Investigating the magnetic inclination angle distribution of $γ$-ray-loud radio pulsars

Several studies have shown the distribution of pulsars' magnetic inclination angles to be skewed towards low values compared with the distribution expected if the rotation and magnetic axes are placed randomly on the star. Here we focus on a sample of 28 $\gamma$-ray-detected pulsars using data taken as part of the Parkes telescope's \emph{FERMI} timing program. In doing so we find a preference in the sample for low magnetic inclination angles, $\alpha$, in stark contrast to both the expectation that the magnetic and rotation axes are orientated randomly at the birth of the pulsar and to $\gamma$-ray-emission-model-based expected biases. In this paper, after exploring potential explanations, we conclude that there are two possible causes of this preference, namely that low $\alpha$ values are intrinsic to the sample, or that the emission regions extend outside what is traditionally thought to be the open-field-line region in a way which is dependent on the magnetic inclination. Each possibility is expected to have important consequences, ranging from supernova physics to population studies of pulsars and considerations of the radio beaming fraction. We also present a simple conversion scheme between the observed and intrinsic magnetic inclinations which is valid under the assumption that the observed skew is not intrinsic and which can be applied to all existing measurements. We argue that extending the active field-line region will help to resolve the existing tension between emission geometries derived from radio polarisation measurements and those required to model $\gamma$-ray light curves.