Research

Previous observations with the Chandra X-ray satellite indicated a rapid flux and temperature decline of the central compact object located in the Cas A supernova remnant. Such rapid cooling has profound implications for our understanding of the neutron star interior. However, the X-ray observations used for the previous investigations suffered from several instrument calibration issues, for example pileup (the CCD can not process quickly enough the incoming X-ray photons).  In our new work, my collaborators and I used  two Chandra observations where these effects have been minimized. We considered different atmosphere models for the neutron star, for example, carbon atmosphere models in our analysis. We did not find any significant temperature decrease and concluded that the previous findings are likely the result of larger-than- expected systematic errors. We found, however, an indication for a flux decrease in a small energy range and speculated whether this might be due to either an imperfect calibration or a changing amount of matter along the line of sight (the supernova remnant and its environment  are highly dynamic).   The image on the left shows in the upper two rows the previous results for observations suffering from the strong calibration uncertainties, the lower points indicate our results for the two observations minimizing the calibration issues. The two colors indicate two different models (the amount of material along the line of sight changes or not).  

Open questions are: How common are these fallback disks and how easily can such disks be destroyed? Since neutron stars are very extreme objects with a violent birth, energetic radiation and particle output, several destruction scenarios are possible. The generally fast movement of pulsars through the interstellar medium has an impact on any such disks too. Investigating submillimeter observations with the Atacama Pathfinder Experiment, we found that  a  disk around  RX J1856.5-3754, for example, could have been easily destroyed during the pulsar's fast passage through a nearby dense molecular cloud fragment (right picture).

The X-ray emission of neutron stars is usually dominated by non-thermal emission processes in their powerful magnetospheres. Thermal emission from the stellar surface is often buried below the non-thermal X-ray spectrum and/or difficult to disentangle from it. There are only few neutron stars with predominately thermal X-ray spectra. The "Magnificent Seven" show nearly perfect  thermal X-ray spectra.  They were discovered with the ROSAT satellite and are interesting because, first, they allow a direct view to the neutron star surface, and second, they represent a significant fraction (nearly half!) of the local neutron star population. Furthermore, these objects are hotter than one would expect from simple cooling models, and heating from magnetic field decay is discussed as one explanation. Previous accretion from a supernova fallback disk might be another option to explain the properties of these X-ray pulsars.Together with my collaborators I have worked on population synthesis models to explain the observed number of these neutron stars and to identify sky regions where we expect to find more of these objects. A webtool of our population synthesis can be found here. Using the absorption of their thermal X-rays we were able to obtain distance constraints for some of the Magnificent Seven. We also investigated the Magnificent Seven at other wavelengths in order to understand their overall peculiar nature.

The very first extrasolar planets which were discovered were planets around a pulsar. Many radio pulsars are very reliable "clocks", their radio pulses are very regular. If such a pulsar has a companion object, the effects of gravitation lead to distortions in the pulse regularity. Modeling these pulse irregularities one can infer the properties of the companion object.Through this indirect method the first pulsar planets were discovered.We tried to detect possible companions directly. A warm companion is expected to be detectable in the Infrared and can be identified in multi-epoch images by its co-motion with the (normally fast moving) pulsar in the sky plane. The current instrumentation allows to probe only the closest and reasonably young neutron stars for such companions. So far, we have investigated several epochs of four neutron stars using ESO's Very Large Telescope. The image on the left shows the pixel shifts of objects in the field of RX J1856.5-3754. They were obtained by comparing the positions of these sources in two observing epochs. The red point marks the pixel shift an object would have which moves with the known pulsar proper motion. Since no such source was found we concluded that RX J1856.5-3754 has no companion with a mass larger than 11 Jupiter masses.