The pulsating magnetosphere of the extremely slowly rotating magnetic β Cep star ξ1 CMa

ξ1 CMa is a monoperiodically pulsating, magnetic βCep star with magnetospheric X-ray emission that, uniquely amongstmagnetic stars, is clearly modulated with the star's pulsation period.The rotational period Prot has yet to be identified, withmultiple competing claims in the literature. We present an analysis of alarge ESPaDOnS data set with a 9 yr baseline. The longitudinal magneticfield 〈Bz〉 shows a significant annualvariation, suggesting that Prot is at least of the order ofdecades. The possibility that the star's H α emission originatesaround a classical Be companion star is explored and rejected based uponVery Large Telescope Interferometer AMBER and PIONIER interferometry,indicating that the emission must instead originate in the star'smagnetosphere and should therefore also be modulated withProt. Period analysis of H α equivalent widths measuredfrom ESPaDOnS and CORALIE spectra indicates Prot > 30 yr.All evidence thus supports that ξ1 CMa is a very slowlyrotating magnetic star hosting a dynamical magnetosphere. H α alsoshows evidence for modulation with the pulsation period, a phenomenonthat we show cannot be explained by variability of the underlyingphotospheric line profile, I.e. it may reflect changes in the quantityand distribution of magnetically confined plasma in the circumstellarenvironment. In comparison to other magnetic stars with similar stellarproperties, ξ1 CMa is by far the most slowly rotatingmagnetic B-type star, is the only slowly rotating B-type star with amagnetosphere detectable in H α (and thus, the coolest star withan optically detectable dynamical magnetosphere), and is the only knownearly-type magnetic star with H α emission modulated by bothpulsation and rotation.