2015ApJ...802L...5F

The recent discovery of a neutron star accretor in the ultra-luminous X-ray (ULX) 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_☉, while the NS's progenitor star had an initial mass in the 8-25 M_☉ range. The donor star still currently resides on the main sequence, and is capable of continued MT on the thermal timescale, while in the ULX regime, for as long as 400,000 yr.