Summary

This book documents the first major X-ray astronomy satellite, the Einstein Observatory (HEAO-2), and its revolutionary impact on astrophysics. Giacconi, the mission’s principal investigator, argues that observing the universe in X-rays—rather than just visible light—reveals high-energy phenomena invisible to optical telescopes, such as black holes, neutron stars, and supernova remnants. The book details the observatory’s design, including its grazing-incidence X-ray mirrors, and presents early results from its 1978-1981 mission. Key findings include the first X-ray images of supernova remnants (e.g., Cassiopeia A), detection of X-ray emission from normal stars, and evidence for hot gas in galaxy clusters. Readers gain a concrete understanding of how X-ray astronomy opened a new observational window, transforming knowledge of cosmic processes like accretion onto compact objects and the evolution of galaxies.

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Key concepts

Grazing-incidence X-ray mirrors — Mirrors that reflect X-rays at very shallow angles to focus them, enabling the first high-resolution X-ray images of celestial sources.
Accretion-powered X-ray sources — Binary systems where a compact object (neutron star or black hole) pulls matter from a companion star, heating it to millions of degrees and emitting X-rays.
Supernova remnant X-ray emission — X-rays produced by shock waves from exploded stars heating surrounding gas to millions of Kelvin, revealing the composition and dynamics of the remnant.
Hot intracluster medium — Diffuse, X-ray-emitting gas at temperatures of 10-100 million Kelvin filling the space between galaxies in clusters, providing evidence for dark matter’s gravitational binding.
X-ray background — A diffuse, uniform X-ray glow across the sky, later resolved into distant active galactic nuclei, which the Einstein Observatory began to identify as discrete sources.