To solve the origin of the hard XRB, a long standing puzzle in X-ray astronomy, was one of the primary scientific goals to be addressed with ASCA.
From deep surveys, they resolved 35% and 80% the soft XRB into discrete sources.
1] in the 2-10 band, resolving 20-26% of the 2-10 keV XRB.
Soon after the launch, Chandra resolved >75% of the hard XRB above 2 keV into discrete sources.
It is seen that a large fraction (~80%) of the XRB below 10 keV is now clearly resolved into faint, individual sources, mostly AGNs.
In other words, the cosmological evolution of AGNs that constitute a major part of the XRB below [approximately equal to] 10 keV, including the dominant type-2 populations, is now revealed.
The combination of the XLF and absorption functions enables us to construct a so-called "population synthesis model" of the XRB, (53,71,75-77) where the contributions to the XRB spectrum from AGNs with different luminosities, redshifts, and absorptions are quantitatively described.
2014) (12) present the most up-to-date XRB synthesis model based on their XLF and absorption function of AGNs.
At the same time, most of the XRB origin below [approximately equal to] 10keV is now quantitatively explained by superposition of these AGNs.
The spectrum of the XRB gives a boundary condition; in this model, the number ratio of Compton-thick AGNs to Compton-thin absorbed AGNs is constrained to be 50%--160% within an uncertainty in the XRB intensity in the 20-50keV band.
The NuSTAR and ASTRO-H missions will resolve ~30% of the hard XRB into individual AGNs, providing us with new insights on Compton thick populations.