Multi-wavelength observations of variability characterizing magnetic activity in late-type stars.

The presence of magnetic fields in the atmosphere of the late-type stars determines the formation of small scale structures which make the atmosphere inhomogenous, causes excess heating of the atmospheric plasma with consequent excess thermal radiation, and also non-thermal radiation. This is what is referred to as magnetic activity, which gives essential information on the properties of the stellar magnetic fields and internal dynamos. In the first work I present, I characterize the relation between magnetic activity (mainly X-ray activity) and the stellar rotation, which is a proxy for the internal dynamo mechanism of stars. For the first time, the measure of rotation period and of the X-ray activity is carried on in a fully homogeneous way on the whole sample. The results I obtain confirms previous results. In the second work, I present the discovery of strong magnetic activity, in the form of an X-ray flare, from a very young (Class I) YSO, ISO-Oph 85; this latter work is focused on the characterization of this transient X-ray emission and on the classification of he evolutionary stage of ISO-Oph 85. In this works, the study of X-ray activity plays a major role. The X-ray data I use are taken from the XMM-Newton observatory. I take advantage, for the time-resolved analsyis of the X-ray data, of algorithms and products by the EXTraS Project. In the first one of the two works presented, the observations performed by the Kepler mission have a crucial role