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

TL;DR

NGC 1624-2, the O-star with the strongest known surface magnetic field, has a rotation period traditionally taken as 157.99 d, but this period poorly phase-shifts magnetospheric spectral variations across epochs. Using multi-instrument spectroscopic time series and Lomb-Scargle analysis of equivalent widths, the study finds two equally viable solutions: a single-wave period of 153.17 ± 0.42 d and a double-wave period of 306.56 ± 1.19 d, each consistent with the Oblique Rotator Model yet implying different magnetic geometries. The shorter period preserves compatibility with prior magnetic measurements while the longer period predicts visibility of the South magnetic pole and requires additional spectropolarimetry to confirm the true geometry. Reassessment of UV and X-ray data under these periods resolves prior phase inconsistencies and highlights the need for targeted polarimetry to break degeneracy and refine the strong-field magnetospheric framework for this benchmark O-star.

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.

Figures (6)

• Figure 1: The He i$\lambda$5876 line profile variability. These normalized line profiles are plotted as a function of velocity in the stellar rest frame, scaled by 0.3, and shifted vertically according to their rotational phases using the ephemeris of 2012Wade.NGC.1624.2 (the black dashes on the left are aligned with their continuum). The profiles are colored by their rotation number ($N_{\textrm{rot}}$). The profiles are split among the left, center, and right panels by $N_{\textrm{rot}} < 2$, $2 \le N_{\textrm{rot}} < 10$, and $N_{\textrm{rot}} \ge 10$, respectively (also indicated by the black lines on the color bar). For display purposes, the MERCATOR observations were smoothed with a 7 point boxcar averaging.
• Figure 2: a) Periodograms from the He ii$\lambda$4686 time series. The dashed gray line indicates the power above which the False Alarm Probability is below 0.005 for the 1-term LS only. We also show the window spectrum scaled by a factor of 0.3 in gray. We display the periodograms as a function of period to be consistent with Figure 5 of 2012Wade.NGC.1624.2. b) Significant periods identified in the periodogram of the four spectral lines. The black points are measurements from the 2012Wade.NGC.1624.2. The dark gray line and shaded area represent the original $157.99\pm0.94$ d period. The purple and blue points are our 1-term and 2-term Lomb-Scargle (LS) derived periods, respectively. The dashed lines and shaded areas show the standard mean of the periods and their associated standard deviations.
• Figure 3: The variability of each EW time series using different periods. In the upper panels, the time series are phase-folded according to the 2012Wade.NGC.1624.2 rotational period. In descending order of row, the four time series are phased by 153.54 d, 153.49 d, 153.15 d, and 152.49 d, which are all 2-term LS periods. The time series are vertically flipped and normalized so that 1 represents the most negative EWs (i.e. strongest emission/high state) and 0 indicates the opposite (ie. weakest emission/ low state; recall that the low state He i profiles are complex). The measurements are colored according to their rotation numbers. To visually highlight the variation, we add unfilled duplicates of the first and last quarters of the time series.
• Figure 4: a) Same as Figure \ref{['fig:HE1_FIG']}, but the panels use $P =$ 153.17 d to phase the profiles. b) Same as Figure \ref{['fig:HE1_FIG']}, but the panels use $P =$ 306.56 d. Both figures share the same color, and the profiles are divided into the panels by $N_{\textrm{rot}} < 6$.
• Figure 5: a)The variability of the normalized EW time series (Hei, Heii, H$\alpha$, and H$\beta$; top panels) and the $\langle B_\textrm{z} \rangle$ measurements from 2021MNRAS.501.2677D and 2021MNRAS.501.4534J (bottom panels) using the 157.99 d, 153.17 d, and 306.56 d periods. The solid curves are 1st-order sinusoidal fits to the $\langle B_\textrm{z} \rangle$ measurements using each period. The data are replicated over two rotation cycles for visual enhancement. b) Relationship between $i$ and $\beta$ that is implied by the min-to-max ratio of the best fitting sinusoid curves to the $\langle B_\textrm{z} \rangle$ measurements. The three types of dashed curves show the $i$-$\beta$ curves derived for the 2021MNRAS.501.2677D (black) and 2021MNRAS.501.4534J (gray) datasets using each period. The colored curves are the $i-\beta$ curves resulting from samples drawn from the MCMC fit posterior distribution; the pink and purple curves are from fits to 2021MNRAS.501.2677D and 2021MNRAS.501.4534J, respectively; the set of lower curves is from using the single-wave periods, and the set of higher curves is from the double-wave period.