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Stars of stellar type later than about M3.5 are believed to be fullyconvective and therefore unable to support magnetic dynamos like the one thatproduces the 11-year solar cycle. Because of their intrinsic faintness, veryfew late M stars have undergone long-term monitoring to test this prediction,which is critical to our understanding of magnetic field generation in suchstars. Magnetic activity is also of interest as the driver of UV and X-rayradiation, as well as energetic particles and stellar winds, that affect theatmospheres of close-in planets that lie within habitable zones, such as therecently discovered Proxima b. We report here on several years of optical, UV,and X-ray observations of Proxima Centauri (GJ 551; dM5.5e): 15 years of ASASphotometry in the V band (1085 nights) and 3 years in the I band (196 nights),4 years of Swift XRT and UVOT observations (more than 120 exposures), and 9sets of X-ray observations from other X-ray missions (ASCA, XMM-Newton, andthree Chandra instruments) spanning 22 years. We confirm previous reports of an83-day rotational period and find strong evidence for a 7-year stellar cycle,along with indications of differential rotation at about the solar level.X-ray/UV intensity is anti-correlated with optical V-band brightness for bothrotational and cyclical variations. From comparison with other stars observedto have X-ray cycles we deduce a simple empirical relationship between X-raycyclic modulation and Rossby number, and we also present Swift UV grism spectracovering 2300-6000 {\AA}.

Optical, UV, and X-Ray Evidence for a 7-Year Stellar Cycle in Proxima Centauri