VizieR Online Data Catalog: Hard spectrum ROSAT sources. I. (Page+, 2000)

We present a catalogue of 147 serendipitous X-ray sources selected to have hard spectra ({alpha}<0.5) from a survey of 188 ROSAT fields. Such sources must be the dominant contributors to the X-ray background at faint fluxes. We have used Monte Carlo simulations to verify that our 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 0.5-2keV flux level of 10^-14erg/cm²/s. The distribution of best-fitting spectral slopes of the hard sources suggests that a typical ROSAT hard source in our survey has a spectral slope {alpha}~0. The hard sources have a steep number flux relation (dN/dS_{mu}S^-{gamma} with a best-fitting value of {gamma}=2.72+/-0.12) and make up about 15 per cent of all 0.5-2keV sources with S>10^-14erg/cm²/s. If their N(S) continues to fainter fluxes, the hard sources will comprise ~40 per cent of sources with 5x10^-15-14erg/cm²/s. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S<10^-14erg/cm²/s. They probably make a strong contribution to the X-ray background at faint fluxes and could be the solution to the X-ray background spectral paradox.

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