Magnetic activity entails various observable phenomena such as
Sun Spots, chromospheric utraviolet emission, highly structured
coronal X-ray emission, and coronal mass ejections. Whether or
not any of these phenomena we know from the Sun are typical
can only be decided by taking samples of other stars.
My PhD thesis has focused on the coronal X-ray emission from other stars that can be detected at Earth, albeit not spatially resolved. Plasma diagnostics from line-resolving X-ray spectrometers allow determinations of temperatures and densities.
Highlights of my work:
Classical Novae are thermonuclear explosions on the surface of White Dwarfs.
Evolution of a solar-like star ends with a White Dwarf after central
burning has ceased. Without the central energy source, the star can no
longer remain its equilibrium between gravitational forces and radiation
pressure from the centre and collapses to a dense object. The collapse
stops when the Pauli Principle would have to be violated.
Since all the central hydrogen has been converted to helium during the stellar life time, White Dwarfs are hydrogen deficient, but if a companion star orbits close to a White Dwarf that hydrogen-rich matter from that star can be transferred to the White Dwarf, it gains a new energy source. The hydrogen accumulate on the surface of the White Dwarf until temperature and pressure have increased to the point allowing nuclear burning again. This happens in an uncontrolled way (thermonuclear runaway), and the explosion leads to the ejection of mass, driven by radiation pressure. The high-energy radiation, produced while nuclear burning continues, is obscured by the ejecta, and radiation transport transforms it into optical emission. For observers from Earth, they look like a new star with the spectral type of an F giant. As the expansion continues and the ejecta become thinner, they become more transparent to high-energy radiation, allowing successively hotter plasma layers to become visible.
My research focuses on studies of X-ray emission of novae that is visible once the ejecta have cleared to the point that nuclear burning on the surface of the White Dwarf can be seen. This phase is called SuperSoftSource (SSS) phase, and high-resolution SSS X-ray spectra are atmospheres with blackbody-like continuum emission and deep absorption lines.
Highlights of my work:
Since X-ray photons have much higher energies than optical light,
their number is much smaller for a given energy budget. X-ray
observations thus need to be much longer than optical observations.
If observing a planet, significant motion of the planet over the
chip complicates the analysis. Since X-ray observations rely on
photon counting with information of position, time, and energy
available for each photon, a transformation can be performed
that concentrates all photons from the planet into a small region
on the chip.
Until 2000, no X-rays from Saturn had been observed, and a careful analysis of a ROSAT observation taken in 1992 revealed a weak but significant signal of 22 photons on a background of 8.
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Highlights of my work:
First discovery of X-rays from Saturn
Confirmation of X-ray emission from Saturn with Chandra. Enough counts were found to study the spatial distribution over the disk. The X-ray emission is concentrated towards the centre of the visible disk. Since the planet was inclined, we could conclude that the equator is not the centre of X-ray emission but instead solar X-rays are reflected. This was later confirmed by Bhardwaj et al. (2005). In contrast to Jupiter, Saturn has no X-ray emission from the poles, thus auroral emission plays a small role.
Clear detection of X-rays with XMM-Newton with enough signal to perform rough spectral diagnostics
Our Galaxy consists of 100 billion stars which move about the
Galactic Centre. Each star is characterized by it location relative
to the Galactic Centre, its momentum (thus mass and velocity),
and its angular momentum. Their orbits are determined by Newton's
laws of motion and their gravitational attraction. In a computer
simulation, the motion of each single star can be computed taking
into account its own motion and the combined attraction by all
other stars. This is computationally too expensive for 100 billion
stars but sophisticated approximations have been developed to
operate in a feasible domain.
My Diploma Thesis builds on previous work: