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The origin of the cosmic X-ray background: bridging observations with theory

Final Activity Report Summary - THE XRB ORIGIN (The origin of the cosmic X-ray background: bridging observations with theory)

Since its discovery 40 years ago, the origin of the diffuse X-ray background radiation (XRB), that fills every direction on the Sky, remains a mystery. Models predict that the XRB is produced from the conglomeration of the X-ray emission of discrete sources, the vast majority of which are Active Galactic Nuclei (AGN) powered by accretion of material on a massive black-hole. Recent surveys using the largest X-ray telescopes have confirmed this fundamental prediction. However, the observed properties of the AGN are very different from those predicted by the models. This suggests that either some of the model parameters have to be modified and/or that the observations provide a twisted view of the real Universe because of selection effects that we still do not fully understand. During the EIF both these possibilities have been explored and it has been found that both are responsible for the apparent discrepancy between observations and the model predictions.

The backbone of the XRB synthesis models is the AGN unification scheme. In this picture the central massive black hole is surrounded by a donut-shaped torus of dust and gas clouds which absorb the photons emitted by the central engine. An observer will see an obscured AGN if the line of sight happens to intersect the torus. In this scenario the fraction of obscured AGN in the Universe depends only on the opening angle of the torus. Our work has shown that this paradigm has to be modified to include the effect of the energy output from the AGN. The more powerful the central engine - the smaller the size of the torus, and hence the lower the fraction of obscured sources -in broad agreement with the observations.

A complementary way of finding obscured AGN are infrared (3.6-160 microns) observations. The energy emitted at X-ray wavelengths is absorbed by the torus of dust and gas clouds and emerges thermally reprocessed in the infrared. Therefore, the combination of X-ray and infrared observations has the potential to provide a complete picture of AGNs, the prime constituents of the XRB, over a wide range of obscuring cloud densities. Infrared observations have indeed revealed a large population of AGN that are not detected at X-rays, most likely because the X-ray photons emitted by the central engine cannot penetrate the dense torus and are not visible to the observer.

The findings above, i.e. modification of the standard AGN paradigm and observational selection effects associated with very dense clouds in the vicinity of the massive black hole, can potentially fully reconcile the apparent discrepancy between model predictions and experiment.