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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 .

Tue Jun 11 16:18:41 EDT 1996