X-ray evidence for flare density variations and continual chromospheric evaporation in Proxima Centauri

Using the XMM-Newton X-ray observatory to monitor the nearest star to the Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically active star ever observed. Correlated X-ray and optical variability provide strong support for coronal energy and mass supply by a nearly continuous sequence of rapid explosive energy releases. Variable emission-line fluxes were observed in the He-like triplets of O VII and Ne IX during a giant flare. They give direct X-ray evidence for density variations, implying densities between 2 x 10(10) and 4 x 10(11) cm(-3) and providing estimates of the mass and the volume of the line-emitting plasma. We discuss the data in the context of the chromospheric evaporation scenario.