Activity and rotation of the X-ray emitting Kepler stars

The relation between magnetic activity and rotation in late-type stars provides fundamental information on stellar dynamos and angular momentum evolution. Rotation/activity studies found in the literature suffer from inhomogeneity in the measure of activity indexes and rotation periods. We overcome this limitation with a study of the X-ray emitting late-type main-sequence stars observed by XMM-Newton and Kepler. We measure rotation periods from photometric variability in Kepler light curves. As activity indicators, we adopt the X-ray luminosity, the number frequency of white-light flares, the amplitude of the rotational photometric modulation, and the standard deviation in the Kepler light curves. The search for X-ray flares in the light curves provided by the EXTraS (Exploring the X-ray Transient and variable Sky) FP-7 project allows us to identify simultaneous X-ray and white-light flares. A careful selection of the X-ray sources in the Kepler field yields 102 main-sequence stars with spectral types from A to M. We find rotation periods for 74 X-ray emitting main-sequence stars, 22 of which without period reported in the previous literature. In the X-ray activity/rotation relation, we see evidence for the traditional distinction of a saturated and a correlated part, the latter presenting a continuous decrease in activity towards slower rotators. For the optical activity indicators the transition is abrupt and located at a period of ~ 10 d but it can be probed only marginally with this sample which is biased towards fast rotators due to the X-ray selection. We observe 7 bona-fide X-ray flares with evidence for a white-light counterpart in simultaneous Kepler data. We derive an X-ray flare frequency of ~ 0.15 d^{-1} , consistent with the optical flare frequency obtained from the much longer Kepler time-series.Comment: Accepted for publication in A&A. 31 pages, 19 figure

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

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

EXTraS discovery of an X-ray superflare from an L dwarf

We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331−27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331−27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331−27 to be L1. The X-ray flare has an energy of EX, F ∼ 2 × 1033 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Ms of XMM-Newton data we derive an upper limit of LX, qui <  1027 erg s−1. The flare peak luminosity (LX, peak = 6.3 × 1029 erg s−1), flare duration (τdecay ≈ 2400 s), and plasma temperature (≈16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ∼2100 K. However, the absence of any other flares above the detection threshold of EX, F ∼ 2.5 × 1032 erg in a total of ∼2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE ∼ E−2, suggesting that magnetic energy release in J0331−27 – and possibly in all L dwarfs – takes place predominantly in the form of giant flares