2025ApJ...991...42K

Most X-ray pulsars in high-mass X-ray binary (HMXB) systems exhibit both global and local trends of spin acceleration (spin-up) and deceleration (spin-down). Moreover, decades-long monitored objects display even more general spin-up trends in their spin evolution and simultaneously demonstrate a decrease in the orbital period, so-called orbital decay. Although obvious, this general behavior leaves open the question of the energy source and the mechanism of angular momentum transfer that ensures the neutron star's (NS's) spin acceleration. We hypothesize that the observed spin acceleration of X-ray pulsars in HMXB (Roche lobe overflow and sgXRB) systems results from orbital angular momentum transfer via tidal interactions with their massive companions. To study these phenomena, we sample five well-studied objects with the longest observational history of their spin periods from the population of known Galactic persistent X-ray pulsars in HMXB systems. We obtain the spin frequency change from the observational data and estimate energy loses for orbital decay and pulsars' spin acceleration. We propose a new term of super-global spin-up trends, and show that their observable values do not exceed theoretical upper limits predicted within the scenario of orbital decay due to the tidal interaction between the NS and its massive companion.