EXOSAT and Ginga observations of NGC 4151.

The archetypal Seyfert galaxy NGC 4151 was monitored over twenty times by EXOSAT in the period July 1983 to March 1986, and more recently has been observed several times by Ginga. The observations are interpreted in terms of the so-called warm absorber model in which the opacity of the line-of-sight gas decreases as the central X-ray source brightens due to an increasing degree of photoionization of the absorbing gas.