MG14 - Talk detail

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Ultra-luminous X-ray sources (ULX) are among the most extreme phases of binary evolution, characterized by X-ray luminosities exceeding the Eddington luminosity for compact objects formed via stellar evolution. Traditionally, two classes of models have been proposed in the literature for the formation of these systems: (i) under the assumption of spherical, Eddington-limited accretion, one class of models employs intermediate-mass black holes with masses exceeding ∼100 Msolar, and (ii) the second method employs some combination of thin accretion disks around stellar-mass black holes (~10 Msolar), whose luminosity can exceed the Eddington limit without being disrupted by radiation pressure, and/or anisotropic X-Ray emission The recent discovery of a neutron star accretor in the ULX M82 X-2 challenges our understanding of high-mass X-ray binary formation and evolution. Apart from the specific emission mechanism and the magnitude and influence of the neutron star's magnetic field, the measured orbital period (~2.5 days) and the lower limits on the donor mass (>5.2 Msolar) and radius (>5.2 Msolar) 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 ULX 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 days 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, or, in other words, their formation rate is ~0.03 yr/Msolar. This number is an order of magnitude lower compared to predictions in the literature for the formation rate of ULXs with black-hole accretors, which suggest a rate of ~0.2-1.0 yr/Msolar at solar metallicity. We note however, that for a neutron star ULX to be identified as such, the requirement of a highly magnetised neutron star, that allows the production of X-ray pulses, should be taken into account. The latter makes their identification much more difficult, which is consistent with the fact that only one such ULX has been observed todate.

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