Accretion-induced spin-up: Implications for mass constraints of AMXPs

Abstract

We investigate the influence of the global structure of accreting millisecond X-ray pulsars (AMXPs) on accretion-induced spin-up, using three equations of state (EoS) models representing neutron stars, quark stars, and strangeon stars. By applying the classical accretion torque formalism, and deriving the accretion rate and magnetic field from observations of three AMXPs --- XTE J1751-305, SAX J1808.4-3658, and IGR J00291+5934 --- we derive mass contours in the plane of luminosity versus spin-up rate. Our results show that the inferred masses are broadly consistent across the three EoS models, indicating that the method is insensitive to the specific EoS and can serve as an evolutionary channel for constraining the masses of pulsar-like compact stars. Notably, we find that SAX J1808.4-3658 and IGR J00291+5934 are constrained to very low masses, while XTE J1751-305 yields a mass consistent with the typical range for pulsars. Our analysis suggests that future improvements in distance and spin-up measurements would refine these mass constraints, which could offer crucial evidence to distinguish different EoS models. This paper also presents the idea that using EoS-specific parameters could yield new insights.