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X-rays and extreme UV-photons are absorbed by the atmosphere of the Earth, therefore observations at these wavelengths can only be performed with balloon or space-born instruments. The field of cosmic X-ray astronomy was started in 1962 with the famous Aerobee rocket flight that eventually lead to the award of the Nobel prize to the "father" of X-ray astronomy, R. Giacconi, in 2002. While the first detections of astrophysical X-ray sources were made with rocket-borne instrumentation, today satellites are almost exclusively used to study the high-energy sky. While X-ray emission from our Sun was detected shortly after World War II, the detection and investigation of stellar X-ray emission had to await the launch of the Einstein (1978 - 1981) and ROSAT (1990 - 1998) observatories, which demonstrated that most types of stars do in fact produce X-ray emission, often at levels exceeding that of the Sun by many orders of magnitude and that different X-ray generating mechanisms must operate in different types of stars. The image below shows a comparison between the optical and the X-ray image of the Orion region, an area of ongoing star formation where many young X-ray bright stars are found,



Stellar X-ray astronomy in Hamburg

The Stars & Exoplanets group at the Hamburg Observatory has been actively involved in most major X-ray missions and has used specifically the XMM-Newton (ESA) and Chandra (NASA) observatories to carry out a variety of scientific observations in the field of stellar X-ray astronomy.
Since stellar X-rays are typically produced by very hot plasma at temperatures (often substantially) above 1 MK, X-ray emission specifically allows to study energetic phenomena in the diverse stellar populations. The hot thermal plasmas of stellar coronae are optically thin and produce continuum emission as well as strong emission lines from highly ionized atoms. These X-ray lines provide a wealth of spectroscopic diagnostics, that allow to determine e.g. temperature structure, density or chemical composition of the plasma and thus provide important information on the physical properties of the underlying plasma.
In the Hamburg stellar X-ray group we investigate, among other topics, the birth of stars and the high-energy environment of planet formation, magnetic activity and its evolution in cool stars, the physics of hot plasma and the solar-stellar connection



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With modern X-ray satellites like the above mentioned XMM-Newton and Chandra observatories one can obtain X-ray images, light curves and spectra for a large variety of objects. High-resolution X-ray spectra like the one shown above allow to study individual emission lines from stellar sources. These X-ray data opened new windows to stellar astrophysics and is fundamental for the investigation of stellar coronae and other high-energy emission generating phenomena as well as their underlying physical processes. Our group has also studied stars in multi-wavelength campaigns, where X-ray data is taken simultaneously with e.g. optical observations. The X-ray flare light curve obtained during such a campaign on a low mass star can help to reveal energy release mechanisms related to magnetic activity.
The Hamburg X-ray group is also participating in future X-ray missions like the German-Russian eROSITA/SRG project. eROSITA will perform an imaging all-sky survey in the medium energy X-ray range with an unprecedented spectral and angular resolution that is expected to detect about half a million X-ray emitting stars.