X-Ray Astronomy

X-ray astronomy is an observational branch of astronomy dealing with the study of X-ray emission from celestial objects. X-ray astronomy is an achievement of the space age as the Earth’s atmosphere is completely opaque at photon energies beyond the ultraviolet region.

Like Gamma Ray Astronomy, the field of x-ray astronomy was pioneered by physicists. Herb Friedman, who detected the first solar coronal x-rays in 1949, had been working previously in ionospheric physics; Giacconi and Rossi, who led the group discovering Scorpius X-1 and the x-ray background in 1962, were nuclear and cosmic ray physicists.

In 1949 the first X-Rays from the solar corona were detected by a Geiger counter on a V-2 rocket. In 1962 the first x-ray source outside the solar system — Scorpius X-1 was discovered. With the same rocket experiment an apparently diffuse x-ray background was found. Since these early days x-ray astronomy has developed at an enormous pace. Today, we know more than 150 000 x-ray sources in the sky and they include almost all astrophysical species—from the nearby comets to the most distant quasars at the edge of the universe, from the tiny Neutron Stars to the clusters and super-clusters of galaxies as the largest physical formations in the cosmos. Some species radiate most of their power in x-rays, for instance black holes or neutron stars accreting matter from a binary companion, as well as supernova remnants and single, cooling neutron stars.

Many of the known objects shine in x-rays because they are hot—having temperatures of millions to billions of Kelvin. Other emission mechanisms are synchrotron radiation of extremely energetic electrons spiraling in magnetic fields or the inverse Compton  effect which occurs when high-energy electrons scatter at low-energy photons, e.g. from stellar light or from the 2.7 K cosmic background radiation. In any case, the emission of x-rays points to extreme physical conditions in the source region. Also, these x-rays carry information which is not available from observations in other spectral bands. In this article we shall give a brief summary of the development of x-ray astronomy, describe the evolution of experimental techniques and x-ray space missions and highlight some of the results which have had a major impact on astrophysics.

Most significant progress in x-ray astronomy came with the advent of satellite observatories. Their ancestor, the first satellite entirely devoted to x-ray astronomy, was

Uhuru. Launched in 1970 it was a spinning spacecraft with a simple, but very powerful, instrument package: an array of proportional counters of 840 cm² area working in the 2–20 keV band. It performed the first all-sky survey and located 339 objects, mostly X-Ray Binaries and supernova remnants, showing a strong clustering near the galactic plane.

Questions to Ponder

• What is the typical wavelength of X-ray used in X-Ray Astronomy?
• What are the advantages of X-ray astronomy over Radio Astronomy?