On the Formation of Ultraluminous X-ray Sources with Neutron Star Accretors: the Case of M82 X-2

TL;DR

This study explains the formation of the neutron star ultraluminous X-ray source M82 X-2 through population synthesis and mass-transfer models, showing such systems are plausible and can last hundreds of thousands of years.

Contribution

It demonstrates that non-conservative mass transfer models can account for the formation of M82 X-2, providing new insights into neutron star ULX formation.

Findings

M82 X-2's parameters align with population synthesis predictions.

Such systems are expected in about 13% of similar star-forming galaxies.

The system is likely less than 50 million years old with a donor star of 8-10 solar masses.

Abstract

The recent discovery of a neutron star accretor in the ultra-luminous X-ray source M82 X-2 challenges our understanding of high-mass X-ray binary formation and evolution. By combining binary population synthesis and detailed mass-transfer models, however, we show that the binary parameters of M82 X-2 are not surprising provided non-conservative mass transfer is allowed. Specifically, the donor-mass lower limit and orbital period measured for M82 X-2 lie near the most probable values predicted by population synthesis models, and systems such as M82 X-2 should exist in approximately 13% of the galaxies with a star-formation history similar to M82. We conclude that the binary system that formed M82 X-2 is most likely less than 50 Myr old and contains a donor star which had an initial mass of approximately 8-10 M$_\odot$, while the NS's progenitor star had an initial mass in the $8-25\,\rm M_{\odot}$ range. The donor star still currently resides on the main sequence, and is capable of continued MT on the thermal timescale, while in the ultra-luminous X-ray regime, for as long as 400,000 years.