We report the latest set of spectropolarimetric observations of the magnetic $beta$ Cep star $xi^1$ CMa. The new observations confirm the long-period model of Shultz et al. (2017), who proposed a rotational period of about 30 years and predicted that in 2018 the star should pass through a magnetic null. In perfect agreement with this projection, all longitudinal magnetic field $langle B_z rangle$ measurements are close to 0 G. Remarkably, individual Stokes $V$ profiles all display a crossover signature, which is consistent with $langle B_z rangle sim 0$ but is {em not} expected when $vsin{i} sim 0$. The crossover signatures furthermore exhibit pulsationally modulated amplitude and sign variations. We show that these unexpected phenomena can all be explained by a `radial crossover effect related to the stars radial pulsations, together with an important deviation of the global field topology from a purely dipolar structure, which we explore via a dipole+quadrupole configuration as the simplest non-dipolar field.
Download