X-ray background

Lore & Background

In 1962, a project led by Riccardo Giacconi, Herbert Gursky, Frank R. Paolini, and Bruno B. Rossi used Geiger counters mounted on a rocket to detect a 'diffuse background radiation' of X-ray flux spanning all directions covered by the rocket's rotation. This X-ray background flux was understood to consist primarily of X-rays with energies in the 2-10 keV range. Analysis showed that the flux originated from outside the solar system, but it was unclear whether such sources are galactic or extragalactic.

This background X-ray flux was later characterized as an isotropic background of hard X-rays (with energies greater than 2 keV) of extragalactic origins, while other rocket-based studies detected an anisotropic X-ray flux of energies around 0.2 keV, believed to be emitted by sources within the Milky Way. With improved angular resolution and longer observation times from X-ray satellite missions since the 1970s, the nature of both components became better understood.

Observations from the Einstein Observatory revealed that the cosmic X-ray background originates from discrete distant sources, later confirmed to be active galactic nuclei (AGN). The galactic component was surveyed by ROSAT, which revealed resolved structures of galactic X-ray-emitting sources. Chandra and XMM-Newton further showed that around 80-90% of CXB emissions in soft X-rays (0.5-2 keV) and 70-80% in hard X-rays (2-10 keV) could be attributed to resolved point sources such as AGN, quasars, and thermal bremsstrahlung emissions from galaxy clusters.

Reader's Guide

The X-ray background is significant as a fundamental observational feature of the X-ray sky, comprising both a galactic component (below 0.3 keV) and a cosmic component (above 0.3 keV). Its discovery in 1962 by Giacconi and colleagues marked a milestone in X-ray astronomy, revealing a diffuse flux that initially posed questions about its origin. Subsequent missions, including the Einstein Observatory, ROSAT, Chandra, and XMM-Newton, resolved the cosmic X-ray background into discrete sources, primarily active galactic nuclei, quasars, and galaxy clusters, accounting for the vast majority of its emission. The galactic component, mapped by ROSAT, arises from structures within the Milky Way. The study of the X-ray background has thus advanced understanding of both galactic and extragalactic X-ray sources, and its unresolved fraction continues to drive research into faint or diffuse X-ray populations. Its legacy includes informing models of supermassive black hole growth and the large-scale structure of the universe.

Did You Know?

• The X-ray background was first detected in 1962 by a rocket-borne experiment led by Riccardo Giacconi, Herbert Gursky, Frank R. Paolini, and Bruno B. Rossi.
• The cosmic X-ray background above 0.3 keV is thought to result from a combination of numerous X-ray point sources outside the Milky Way.
• The galactic component of the X-ray background, below 0.3 keV, was surveyed by ROSAT, which revealed resolved structures of galactic X-ray-emitting sources.

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