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

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 days) 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 ± 0.42 days and 306.56 ± 1.19 days 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-day 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.