Stellar X-ray flares have now been observed for over 8 years with Chandra and XMM-Newton. Both satellites carry high-resolution X-ray spectrometers. We compare soft X-ray spectra of active M-type stars taken during high and low flaring activity. In particular, we focus on an observation of a dMe star which was quiescent for the first part and had many flares during the second. Because flares usually last for at most up to one hour, only very bright flares will produce enough photons for a sufficiently well exposed spectrum. As the chances of observing such bright flares within a typical ~10 hour observing slot are very small, we combine the times with the flaring activity into one averaged active spectrum, to be compared with the quiescent one.
During flares the plasma is known to become hotter, but also changes in density are anticipated, as flares will rather be compact and dense than large and tenuous. Such temperature and density changes can be identified from spectral line ratios. With careful statistical analysis, we find that the intercombination line of the He-like O VII-triplet at 21.8 Angstrom is enhanced during the flares, while other lines are not. This leads to a change in the R-ratio and suggests a highly increased density during the flares.
However, the line ratios are not only dependent on temperature, density and of course element abundance, but also on the plasma type, whether the plasma is in thermal equilibrium, photoionised, or in a non-equilibrium state. While, as expected, the flaring spectrum is dominated by hotter lines than the quiescent one, we also find that certain line ratios of the flaring spectrum are incompatible with a "coronal" plasma in thermal equilibrium. However, they are consistent with a highly ionised, rapidly cooling plasma. These findings have major consequences for interpreting the plasma of active stellar coronae.