Institute for Astronomy and Astrophysics

Compact Objects

Astronomical objects which have an extremely high density (high mass and small size) are called compact objects. Their compactness gives them many extreme properties which make them relevant for the high energy astrophysics, as for example the emission of X-rays and Gamma-rays or jets.

In general, these sources are remnants of stellar explosions, such as neutron stars, white dwarfs and black holes. Due to their extraordinary properties, compact objects form unique laboratories for understanding the physics of extreme environments. In addition, surprisingly many questions about their origin, composition and evolution are still open today, more than 50 years after they were first discovered.

The main study focus in the High Energy Astrophysics group Tübingen is X-ray binaries. These binary systems consist of a star and a compact object orbiting one another. In certain configurations and evolutionary states, the gravitational interactions of both objects leads to matter transfer from the outer envelope of the star onto the compact object. When this happens, the kinetic energy of the in-falling matter is converted into heat once it reaches the surface and X-rays can be observed. This physical process is called accretion.

In particular, so-called accreting pulsars are investigated by scientists in our group: X-ray binaries which include a neutron star with a very high magnetic field. Among other things, the following questions are examined:

  • Where does the intense X-ray radiation of these object originate and how is it emitted?
  • What are the properties of cyclotron resonant scattering features, which are observed in their spectra and what can we learn by observing them about the magnetic fields of neutron stars?
  • How are accretion disks formed and what are their properties?

We are also working on enhancing our understanding of X-ray binaries containing black holes as compact objects. In particular, we are interested in:

  • what drives different accretion regimes (so-called states) and how different accretion and ejection flows (accretion disks, disk winds, jets and coronae) interplay with each other
  • whether jets contribute to the observed X-ray emission and how much
  • measurements of spins accreting black holes

X-ray binaries where the companion star is a O or B supergiant can also be used to study the clumpy winds of massive stars.