Spin-orbit alignment hypothesis in millisecond pulsars

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000, Strasbourg, France
² Université de Strasbourg, CNRS, IRMA, UMR 7501, F-67000, Strasbourg, France
³ INRIA, équipe MACARON: Apprentissage automatique pour des méthodes numériques optimisées, F-67000, Strasbourg, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 July 2025
Accepted: 10 February 2026

Abstract

Context. Millisecond pulsars (MSPs) are spun up during their accretion phase in a binary system. The exchange of angular momentum between the accretion disk and the star tends to align the spin and orbital angular momenta on a very short timescale compared to the accretion stage.

Aims. In this work, we study a sub-set of γ-ray MSPs in binaries for which the orbital inclination angle, i, has been accurately constrained thanks to the Shapiro delay measurements. Our goal is to constrain the observer viewing angle, ζ, and to check whether it agrees with the orbital inclination angle, i, in other words, whether ζ ≈ i.

Methods. We used a Bayesian inference technique to fit the MSP γ-ray light curves based on the third γ-ray pulsar catalogue (3PC). The emission model relies on the striped wind model deduced from force-free neutron star magnetosphere simulations.

Results. We found a good agreement between the two angles, i and ζ, for a significant fraction of our sample (i.e. about four-fifths), confirming the spin-orbit alignment scenario during the accretion stage. However, we find that about one-fifth of our sample deviates significantly from this alignment. The possible reasons are manifold, namely, either the γ-ray fit is not reliable or some precession and external torque act to avoid an almost perfect alignment.