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 <0.5) from a survey of 188 ROSAT fields. Such sources m ust be the dominant contributors to the X-ray background at faint fluxes. W e have used Monte Carlo simulations to verify that our technique is very ef ficient at selecting hard sources: the survey has greater than or equal to 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 typica l ROSAT hard source in our survey has a spectral slope alpha similar to0. T he hard sources have a steep number flux relation (dN/dS proportional toS(- gamma) with a best-fitting value of gamma =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 simi lar 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 coul d be the solution to the X-ray background spectral paradox.