Search

We present preliminary results from the analysis of a long duration flare that was observed in a serendipitous XMM-Newton detection of the M star CD-39 7717 B (TWA 11 B), member of the young stellar association TW Hya. Only the rise phase (with a duration of ~35 ks) and possibly the flare peak are present in the light-curve. The decay phase was not monitored. The fluorescent iron emission line at 6.4 keV was observed during this event. As far as we are concerned, since TWA 11 B seems to have no disk, this is only the third detection of Fe photospheric fluorescence. During the flare, the X-ray flux increased a factor of ~4. Taking the light curve and the evolution of the hardness ratio into account, we interpret the rise phase as resulting from the ignition of a first group of loops (part 1) which triggered a subsequent two-ribbon flare (part 2). Part 1 is analysed using the Reale's (2007) model, since it has been assumed to be dominated by a single loop. For part 2, the diagnostic method for two-ribbon flares developed by Kopp & Poletto (1984) is applied. Loop semi-lengths of about 2.5-3.0 stellar radii are obtained. These large structures were previously detected only in very young objects, this is the first time that are detected in a more evolved (~8 Myr) star.

The solar corona is a template to study and understand stellar activity. However the solar corona differs from that of active stars: the Sun has lower X‐ray luminosity, and on average cooler plasma temperatures. Active stellar coronae have a hot peak in their emission measure distribution, EM (T), at 8–20 MK, while the non‐flaring solar corona has a peak at 1–2 MK. In the solar corona significant amounts of plasma at temperature ∼10 MK are observed only during flares.To investigate what is the time‐averaged effect of solar flares we measure the disk‐integrated time‐averaged emission measure, EMF (T), of an unbiased sample of solar flares. To this aim we analyze uninterrupted GOES/XRS light curves over time intervals of one month. We also obtain the EMQ (T) of the quiescent corona for the same time intervals from Yohkoh/SXT data. To investigate variations due to the solar cycle we evaluate EMF (T) and EMQ (T) at different phases of the cycle between December 1991 and April 1998.Irrespective of the solar cycle phase, EMF (T) appears as a peak in the distribution, and it is significantly larger than the values of EMQ (T) for T∼5–10 MK. Adding EMF (T) and EMQ (T) we obtain for the first time a time average EM (T) of the entire solar corona: it is double‐peaked, with the hot peak, due to time‐averaged flares, being located at temperatures similar to those of active stars, but less enhanced.In the assumption that the heating of the corona is entirely due to flares, from nano‐ to macro‐flares, a two peak EM (T) distribution suggests that then either the flare distribution or the confined plasma response to flares, or both, are bimodal. Moreover the EMF (T) shape supports the hypothesis that the hot EM (T) peak of active coronae is due to unresolved solar‐like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity.

We model hydrodynamic interactions of an old supernova remnant shock wave with a small interstellar gas cloud, taking into account the effects of thermal conduction and radiative losses. In particular, we consider a representative case of a Mach 30 shock impacting on an isolated cloud with density contrast χ = 10 with respect to the ambient medium. Thermal conduction appears to be effective in suppressing the Kelvin-Helmholtz and Rayleigh-Taylor instabilities which would develop at the cloud boundaries. We demonstrate that the radiative losses play a crucial role in the dynamics of the shock-cloud interaction, dominating evolution of the shocked cloud medium.

In response to the Italian Space Agency announcement "New Ideas for Space Missions", we have proposed an observatory "CALorimetri per Osservazioni Solari" (CALOS) that will perform spatially resolved (Deltatheta similar to 2) X-ray spectroscopy of the solar corona over the 0.1 - 10 keV band using an array of NTD germanium microcalorimeters. The observatory will also include an X-ray polarimeter of radically new design that will study the hard X-ray solar emission and its polarization and will serve as a flare alarm.