Nature of the Extreme Ultraluminous X-ray Sources

TL;DR

This study shows that binary systems can produce extreme ultraluminous X-ray sources with luminosities exceeding 10^42 erg/s through high mass transfer rates, challenging traditional limits and suggesting such sources may be short-lived phases.

Contribution

It introduces a proof-of-concept model demonstrating that high mass transfer in binaries can produce extreme ULXs, including those with neutron stars, beyond classical Eddington limits.

Findings

High mass transfer rates can produce ULXs with Lx>10^42 erg/s.

Both black hole and neutron star binaries can become extreme ULXs.

Such systems are likely short-lived phases in binary evolution.

Abstract

In this proof-of-concept study we demonstrate that in a binary system mass can be transferred toward an accreting compact object at extremely high rate. If the transferred mass is efficiently converted to X-ray luminosity (with disregard of the classical Eddington limit) or if the X-rays are focused into a narrow beam then binaries can form extreme ULX sources with the X-ray luminosity of Lx>10^42 erg/s. For example, Lasota & King argued that the brightest known ULX (HLX-1) is a regular binary system with a rather low-mass compact object (a stellar-origin black hole or a neutron star). The predicted formation efficiencies and lifetimes of binaries with the very high mass transfer rates are large enough to explain all observed systems with extreme X-ray luminosities. These systems are not only limited to binaries with stellar-origin black hole accretors. Noteworthy, we have also identified such objects with neutron stars. Typically, a 10 Msun black hole is fed by a massive (10 Msun) Hertzsprung gap donor with Roche lobe overflow rate of 10^-3 Msun/yr (2600 MEdd). For neutron star systems the typical donors are evolved low-mass (2 Msun) helium stars with Roche lobe overflow rate of 10^-2 Msun/yr. Our study does not prove that any particular extreme ULX is a regular binary system, but it demonstrates that any ULX, including the most luminous ones, may potentially be a short-lived phase in the life of a binary star.