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Abstract:
X-ray astronomy is a relatively young field, its date of birth (1962) still alive in the memory of many pioneers of space astronomy. Early experiments used collimators to channel X-rays onto large gas-filled detectors. Focusing X-ray optics later enabled a leap in sensitivity, culminating in the flagship observatories such as NASA’s Chandra and ESA’s XMM-Newton that dominated the landscape of high-energy astrophysics in the first part of this century.
What kind of sources do these observatories reveal? X-rays trace physics in extreme astronomical sources. Most stars are relatively weak in X-rays, so we see only examples that are particularly active, or those that are being slowly consumed in a binary system by a companion white dwarf, neutron star or black hole. Outside the Milky Way, the most populous sources of X-ray emission are not galaxies, but the supermassive black holes bubbling away at their centres, whose growth may strongly influence their host’s formation and subsequent evolution. The majority of the feeble X-ray sky background is produced by the ensemble of these accreting black holes throughout cosmic time.
X-rays also offer the potential to measure the dark components that apparently dominate our cosmos. The primary emission of the hot, tenuous gas that permeates clusters of galaxies is in the X-ray band. In the current cosmological model, clusters signpost the largest concentrations of dark matter in the Universe, forming over time by aggregation from smaller galaxies and groups. Their number density as a function of mass and redshift therefore depends on the competition between the gravitational pull of dark matter and expansionary effects of dark energy. The evolution of the X-ray cluster mass function has been used successfully to measure cosmological parameters, including dark energy, in a way that is highly complementary to the measurements based on the Cosmic Microwave Background and other techniques. The possible tensions between orthogonal precision cosmological measurements are perhaps the best way to seed new physical ideas and models.
The sample sizes used in X-ray cosmology studies range from a few tens to a few hundreds of objects. The limited field of view of many X-ray telescopes makes it difficult to map large volumes of the Universe in a reasonable amount of time. A notable exception was the ROSAT all-sky survey, performed over six months in 1990, which discovered more than 10% of all X-ray sources known today, yielding — among many other riches — large samples of X-ray galaxy clusters that are still used for cosmological studies today. To take the next step in X-ray cosmology, we need a combination of ROSAT’s all-sky scanning strategy, with the kind of sensitivity enabled by the technology of more modern X-ray telescopes such as XMM-Newton and Chandra.
