The multiple facets of millisecond pulsar binaries

TL;DR

Millisecond pulsar binaries offer extreme environments to study particle acceleration and magnetospheric-disk-wind interactions. The paper synthesizes optical pulsation detections from MSPs in accretion-powered, rotation-powered, and intermediate disk states, highlighting that optical pulsed emission can be far more efficient than simple rotation-powered predictions, implying disk-enhanced acceleration ($\eta$ much larger than expected) and a coupling between accretion flows and magnetospheric processes. It argues for a coordinated, time-domain observational program employing ultra-fast photometry, time-resolved spectroscopy, and phase-resolved polarimetry to map disk structure, intrabinary shocks, and mass-transfer dynamics, across MSP evolutionary stages. The work emphasizes the need for next-generation (25-m class) facilities to achieve the required time resolution and sensitivity, enabling a systematic exploration of optical pulsars and informing models of particle acceleration and binary evolution in MSPs. These efforts will advance our understanding of how disks, winds, and shocks shape MSP emission and evolution, with broad implications for high-energy astrophysics and time-domain astronomy ($\dot{E}$, $\eta$).

Abstract

Millisecond pulsar (MSP) binaries are unique laboratories for studying matter and radiation under extreme conditions that are unattainable on Earth. Recent detections of optical millisecond pulsations from three systems in distinct evolutionary stages have opened an entirely new observational window to investigate particle acceleration, pulsar-disk interplay, and intrabinary wind interactions. These discoveries reveal unexpectedly diverse emission mechanisms across accretion regimes, with optical efficiencies in some systems far exceeding those predicted by rotation-powered models. Despite decades of research, key questions remain unresolved: are optical pulsations a universal property of MSPs? How does the presence of an accretion disk boost the conversion of spin-down power into coherent optical emission? What physical processes drive the observed fast variability, and how do pulsar and companion winds regulate mass transfer and binary evolution? Addressing these fundamental problems requires high-time-resolution optical observations, rapid-response observing capabilities, and time-resolved spectroscopy at moderate spectral resolution to map disk and intrabinary-shock variability. Future facilities dedicated to time domain astronomy present a unique opportunity to perform a systematic exploration of optical pulsars across all MSP evolutionary stages for the first time and answer the above-mentioned fundamental questions.