The spin-orbit alignment hypothesis in millisecond pulsars

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 $\gamma$-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 $\zeta$ and to check whether it agrees with the orbital inclination angle $i$, in other words if $\zeta \approx i$. We use a Bayesian inference technique to fit the MSP $\gamma$-ray light curves based on the third $\gamma$-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 $\zeta$ 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 $\gamma$-ray fit is not reliable or some precession and external torque avoid an almost perfect alignment.