Ross 128 - GL 447. A possible activity cycle for a slow-rotating fully convective star

Context. Long-term chromospheric activity in slow-rotating fully convective stars has scarcely been explored. Ross 128 (Gl 447) is a slow-rotator and inactive dM4 star that has been extensively observed. It hosts the fourth closest extrasolar planet.
Aims: Ross 128 is an ideal target to test dynamo theories in slow-rotating low-mass stars.
Methods: To characterize the magnetic activity of Ross 128, we studied the S_K-indexes derived from CASLEO, HARPS, FEROS, UVES, and X-shooter spectra. Using the generalized Lomb-Scargle and CLEAN periodograms, we analyzed the whole S_K time-series obtained between 2004 and 2018. We performed a similar analysis for the Na I-index, and we analyzed its relation with the S_K-index.
Results: From both indexes, we obtain a possible activity cycle with a period of about five years, which is one of a small handful of activity cycles that have been reported for a slow-rotating fully convective star.