Investigating binary systems with neutron stars in different accretion regimes

Binary systems with neutron stars (NSs) can display very different emission properties depending on the accretion regime and magnetic field. In this thesis, we focus on two extreme cases: Pulsating Ultraluminous X-ray sources (PULXs), with high accretion rates, strong magnetic fields and massive companions, and accreting millisecond pulsars (MSPs), with weaker fields, lower accretion rates and low-mass donors. Studying these systems provides insights into the evolutionary paths of NSs in binaries. For PULXs, which emit above the Eddington luminosity thanks to the suppression of opacity in strong magnetic fields (B ~ 10¹⁴ G), we developed a simplified emission model including disk and magnetospheric envelope contributions. By comparing simulations with observations of M51 ULX-7 and NGC 7793 P13, we constrained viewing geometries, pulsed fractions and disk temperatures, and showed that polarization measurements can provide key additional information. For transitional millisecond pulsars (tMSPs), we analyzed the long-term evolution of PSR J1023+0038, using optical (Aqueye+/Iqueye, 2021–2025) and X-ray (NICER, 2023–2025) data. We measured an increasing delay in the time of passage at the ascending node (Tasc), consistent with orbital expansion driven by non-conservative mass transfer. Simultaneous optical and X-ray observations revealed phase lags of 60–137 μs, supporting a common origin of the pulsations. We also report the detection of a short, intense optical pulse in February 2025, whose nature remains uncertain.