A new way to probe extreme physics
When astronomers observe the universe at X-ray wavelengths, two types of astronomical objects dominate. Black holes, sitting at the centres of large galaxies, ferociously devour the surrounding material. On the other hand, in binary systems, a neutron star or a black hole strips off matter from its companion star. In both cases, however, gas forms a swirling disc around the very dense central object. Friction in the disc causes the gas to heat up and emit light with a peak in X-rays. Since their discovery in the 1960s, X-ray binaries have been studied intensively to understand accretion processes at work under extreme conditions. The EU-funded project EXTREMEPHYSICS (The slowest accreting neutron stars and black holes: New ways to probe fundamental physics) focused on X-ray binaries that are a hundred times fainter than ordinary sources. Researchers thought that these subluminous X-ray sources are special binary systems. When the project began, only a handful of very faint X-ray binaries persistently accreting at very low rates were known. Utilising the wealth of observations from X-ray satellites currently in orbit, the EXTREMEPHYSICS team discovered new systems and observed very faint X-ray transients. Our galaxy harbours many X-ray transients that are in a dim, quiescent state but occasionally experience bright X-ray outbursts. The low outburst luminosities of very faint X-ray transients observed were characteristic of very low mass accretion rates that challenged scientists' understanding of their evolution. Assuming that they were dealing with accreting neutron stars and black holes, scientists estimated the time average accretion rate of the transients. This is an important input parameter for binary evolution models that attempt to explain the nature of subluminous X-ray sources. Multi-wavelength studies showed that, at the lowest accretion rate, the gas flow geometry changes significantly, thus offering new input to accretion models. In addition, black holes behave differently than neutron stars at low accretion rates, mainly because black holes do not have surfaces. New insights into the crust structure of neutron stars in binary systems resulted in new theoretical paths to explore in the study of the heating and cooling of accreting neutron stars. In particular, observations on thermonuclear X-ray bursts are expected to lead to new theories tailored to very low accretion rates. With a broader sample of very faint X-ray sources and multi-wavelength observations, the EXTREMEPHYSICS team was successful in uncovering physical mechanisms that drive these powerful objects. To investigate further, data archives of X-ray satellites will need to be scrutinised for more sources of this type.
Keywords
Neutron stars, black holes, X-ray binaries, EXTREMEPHYSICS, mass accretion