The existence of a diffuse XRB was discovered more than 40 years ago . The aim of the experiment was to measure the X-ray emission from the Moon, but the data showed a strong X-ray source about 30° away from the Moon (Sco X-1) and a diffuse emission approximately constant from all the directions observed during the flight. In 2002, Riccardo Giacconi received the Nobel prize for this discovery, among others.
Improvements in our understanding of the XRB have been achieved with the first all-sky surveys (Uhuru and Ariel V) at the beginning of the seventies. The high degree of isotropy revealed by these surveys immediately led to realize that the origin of the XRB has to be mainly extragalactic. Active galactic nuclei (AGNs) were already considered the most likely candidates for the production of the XRB .
In the same years a number of experiments were set up to measure the spectrum of the XRB over a large energy range. At the beginning of the eighties, the HEAO-1 data showed that in the energy range 3-50 keV the shape of the XRB is very well fit by an isothermal bremsstrahlung model corresponding to an optically thin, hot plasma with kT of the order of 40keV . It was also shown that essentially all Seyfert 1 galaxies with reliable 2-20 keV spectra were well-fit by a single power law with an average spectral index of the order of 0.65, significantly different from the slope of the XRB in the same energy range , the so called "spectral paradox," which was originally taken as additional support of the hypothesis, that the XRB may be of diffuse thermal origin.
Observations with the Einstein Observatory were painting a different picture: pointed observations of previously known objects very soon showed that AGNs, as a class, are luminous X-ray emitters . Deep Einstein surveys showed that about 20% of the soft XRB (1-3 keV) is resolved into discrete sources [22,26] at fluxes of the order of a few x 10~14ergcm~2 s_1. A large fraction of these faint X-ray sources were identified with AGNs. Subtraction of the flux already resolved into discrete objects from the total XRB spectrum would destroy the apparent thermal nature of the X-ray background spectrum .
The debate between the supporters of the discrete source vs. thermal plasma hypothesis continued until the final resolution of the controversy came from the very convincing results obtained with the FIRAS instrument on board COBE: the absence of any detectable deviation from a pure black body of the cosmic microwave background set an upper limit to the comptonization parameter y < 2.5 x 10~5, much smaller than the value required by the hot intergalactic gas model , showing that a uniform, hot intergalactic gas produces at most 10~4 of the observed XRB .
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