X-ray observations of active galaxies.

The present state of knowledge about activity in the nuclei of galaxies is reviewed with special reference to x-ray observations, from both an observational and a theoretical point of view. In particular, specific methods of confront ins theory with observation are outlined, and used in succeeding chapters to interpret observations made with the Sky Survey Instrument on Ariel V. The Ariel V satellite and the sky Survey Instrument are described in detail, as is the data analysis scheme employed. An x-ray light curve of the radio galaxy NGC 5128 covering ten years is presented, and shown to exhibit two characteristic variability timescales, a "glow outburst" over years and "flaring" over days. Observations of the giant radio lobes surrounding NGC 5128 are presented, showing extended x-ray emission which is interpreted as photons of the 30 K background Compton scattered to x-ray energies by the same relativistic electrons which produce the radio emission by synchrotron radiation. This allows the first determination of the magnetic field in an extended extragalactic radio source. Observations of "187show that x-rays are emitted not only from this active galaxy, but also from the Virgo cluster in which it is embedded. The cluster is elongated, reflecting the local galaxy distribution. A four year x-ray light curve of the Seyfert galaxy NGC 4151 is presented, and shown to exhibit frequent flaring on a timescale of days. An observation lasting 60 days is analysed in terms of a shot noise model for x-ray variations. Arguments are forwarded regarding the fuelling, efficiency of energy production, and size, of the x-ray source. Finally, a search for very fast infra-red and optical variations in the nucleus of NGC 4151 is described. None were found. Imlications for the source structure are briefly discussed.