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 (alpha 10(-14) erg cm(-2) s(-1). If their N(S) continues to fainter fluxes, the hard sources will comprise similar to 40 per cent of sources with 5x10(-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.