It has been almost 40 years since Giacconi et al. (1962) discovered the X-ray Background (XRB), the first cosmic background detected. Nevertheless its origin is still not fully understood. At a flux limit of ∼ 10 −15 erg cm −2 s −1 70 −80 % o
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.