New Insights from Revisiting the Rotation Period of the Strongly Magnetic O Star, NGC 1624-2

TL;DR

This study revisits the rotation period of the magnetic O star NGC 1624-2, proposing a new period that better explains spectral variations and suggests a different magnetic geometry than previously thought.

Contribution

The paper identifies a new rotation period for NGC 1624-2 that aligns spectral and magnetic data, challenging the previously accepted period and magnetic geometry.

Findings

The star's rotation period is likely 306.56 days, not 157.99 days.

The magnetic geometry may be different, with the south pole not yet observed.

Spectral and magnetic data are better phased with the new period.

Abstract

NGC 1624-2 hosts the strongest surface magnetic field found on an O star thus far. When applied across several epochs of observations, the star's currently accepted rotation period (157.99 d) does not coherently characterize the variations of spectral lines of magnetospheric origin. We analyze Lomb-Scargle periodograms produced with new and archival, multi-instrument spectroscopic time series of Balmer H and He spectral lines. We find that 153.17 $\pm$ 0.42 d and 306.56 $\pm$ 1.19 d are both equally suitable periods at phasing the spectral and magnetic time series data in a manner consistent with the Oblique Rotator Model. The 306.56 d period implies a magnetic geometry for NGC 1624-2 that is quite different from the previously accepted one, for which both magnetic poles should be observed during a full rotational cycle. If this is the case, the star's magnetic South pole has yet to be observed, and additional spectropolarimetric observations should be acquired in order to confirm whether or not the south pole is in fact observable.