Confirmation of ξ1 CMa's ultra-slow rotation: magnetic polarity reversal and a dramatic change in magnetospheric UV emission lines

The magnetic β Cep pulsator ξ¹ CMa has the longest rotational period of any known magnetic B-type star. It is also the only magnetic B-type star with magnetospheric emission that is known to be modulated by both rotation and pulsation. We report here the first unambiguous detection of a negative longitudinal magnetic field in ξ¹ CMa (⟨B_z⟩ = -87 ± 2 G in 2019 and ⟨B_z⟩ = -207 ± 3 G in 2020), as well as the results of ongoing monitoring of the star's H α variability. We examine evidence for deviation from a purely dipolar topology. We also report a new HST UV spectrum of ξ¹ CMa obtained near magnetic null that is consistent with an equatorial view of the magnetosphere, as evidenced by its similarity to the UV spectrum of β Cep obtained near maximum emission. The new UV spectrum of ξ¹ CMa provides additional evidence for the extremely long rotation period of this star via comparison to archival data.