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Ultra-luminous X-ray sources (ULXs) are sources observed to exceed theEddington limit of a stellar-mass black hole (BH). A fraction of ULX sourcesshow X-ray pulses which are evidence for accreting neutron stars (NSs).Theoretical studies have suggested that NSs dominate the compact objects ofintrinsic ULXs, even though the majority of observed sample is non-pulsating,implying that X-ray pulses from many NS ULXs are unobservable. We use POSYDONto generate and study X-ray binary populations spanning starburst ages 5 to1000Myr. Following theoretical predictions for the alignment of the NS spinaxis with the accretion disc, we estimate the required accreted mass in ULXs sothat the alignment suppresses observable X-ray pulses. While the properties ofULXs are sensitive to model assumptions, there are certain trends that thepopulations follow. Young and old stellar populations are dominated by BH andNS accretors, respectively. The donors go from massive H-rich main-sequence(MS) stars in young populations (<100Myr) to low-mass post-MS H-rich stars inolder populations (>100Myr), with stripped He-rich giant stars dominating thepopulations at around 100Myr. In addition, we find that NS ULXs exhibitstronger geometrical beaming than BH ULXs, leading to an under-representationof NS accretors in observed populations. Coupled with our finding that X-raypulses are suppressed in at least 60% of the NS ULXs, we suggest that theobserved fraction of ULXs with detectable X-ray pulses is very small, inagreement with observations. This study investigates the effects of age on ULXsas well as the effects of different model assumptions on ULX demographics. Weshow that geometrical beaming and the mass-accretion phase are critical aspectsof understanding ULX observations. Our results suggest that even though mostULXs have accreting NSs, those with observable X-ray pulses would be very few.

Exploring the nature of ultra-luminous X-ray sources across stellar population ages using detailed binary evolution calculations