ROSAT Hard-Spectrum X-Ray Source Catalog

Such sources must be the dominant contributors to the X-ray background at faint fluxes. The authors have used Monte Carlo simulations to verify that their technique is very efficient at selecting hard sources: the survey has >= 10 times as much effective area for hard sources as it has for soft sources above a 0.5 - 2 keV flux level of 10^-14 erg cm^-2 s^-1.

The distribution of best-fitting spectral slopes of the hard sources suggests that a typical ROSAT hard source in this survey has a spectral slope Alpha ~ 0. The hard sources have a steep number-flux relation (dN/dS ~ _S^-Gamma, with a best-fitting value of Gamma = 2.72 +/- 0.12), and make up about 15% of all 0.5 - 2 keV sources with S > 10^-14 erg cm^-2 s^-1. If their N(S) continues to fainter fluxes, the hard sources will comprise ~ 40% of sources with 5 x 10^-15 < S < 10^-14 erg cm^-2 s^-1.