U. Mitra-Kraev
Flares on active M-type stars observed with XMM-Newton and Chandra
2007 (Supervisor: L. K. Harra)

M-type red dwarfs are among the most active stars. Their light curves display random variability of rapid increase and gradual decrease in emission. It is believed that these large energy events, or flares, are the manifestation of the permanently reforming magnetic field of the stellar atmosphere. Stellar coronal flares are observed in the radio, optical, ultraviolet and X-rays. With the new generation of X-ray telescopes, XMM-Newton and Chandra, it has become possible to study these flares in much greater detail than ever before. This thesis focuses on three core issues about flares: (i) how their X-ray emission is correlated with the ultraviolet, (ii) using an oscillation to determine the loop length and the magnetic field strength of a particular flare, and (iii) investigating the change of density sensitive lines during flares using high-resolution X-ray spectra.

(i) It is known that flare emission in different wavebands often correlate in time. However, here is the first time where data is presented which shows a correlation between emission from two different wavebands (soft X-rays and ultraviolet) over various sized flares and from five stars, which supports that the flare process is governed by common physical parameters scaling over a large range.

(ii) As it is impossible to spatially resolve any but a very few giant stars, the only information on spatial dimensions as well as the magnetic field strength of stellar coronae has to come from indirect measurements. Using wavelet analysis, I isolated the first stellar X-ray flare oscillation. Interpreting it as a standing coronal flare loop oscillation, I derived a flare loop length as well as the magnetic field strength for this X-ray flare.

(iii) The high-resolution soft X-ray spectra of Chandra and XMM-Newton allow us to determine temperatures, densities and abundances of the stellar coronae. Despite a low signal-to-noise ratio because of the relatively short duration of a flare, we find that, if adding up the photons of several flares, certain density sensitive spectral lines change significantly between quiescent and flaring states. This project led on to investigate the flaring spectrum further, and it is found that the plasma is no longer in collisional ionisation equilibrium, but that it is dominated by recombinations.