2025AJ....170....7P

In Patil et al., we developed a power spectrum estimation method, mtNUFFT (or multitaper nonuniform fast Fourier transform), for analyzing time series with quasi-regular spacing, and showed that it not only improves upon the statistical issues of the Lomb-Scargle (LS) periodogram, but also provides a factor of 3 speedup in some applications. In this paper, we extend mtNUFFT to include a multitaper F-test, a hypothesis test to assess whether a strictly periodic signal or its harmonic (as opposed to, e.g., a quasi-periodic signal) is present at a given frequency. This extension is possible because the F-test is an accompaniment to the multitaper power spectrum estimator (as opposed to other estimators such as the LS periodogram). The mtNUFFT/F-test combination allows detection of strictly periodic signals embedded in noise and precise estimation of their frequencies, in addition to power spectrum estimation. Using asteroseismic time-series data for the Kepler-91 red giant, we show that the F-test automatically picks up the harmonics of its transiting exoplanet as well as certain dipole (l = 1) mixed modes. We use this example to highlight that we can distinguish between different types of stellar oscillations, e.g., transient (damped, stochastically excited) and strictly periodic (undamped, heat driven). We also illustrate the technique of dividing a time series into chunks to further examine the transient versus periodic nature of stellar oscillations. The harmonic F-test combined with mtNUFFT is implemented in the public Python package tapify, which opens opportunities to perform detailed investigations of periodic signals in time-domain astronomy.