Author(s): , , ,
Institution(s): 1. Northwestern University, 2. University of Chicago, 3. University of Geneva
Ultra-luminous X-ray Sources are among the most extreme phases of binary evolution, characterized by X-ray luminosities exceeding 1e39 erg/s (approximately the Eddington limit for a stellar mass black hole). 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 binary evolution in general. Apart from the specific emission mechanism and the magnitude and influence of the neutron star's magnetic field, the measured orbital period and the lower limits on the donor mass and radius are both telling and puzzling at first glance: (1) the high inferred accretion rate onto the neutron star requires that the donor is in Roche-lobe overflow, as wind-fed X-ray binaries with neutron star accretors are expected to have X-ray luminosities orders of magnitude below the ultr-luminous X-ray source range, (2) the donor star must be hydrogen rich, as a helium star with mass >5 Msolar cannot fill its Roche lobe in a 2.5 day period orbit, independent of its evolutionary stage, (3) the evolutionary mechanism must have either a long lifetime, or a high formation rate, in order for this one system to exist in the local universe.
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 Msolar, while the neutron star's progenitor star had an initial mass in the 8-25 Msolar 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.