A survey of hard spectrum ROSAT sources – I. X-ray source catalogue

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 0.5–2 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 our survey has a spectral slope α ～0. The hard sources have a steep number flux relation (d N /d S ∝ S ^{? γ} with a best-fitting value of γ =2.72±0.12) and make up about 15 per cent 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 per cent of sources with 5×10^?15< S <10^?14 erg cm^?2 s^?1. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S <10^?14 erg 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.