The CXB covers a range of photon energies, with a peak around 40 kiloelectron volts (keV). The soft X-ray part of the background - photon energies between about 0.5 and 2 keV - was extensively studied with ROSAT satellite, and most of it has been resolved into individual objects, mostly distant active galactic nuclei (AGN) that contain a massive black hole at their centre. However, these sources cannot explain the whole of the CXB. In particular they would result in a flat rather than peaked background spectrum.

Now observations with the Chandra satellite, which was launched last year, have detected a large number of sources which can account for three-quarters of the hard X-ray background (between about 2 and 10 keV), and which also match the overall shape of the background spectrum. Most of these sources can be matched unambiguously with either active galactic nuclei or faint optical sources. The latter are likely to be either dust enshrouded active galactic nuclei or extremely distant quasars.

The next step is to combine the Chandra results with data from other telescopes - including the recently launched XMM Newton X-ray telescope, and other telescopes that are still on the drawing board - to achieve what the Chandra team call "a complete cosmic census of the population of galaxies and active galactic nuclei."