Periodic low-amplitude variations in the brightness of Proxima Centauri

Using data acquired with Fine Guidance Sensor No. 3 on board the Hubble Space Telescope we find that the brightness of Proxima Centauri varies with an amplitude of 0.01 mag and a period P = 41.6 +/- 0.6 days. We interpret this variation as rotational modulation of star spots or bright regions in the photosphere of Proxima Cen. In a total of 89.36 min of monitoring over 212 days, we detected one major flare and three post-flare brightenings.