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- The expected count rate from a source with a blackbody spectrum (perhaps
a neutron star) needs to be estimated. The source has L
ergs s
in the ROSAT energy band (0.1 to 2.4 keV), and is
at a distance of 2 kpc with
cm
(log(
). The assumed temperature is equivalent to 0.1 keV.
The unabsorbed flux for this source is
ergs cm
s
, or in units of
the flux is
. From the Table 11.5
or
Figure 11.7
the
appropriate ECF is
, and the expected count rate is
counts s
. We estimate that a detection of this source
at the
level will require an observation time of about 4,000 s. - Consider a distant AGN with a power law spectrum of energy slope 0.5
which is at a luminosity distance of 1000 Mpc and is viewed through an
absorption column of
cm
(log(
).
The AGN luminosity in the ROSAT band is L
ergs s
.
We want to determine how long an observation is required for a
detection. The unabsorbed flux is
ergs cm
s
. The ECF for this spectral model is taken
from Table 11.3
or Figure 11.5
,
it is
, which gives a count rate
counts s
. Using the sensitivity formula
from § 11.2
, we calculate that the observation time needed
to detect this source at the
level is about 12,600 s. - What is the conversion factor from counts per second to energy flux for
a particular spectrum? For example, a Raymond-Smith Plasma with a
temperature corresponding to 1 keV and a column of
cm
(log(
). The ECF for this spectral model
is taken from Table 11.4
or Figure 11.6
,
it is
. That is, a flux of
ergs cm
s
in the ROSAT
energy band (0.1 to 2.4 keV) will give a count rate of
counts s
in the HRI. Inverting this relationship gives the
result that 1 count s
ergs cm
s
. In
general the conversion factor from counts per second to energy flux is
given by: 1 count s
/ECF ergs cm
s
.
Next: Spatial Resolution
Up: Expected Count Rates
Previous: Expected Count Rates
Michael Arida
Tue Jun 11 16:18:41 EDT 1996