Description
Accreting millisecond X-ray pulsars represent a key phase in the formation of rotation-powered
millisecond pulsars, providing a direct observational window onto the interaction between accretion
flows and magnetised neutron stars. Their X-ray pulsations offer a unique probe of how
angular momentum is transferred to the neutron star and how spin evolution proceeds under the
action of accretion torques.
In this talk, I will present a review of the observational properties of accreting millisecond X-ray
pulsars, focusing on their spin distributions, transient behaviour, and accretion regimes. Recent
studies indicate that the observed diversity of these systems cannot be interpreted solely in terms
of instantaneous accretion rates, but likely reflects the imprint of their long-term evolutionary history.
Within this framework, the observed spin properties can be understood as the result of the
interplay among accretion torques, magnetic field strength, and the system’s secular evolution.
I will discuss how X-ray timing observations constrain the physics of the disc–magnetosphere interaction
and the efficiency of spin-up processes, and how these constraints can be connected to
broader evolutionary scenarios. In this context, multi-wavelength observations provide an important
complementary perspective. In particular, the detection of optical pulsations at the neutron
star spin period in a handful of systems indicates that part of the optical emission can be directly
linked to magnetospheric processes operating on millisecond timescales, offering an additional
diagnostic of the accretion–ejection interplay.
This overview highlights how accreting millisecond X-ray pulsars serve as a laboratory for connecting
short-term accretion physics with long-term neutron star spin evolution, offering clear synergies
with ongoing efforts to characterise millisecond pulsars across the electromagnetic spectrum,
including the optical domain explored within the GEMS project.