor ECF. by ECF to obtain the expected ROSAT
PSPC count rate.
Example 1:
The expected count rate from a coronal source with
ergs s at a distance of 50 pc and
cm is to be estimated.
The temperature of the source is assumed to be 
K.
With these parameters an unabsorbed flux of
ergs cm s is calculated. From
Figure 10.3 a ECF of 1.25 is estimated, hence
a count rate of 4.25 counts s is expected. Similarly, using the
boron filter, a ECF of 0.6 is found from
Figure 10.6 , and consequently the count rate is
expected to be 2.0 counts s . Note that no
corrections for dead time, vignetting or scattering out of the detect
cell have been applied. Therefore the curves in
Figures 10.2 - 10.7
apply only to on-axis observations of point sources with sufficiently large
detect cell sizes.
Example 2:
A power law source with photon index 1.7 and an intensity (at 1 keV) of
0.01 keV cm s keV is to be observed through an
absorbing column density of 
cm . With this spectrum
one obtains an unabsorbed flux of
ergs cm s in the pass band 0.1-2.4 keV.
From Figure 10.2 , one
finds a ECF of 0.55, hence a count rate of 2.4 counts s is expected.
Similarly, for the boron filter one calculates an expected count rate
of 1.2 counts s with a ECF of 0.28 taken from
Figure 10.5 .