Confirmation of xi^1 CMa's ultra-slow rotation: magnetic polarity reversal and a dramatic change in magnetospheric UV emission lines

TL;DR

This study confirms xi^1 CMa's ultra-slow rotation, detects magnetic polarity reversal, and observes significant changes in its magnetospheric UV emission lines, providing insights into its magnetic topology and long-term magnetic behavior.

Contribution

It presents the first clear detection of magnetic polarity reversal and detailed UV spectral analysis, advancing understanding of magnetic and rotational properties of xi^1 CMa.

Findings

Detected negative longitudinal magnetic field in 2019 and 2020.

Observed UV spectrum consistent with an equatorial magnetosphere view.

Provided evidence for the star's extremely long rotation period.

Abstract

The magnetic beta Cep pulsator xi^1 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 xi^1 CMa (<Bz>=-87 +/- 2 G in 2019 and <Bz>=-207 +/- 3 G in 2020), as well as the results of ongoing monitoring of the star's Halpha variability. We examine evidence for deviation from a purely dipolar topology. We also report a new HST UV spectrum of xi^1 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 beta Cep obtained near maximum emission. The new UV spectrum of xi^1 CMa provides additional evidence for the extremely long rotation period of this star via comparison to archival data.