The Soft X-ray Spectra of Active Galactic Nuclei

EXOSAT LE and ME data on Seyfert 1 galaxies and quasars are reviewed: the observations indicate that a strong flux of soft X-rays, in excess of the power-law continuum that extends over most of the spectrum, is a common feature of the emission of active galactic nuclei (AGN). This low-energy X-ray spectral component, which is often identified with the emission of an optically thick accretion disc surrounding a central massive black hole, is highly variable, with changes in flux by more than a factor of ten over a few months. This new result and the observation of spectral variability correlated with the changes in flux give clues to the mechanisms of X-ray production in AGN and to the structure of the innermost parts of the accretion disc in these objects.