The cosmic microwave background is one of the key pieces of evidence for the Big Bang model of cosmology. However, it is just one of four cosmic backgrounds. The infrared and optical backgrounds have only been discovered recently, but the cosmic X-ray background (CXB) was actually discovered before the more famous microwave version. Surprisingly, perhaps, there are elements of the CXB that astrophysicists still do not understand, although recent results from the Chandra X-ray Observatory will go a long way to improving our knowledge of the X-ray background (R F Mushotzky et al. 2000 Nature 404 459).
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.”
