SIMBAD references

We present a catalogue of 147 serendipitous X-ray sources selected to have hard spectra (α<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.cm^–2.s^–1. 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 α∼0. The hard sources have a steep number flux relation (dN/dS∝S^–γ with a best-fitting value of γ=2.72±0.12) and make up about 15per cent of all 0.5-2keV sources with S>10^–14erg.cm^–2.s^–1. If their N(S) continues to fainter fluxes, the hard sources will comprise ∼40per cent of sources with 5x10^–15<S<10^–14erg.cm^–2.s^–1. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S<10^–14erg.cm^–2.s^–1. 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.