Astronomical X-ray spectroscopy—ten years on

Abstract

In a wide-ranging review in 'Vistas' ten years ago, Holt (1980) described the impact that relatively crude spectroscopic techniques had already had in clarifying the physical processes occurring in cosmic X-ray sources. The broad-band, low resolution spectra from proportional counter instruments on the Ariel-5, OSO-8 and HEAO-I satellites had been able to establish galaxy clusters as powerful thermal sources of x-radiation, and to yield information on the temperature, composition and quantity of the emitting gas. The same instruments had demonstrated the complex continuum spectra emitted by optically thick plasmas in a variety of X-ray binary systems. At that time most of the more detailed spectra had come from the Einstein Observatory Solid State Spectrometer (SSS), with an energy resolution in the I-I0 keV band a factor of 3-10 better than the proportional counter. As Holt (1980) described, the SSS spectral resolution was sufficient to reveal characteristic thermal emission lines in the X-ray spectra of several bright stars and supernova remnants. Even the SSS resolution, however, left the X-ray spectra of the most powerful and enigmatic cosmic sources, the quasars and Seyfert galaxies (generically, hereafter, active galaxies or AGN) as 'featureless' power law continua.