TheROSAT survey of extreme UV sources: a new window on the universe

Abstract

This paper discusses the first all-sky suvey of cosmic extreme ultra-violet sources, discovered by theROSAT Wide Field Camera. Details of the instrument and the survey are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification program is described, and the major results summarized. I then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV obserevations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified with hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe & Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as “standard candles” in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. I conclude the paper with a discussion of the new magnetic cataclysmic variables discovered at SAAO, which have been the subject of intensive follow-up observations.