Peeking into the obscured Universe

The question of whether the CXB is produced by uniform emission from a diffuse intergalactic gas at a temperature of a few million degrees, or by radiation from many individual sources, was

settled in the 1990s. First, the COBE satellite did not detect a tiny deviation from the perfect 2.7 K blackbody spectrum of the cosmic microwave background (CMB; Fig. 1), which would be

expected if the radiation were seen through a screen of hot intergalactic gas. Second, the majority of the ‘soft’ X-ray background (1 × 1017–5 × 1017 Hz) was resolved into discrete sources

by the X-ray satellite ROSAT. These sources are mostly distant active galaxies (active galactic nuclei, or AGN), containing massive black holes accreting matter from their environment4.

The X-ray background should represent the summed emission of all these discrete sources, but the CXB has a spectral shape completely different from the spectra of individual AGN measured at

X-ray frequencies. The spectrum of the CXB has a characteristic peak at about 1019 Hz (Fig. 1), corresponding roughly to the radiation that medical doctors use to ‘X-ray’ our bones, whereas

the AGN have a flat or slightly concave distribution over the whole X-ray range. A solution to this long-standing puzzle was proposed in 1989, namely that most sources of the X-ray

background could be heavily obscured by gas and dust clouds5, much like our bones absorb the doctor's X-rays. The higher-energy (‘hard’) X-rays can penetrate the obscuring clouds, whereas

‘soft’ photons are absorbed, so that the overall spectrum is different from that of individual AGN.

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