Soft X-ray variability and spectra of Seyfert galaxies.

Sixty-four optically-selected active galaxies were observed with the Einstein Observatory. The fifty detections constitute the largest single sample of Seyfert galaxies studied at X-ray energies. The authors conclude that dramatic intensity changes on time scales of a few hours are rare, and are more usual on time scales of weeks to a year. Most individual spectra are consistent with the canonical power law (α ≡ 0.7), but a few are steeper and the mean spectral index is higher. There is no strong trend for the absorbing column density of cold gas derived from the X-ray spectral fit to be systematically lower than the 21-cm-derived column through the interstellar medium of our galaxy, as would be the case if soft excesses appeared in the 0.25 keV band.