Search

Context. Ultra-luminous X-ray sources (ULXs) are sources observed to have extreme X-ray luminosities exceeding the Eddington limit of a stellar-mass black hole (BH). A fraction of ULXs show X-ray pulsations, which are evidence for accreting neutron stars (NSs). Theoretical studies have suggested that NSs, rather than BHs, dominate the compact objects of intrinsic ULX populations, even though the majority of the observed sample is non-pulsating, implying that X-ray pulses from many NS ULXs are unobservable. Aims. We simulate populations of X-ray binaries covering a range of starburst ages spanning from 5 to 1000 Myr with the aim of comparing the properties of observed ULXs at the different ages. Additionally, we compare two models describing different assumptions for the physical processes governing binary evolution. Methods. We used the new population synthesis code POSYDON to generate multiple populations of ULXs spanning multiple burst ages. We employed a model for geometrically beamed emission from a super-Eddington accretion disk in order to estimate the luminosities of ULXs. Following theoretical predictions for the alignment of the spin axis of an NS with the accretion disk due to mass transfer, we estimated the required mass to be accreted by the NSs in the ULX populations so that the alignment suppresses observable X-ray pulses. Results. While we find that the properties of ULX populations are sensitive to model assumptions, there are certain trends that the populations follow. Generally, young and old stellar populations are dominated by BH and NS accretors, respectively. The donor stars go from being massive H-rich main-sequence stars in young populations (< 100 Myr) to low-mass post-main sequence H-rich stars in older populations (> 100 Myr), with stripped He-rich giant donors dominating the populations at around 100 Myr. In addition, we find that NS ULXs exhibit stronger geometrical beaming than BH ULXs, leading to an underrepresentation of NS accretors in observed populations. Coupled with our finding that X-ray pulses are suppressed in at least 60% of the NS ULXs, we suggest that the observed fraction of ULXs with detectable X-ray pulses is very small, in agreement with observations. Conclusions. We show that geometrical beaming and the mass-accretion phase are critical aspects of understanding ULX observations. Our results suggest that even though most ULXs have accreting NSs, those with observable X-ray pulses would be very few. [ABSTRACT FROM AUTHOR]

We analyse the vertical distribution of high-mass X-ray binaries (HMXBs) in NGC 55, the nearest edge-on galaxy to the Milky Way (MW), based on X-ray observations by Chandra. Adopting a statistical approach, we estimate the difference between the scale height of the vertical distribution of HMXBs and the vertical distribution of star-forming activity between 0.33 and 0.57 kpc. The spatial offsets can be explained by a momentum kick the X-ray binaries receive during the formation of the compact object after a supernova explosion of the primary star. Determining the vertical distribution of HMXBs in the MW using Gaia DR2 astrometry, we find that the corresponding difference is considerably lower at 0.036 ± 0.003 kpc, attributed to the greater gravitational potential of the MW. We also calculate the centre-of-mass transverse velocities of HMXBs in NGC 55, using traveltime information from binary population synthesis codes and for different star formation histories (SFHs). For a flat SFH model (typical of spiral galaxies like NGC 55), we find that HMXBs are moving with a typical transverse velocity between 34 and 48 km s−1, consistent with space velocities of MW HMXBs. For an exponentially declining SFH model, HMXBs are moving at a velocity of 21 km s−1, consistent with the corresponding velocity of HMXBs in the Small Magellanic Cloud and Large Magellanic Cloud. Finally, we estimate the formation efficiency of HMXBs in NGC 55 at 299 |$_{-46}^{+50}$| (systems/M⊙ yr−1), consistent within the errors with the Magellanic Clouds but significantly higher than the MW, a difference that can be explained by the subsolar metallicity of NGC 55. [ABSTRACT FROM AUTHOR]

The formation and evolution of double neutron stars (DNS) have traditionally been studied using binary population synthesis. In this work, we take an alternative approach by focusing only on the second supernova (SN) forming the DNS and the subsequent orbital decay and merger due to gravitational wave radiation. Using analytic and numerical methods, we explore how different NS natal kick velocity distributions, pre-SN orbital separations, and progenitor He-star masses affect the post-SN orbital periods, eccentricities, merger times, systemic velocities, and distances travelled by the system before merging. Comparison with the set of 17 known DNSs in the Milky Way shows that DNSs have pre-SN orbital separations ranging between 1 and 44 |${\, \mathrm{R}_{\odot }}$|⁠. Those DNSs with pre-SN separations ∼1 |${\, \mathrm{R}_{\odot }}$| have merger time distributions that peak ∼10–100 Myr after formation, regardless of the kick velocity received by the NS. These DNSs are typically formed with systemic velocities ∼102 km s−1 and may travel ∼1–10 kpc before merging. Depending on progenitor mass of the second-born NS, the short merger time can account for the r -process enrichment observed in compact stellar systems such as ultrafaint dwarf galaxies. For Milky Way-mass galaxies only DNSs with the tightest pre-SN orbits and large kick velocities (≳102 km s−1) can escape. However, those DNSs that do escape may travel as far as ∼Mpc before merging, which as previous studies have pointed out has implications for identifying the host galaxies to short gamma-ray bursts and gravitational wave events. [ABSTRACT FROM AUTHOR]