Are accreting, highly magnetized neutron stars the engines of ultraluminous X-ray sources?

Accreting, highly magnetised neutron star.

Credit: NASA, Caltech-JPL


The following article is written by Dr. F. Koliopanos.


X-ray binaries (XRBs) are binary star systems in which one of the two stars is a relativistic object. A black hole or a neutron star. Due to the proximity of the two stars, the relativistic star gradually rips apart its companion (which is a “normal star similar to our Sun, albeit potentially more massive) and “consumes” its mass. As the material is transferred onto the black hole (or neutron star) it forms an accretion disk (figure 1) around it. This process heats the accreting material at temperatures exceeding 2,000,000 Kelvins, resulting in the emission of copious amounts of X-ray radiation. The luminosity of XRBs is millions of times higher than the luminosity of our sun. In fact, such is the power of their radiation that their luminosity is often close to the theoretically allowed luminosity. Called the “Eddington limit”, this limiting luminosity corresponds to such strong radiation that its pressure becomes higher than the pressure of the in-falling material. If the Eddington limit is surpassed the radiation becomes so powerful, that it expels the in-falling matter, essentially stopping accretion. The value of the Eddington luminosity, scales linearly with the mass of the black hole (or neutron star). In principle an XRB cannot be more luminous than its Eddington limit.

However, in the past decades, astronomers have detected XRBs that surpass the Eddington limit by tens or even hundreds of times. Dubbed “Ultraluminous X-ray Sources” (ULXs), these objects both fascinate and puzzle astrophysicists. Initially though to be powered the elusive intermediate mass black holes — which are thought to have masses of hundreds or thousands of times that of the Sun — ULXs are now understood as being powered by standard stellar mass black holes that appear to be accreting material well above the Eddington limit. Furthermore, the recent discovery of three pulsating ULXs (pulsations are a telltale sign of magnetized neutron stars), has established the fact that at least a few ULXs are actually powered by much less massive neutron stars, rather than black holes. This discovery has raised questions with regard to the currently prevailing paradigm of black holes powering ULXs

In a recent publication, astrophysicists revisited previously analyzed observations of 18 well known ULXs and demonstrated that the spectra of the sources — previously thought to be powered by black holes — are consistent with accretion onto highly magnetized neutron stars. Building on existing theoretical considerations, the authors showed that the strong magnetic fields of neutron stars, cannot only persistently maintain highly super-Eddington accretion, but can also help “mask” the true nature of the sources, which appear as non-pulsating black hole XRBs. The paper concludes that it is conceivable that not only the few pulsating sources, but the entire ULX catalog may be powered by neutron stars, rather than black holes. This analysis adds to an emerging body of theoretical and observational works that contest the current paradigm of black hole powered ULXs. If these findings are confirmed by further studies — and ULXs are found to be powered by neutron stars rather than black holes — this will question our understanding of the capacity of black holes to accrete matter at super-Eddington rates. Since super-Eddington accretion is one of the key requirement for the creation of the super massive black holes that lie in the center of most galaxies, such a conclusion will have dramatic implications.