The origin of pulsating ultra-luminous X-ray sources: Low- and intermediate-mass X-ray binaries containing neutron star accretors

Abstract

Context. Ultra-luminous X-ray sources (ULXs) are those X-ray sources located away from the centre of their host galaxy with luminosities exceeding the Eddington limit of a stellar-mass black hole (LX > 1039 erg s−1). Observed X-ray variability suggests that ULXs are X-ray binary systems. The discovery of X-ray pulsations in some of these objects (e.g. M82 X-2) suggests that a certain fraction of the ULX population may have a neutron star as the accretor. Aims. We present systematic modelling of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs; donor-star mass range 0.92–8.0 M⊙ and neutron-star accretors) to explain the formation of this sub-population of ULXs. Methods. Using MESA, we explored the allowed initial parameter space of binary systems consisting of a neutron star and a low- or intermediate-mass donor star that could explain the observed properties of ULXs. These donors are transferring mass at super-Eddington rates while the accretion is limited locally in the accretion disc by the Eddington limit. Thus, our simulations take into account beaming effects and also include stellar rotation, tides, general angular momentum losses, and a detailed and self-consistent calculation of the mass-transfer rate. Results. Exploring the initial parameters that lead to the formation of neutron-star ULXs, we study the conditions that lead to dynamical stability of these systems, which depends strongly on the response of the donor star to mass loss. Using two values for the initial neutron star mass (1.3 M⊙ and 2.0 M⊙), we present two sets of mass-transfer calculation grids for comparison with observations of NS ULXs. We find that LMXBs/IMXBs can produce NS-ULXs with typical time-averaged isotropic-equivalent X-ray luminosities of between 1039 and 1041 erg s−1 on a timescale of up to ∼1.0 Myr for the lower luminosities. Finally, we estimate their likelihood of detection, the types of white-dwarf remnants left behind by the donors, and the total amount of mass accreted by the neutron stars. Conclusions. We show that observed super-Eddington luminosities can be achieved in LMXBs/IMXBs undergoing non-conservative mass transfer while assuming geometrical beaming. We also compare our results to the observed pulsating ULXs and infer their initial parameters. Our results suggest that a large subset of the observed pulsating ULX population can be explained by LMXBs/IMXBs in a super-Eddington mass-transfer phase.

Using radial velocities to reveal black holes in binaries: A test case

“…We note that the binary mass ratio is relatively large (∼1.6−1.9). This value approaches (but does not exceed) the critical mass ratio for unstable mass transfer from intermediate-mass donor stars with convective envelopes, as computed in recent simulations (Misra et al 2020). In addition, the asymmetry of the light curve seen at phase 0.8−0.9 (Fig.…”

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Chandra Probes the X-Ray Variability of M51 ULX-7: Evidence of Propeller Transition and X-Ray Dips on Orbital Periods

“…Small beaming is also consistent with the detection of pulsations in ULXPs, as large beaming factors would otherwise result in very small pulsed fractions (Mushtukov et al 2021). The above suggests that strong beaming is not needed for ULXPs, and this should be considered in the framework of ULX population synthesis (e.g., Misra et al 2020;Abdusalam et al 2020;Kuranov et al 2020) and perhaps gravitational wave progenitors, which are thought to go through a ULX phase (Marchant et al 2017).…”

A new candidate pulsating ULX in NGC 7793

“…Misra et al 2020, and references therein). Indeed, for a neutron star ULX with a luminosity about 10 times Eddington, the most likely system is an intermediate mass X-ray binary with a period of the order of days (Misra et al 2020).…”