The softest X-ray sources in the ROSAT pointed catalog: WGACAT

Abstract

We present a selection of X-ray sources with the softest observed spectral characteristics and discuss their nature. These "ultrasoft" sources were detected in 3 years of pointings with the ROSA T. These sources are predominantly found to be white dwarfs, magnetic cataclysmic variables, nearby late-type stars, and very X-ray soft active galactic nuclei. The broadband colors of the unidentified ultrasoft sources suggest that a number of them may have at least two components in their spectrum, as for example, is found in most cataclysmic variables. Some of these objects may represent the extreme of their respective populations, for example, unusual galaxies, "supersoft" sources with steep low-energy spectrum and high X-ray luminosity (~10³⁸ ergs s⁻¹), active galactic nuclei, stars, etc. Follow-up studies in the optical and other wavebands are needed.