X-Ray Stellar Cycles at Low Rossby Numbers

Stellar cycles, like that of our Sun, can be explained by dynamo theory, whereby kinetic energy is converted into magnetic energy via inductive effects of motions in an electrically conducting fluid. Unlike the Sun, however, Proxima Centauri (M5.5) is a fully convective star, and as such is not expected to be able to support a solar-like dynamo. Nonetheless, there is strong evidence in the optical, UV, and X-ray wavebands suggesting that Prox Cen possesses an 8-yr activity cycle. Analysis of this star could therefore provide key insights into the underlying mechanisms responsible for stellar cycles. We use Gaussian process regression (GPR) to model the rotational and cyclical variability of Prox Cen — and three other stars spanning a range of stellar type, rotation rate, and activity — and examine correlations (or their lack) among the different wavebands. GPR is preferred over traditional time-series analysis methods (e.g. fast Fourier transforms, Lomb-Scargle periodograms) as we are analysing quasi-periodic, unevenly sampled light curves, and GPR fits are better at following the data and interpolating over gaps.