2025/07/18 by Lorange, Alexandra, Pétri, Jérôme, Sautron, Mattéo +1
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2507.13893
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 time scale compared to the accretion stage. In this work, we study a subset 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 if ζ≈ i. We use 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. We found good agreement between the two angles i and ζ for a significant fraction of our sample, about four fifth, confirming the spin-orbit alignment scenario during the accretion stage. However about one fifth of our sample deviates significantly from this alignment. The reasons are manifold: either the γ-ray fit is not reliable or some precession and external torque avoid an almost perfect alignment.