3.3.1 Blackbody spectrum example

The expected count rate from a source with a blackbody spectrum (perhaps a neutron star) needs to be estimated. The source has L _x = 10³³ ergs s^-1 in the ROSAT energy band (0.1 to 2.4 keV), and is at a distance of 2 kpc with N _H = 1 x 10²¹ cm^-2(log(N _H) = 21). The assumed temperature is equivalent to 0.1 keV. The unabsorbed flux for this source is F_{S, 0} = 2.09 x 10^-12 ergs cm^-2 s^-1. From Table 11 or Figure 33, the appropriate ECF is 5.63 x 10⁹, and the expected count rate is 0.012 ct s^-1. We estimate that a detection of this source at the 5$\sigma$ level will require an observation time of about 4,000 seconds.