In the X-ray band, as in the radio region, one studies virtually every type of astronomical system. Both channels are good indicators of non-thermal activity, and of the beginnings and ends of the lifecycle of stars and massive black holes. In the early days of X-ray astronomy, when locations of sources were no more precise than the order of a square degree, the coincidence of a strong radio source in the region served as an argument for the X-ray identification, for example, M87 (Bradt 1967) and 3C 273 (Bowyer 1970).
As a theme for this review, I will concentrate on systems where the X-ray and radio observations are telling us about the outflow of material and energy. This is in contrast to what might be called "classical" X-ray astronomy, which studied accretion processes. Infall of gas not only explained the energy source, but also led directly to the calculations of X-ray emission from disks in galactic neutron star, black hole, and white dwarf binaries. Accretion onto supermassive black holes was inferred to power quasars and Active Galactic Nuclei (AGN). In these cases the X-ray and non-thermal optical continuum are most closely related. If accretion disks were the only setting for X-ray emission, there might be few X-ray and radio connections, as the peak radiation frequency corresponds to the temperature in the innermost disk, and therefore does not span the very broad range from 109 Hz radio emission to 1018 Hz X-ray emission.
Figure 1 provides examples of systems representing the three topics I will discuss: pulsars and supernova remnants; cooling flows in clusters of galaxies; and jets in quasars and radio galaxies.
Figure 1. Chandra images of examples of the systems discussed in this review. Credits: NASA/MSFC/SAO/CXC (top row, middle row, left 3 on bottom row); Corbel 2002 (bottom row right).
In clusters of galaxies, the X-ray emission from the hot gas filling the volume between the galaxies was shown to have temperatures consistent with the gravitational potential of the cluster. In a substantial fraction of clusters, the gas was observed to be sufficiently dense that it would cool in much less than a Hubble time, and it was interpreted that massive cooling flows involving hundreds of solar mass per year were condensing onto the cluster centers (Arnaud (1988), Fabian (1994)). This created a great puzzle, as neither the destination of the cooling gas, nor great quantities of gas at temperatures less than 1 to 2 keV have been found. It now appears that powerful radio sources in cD galaxies in the cluster cores (Burns (1990)) provide the energy to offset the cooling.
In contrast to accretion, a major theme of radio astronomy has been the origin of cosmic rays, the acceleration of particles to ultra high energies, and the transport of energy in jets to distances of pc to Mpc away from the nuclei of active galaxies. Radio astronomy has been primarily an imaging rather than spectroscopic science (with some important exceptions which we have heard at this symposium). In this article I therefore emphasize X-ray imaging. For decades radio observations have studied detailed structure in supernova remnants, emission from cluster of galaxies and sources in clusters, and jets in active galaxies. With the half-arcsecond X-ray imaging now available from the Chandra X-ray Observatory, we can finally compare X-ray with GHz radio observations on the same angular scales.
We adopt a flat, accelerating cosmology with Hubble constant H0 = 70 km s-1 Mpc-1, m = 0.3, and = 0.7.