The ROSAT Wide Field Camera all-sky survey of extreme-ultraviolet sources – I. The Bright Source Catalogue

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Abstract
The first all-sky survey for cosmic sources of extreme-ultraviolet radiation has been carried out with the UK Wide Field Camera on ROSAT. A first reduction of the survey data has yielded a catalogue of 383 relatively bright EUV sources, forming the WFC Bright Source Catalogue. This represents a 30-fold increase in the number of astrophysical objects detected in the ~ 60–200 eV energy band and covers a flux range, in each of the two survey bands, of more than 2000. A search of the (typically ~ 1-arcmin) error circles of the WFC sources, using a variety of catalogues and the SIMBAD data base, has identified probable optical counterparts of ~ 73 per cent, including many active stars, white dwarf stars and a variety of other galactic and extragalactic objects. A follow-up programme of optical spectroscopy has since added further identifications, but some 13 per cent of the EUV sources remain unidentified. Details of the EUV source positions and count rates are given, together with optical identifications where known. Considerations of survey completeness allow source counts (log N–log S) to be derived for each survey band. It is found that the log N-log S distributions are unusually flat for the white dwarf stars, but almost Euclidean for the nearby main-sequence late-type stars. This is probably an effect of local ( ≲ 100 pc) interstellar absorption, since the more (EUV) luminous white dwarfs are potentially detected at correspondingly greater distances than the late-type stars. In addition, the sky distribution of identified white dwarfs is highly non-uniform, also suggesting gross variations in the opacity of the interstellar medium within ~ 100 pc.