What are ultraluminous X-ray sources?


Black holes represent the densest form of matter in the universe, and yet are they are simple objects possessing only mass and spin. Studies of many galactic systems, containing these objects, have show that the appearance of their accretion flow should be determined by the parameters associated with black holes, together with their mass accretion rate. This has also been found to be the case for their super massive cousins observed in the centre of galaxies. However, ~30 years ago bright non-nuclear point-like sources were discovered during surveys of other galaxies. Their luminosities (L_X > 10^39 erg/s) exceed the Eddington limit for accretion on to a stellar-mass black hole (M_BH = 3 - 20 M_sun) radiating isotropically. The Eddington limit is the luminosity (set by the objects mass) at which the force of outward radiation would be equal to the force of gravity pulling material in. If a source was to exceed this, theory predicts that radiation pressure would dominate, potentially destroying the accretion disc. The position of these bright objects within host galaxies - along with dynamical friction arguments – also rules out super-massive black holes (M_BH >10^6 M_sun) as a possible source of emission. If the black hole contained within these systems were this massive, it would sink to the centre of the galaxy ULXs are intermediate in luminosity between SMBHs and StMBHs, so an attractive supposition is that they contain intermediate-mass black holes (IMBHs, M_BH = 10^2 – 10^4 M_sun). If this were the case it would have significant implications on our understanding of the formation / evolution of super-massive black holes and galaxies. The alternative is that we are observing stellar mass black holes that are either exceeding the Eddington limit, by undergoing some form of super-Eddington accretion or emitting non-isotropically (e.g. geometric beaming, relativistic beaming). 

See Gladstone (2010) for a more detailed scientific discussion of observational heritage of these systems and how these have developed our understanding of ULXs.

X-ray analysis of ULXs

Early spectral studies of these bright sources, with the current generation of X-ray telescopes, fit the data with a standard canonical disc plus power-law continuum model, a simple model that has been used extensively with X-ray binary systems in our Galaxy to describe accretion in standard states. The resultant fits indicated the presence of a remarkably cool disc (kT ~ 150 eV) temperature, implying a MBH ~ 2x10^4 M_sun. However, recent results have also shown the presence of a spectral break above ~ 3 keV, a feature that is not present in the standard accretion states. Re-evaluation of ULX spectra also revealed that they could be equally well explained by models that infer extreme accretion rates onto a stellar mass black holes (< 100 M_sun). Either option is intriguing and so the nature of these new ultraluminous X-ray sources (ULXs) was the focus of much debate.

Optical & multi-wavelength analysis of ULXs

Multi-wavelength studies of ULX locations have found that they are preferentially located in actively star-forming galaxies, along with starburst, interacting/merging and dwarf galaxies preferably with low metallicities. An observed correlation between the number of ULXs and the global star-formation rate in spirals has also been noted. Such an association may indicate that the companion is a high mass star.

There have been many searches for optical counterparts of ULXs, searching for high mass donor stars. To date, these studies have focused mainly on nearby systems (< 10 Mpc), with individual counterparts observed over the range M_V ∼ 22 - 26 magnitudes. Many of these are found to be blue, suggesting an O or B star in a high mass X-ray binary, or that we are observing reprocessed emission from the accretion disc. These high mass stars would provide a natural origin for the high mass transfer rates required to power the observed luminosities of ULXs.

Optical spectroscopy is the newest frontier in the study of ULXs. The analysis of their source spectra show that the emission is non-stellar, with the majority of emission features being associated with the surrounding nebula. With the highest quality data, it is possible to see evidence for the He II high excitation line at 4686 angstrom. This line has long been associated with emission from the accretion disc, and has been used to gain dynamical mass estimates of Galactic black holes. Such studies of ULXs may yet bring an end to the debate over the mass of the black holes contained within these systems.