Space and Astrophysics – Research

Earth-bound telescopes and particle detectors as well as research satellites provide steadily better pictures and data of the processes in the universe. The goal of our research is to explain these processes with the known laws of nature and physical theories. For this we treat topics from astro-, space and plasma physics. We want to give here only a short overview of some of our research topics. For detailed informations please look up our recent publications.

General radiation processes and physics of active galactic nuclei

Artistic visualisation of an AGN.
Copyright: Paolo Padovani.

In the case a galactic core over-shines its host galaxy we talk of an active galactic nuclei. These object are among the brightest in the universe and show exceptional variability. As a subclass in the unified picture of active galaxies, blazars exhibit a broadband double hump spectrum, which is dominated by the emission from the jet plasma leaving the central region in bi-polar directions. While the lower energetic component is commonly associated with synchrotron emission, the origin of the high energy component is still under debate. We analytically calculate the theoretical expectable spectra for various radiation mechanisms and plasma species. Because especially blazars show strong time variability, we incorporate the time-evolution due the several energy cooling effect on the plasma constituents. The resulting fluxes, light curves and polarization properties serve as vital tools for better understanding of active galaxies and estimations of their intrinsic physical parameters.
Selected recent related publication:

Cosmic Voids, extra galactic background light and the intergalactic magnetic field

Cosmic web
The cosmic web
Copyright: The Millennium Simulation Project.

The large scale structure of the universe exhibits a foam-like structure. Galaxy clusters and super clusters at the knot points are connected via filaments of galaxies surrounding gigantic voids. In these voids high energy photons from active galactic nuclei are converted in electron positron pairs by photon-photon collisions with the extra-galactic background light (EBL). The diffuse EBL consists of direct stellar radiation, or stellar radiation reprocessed by dust. Due to momentum conversion the generated pairs form a collimated beam. In the classical picture the generated particles may inverse-Compton scatter with photons from the cosmic microwave background radiation generating still energetic gamma photos, which can in turn can produce pairs in the ambient radiation fields. An electromagnetic cascade arises, which may cause time delayed echos of flares. Deflection in the inter galactic magnetic field possibly generates halos around point sources like blazars. Our on-going research investigates plasma-effect of the pair beams depressing the evolution of the electromagnetic cascade. For example we investigate consequences of an unmagnetized inter galactic medium where the dissipation of generated electrostatic turbulence can lead to electrostatic bremsstrahlung pair halos from weak blazars.
Related recent Publications:

The solar wind – equipartition and plasma instability thresholds

Solar wind data
In situ data of the solar wind taken by the WIND satellite overlaid with theoretical instability thresholds.

The solar wind is a steady stream of charged particles flowing outwards from the sun. Although cosmic plasmas in general are regarded as collisional-poor, the observed electron and proton velocity distribution functions are close to bi-Maxwellian distributions with different temperature along and perpendicular to the ordered magnetic field direction of the sun. The satellite mission WIND has monitored the properties of this plasma for more than ten years. A parameter plot comparing the temperature anisotropy, meaning the ratio of perpendicular to parallel temperature, to the so-called parallel plasma beta, the ratio of the parallel plasma pressure to the magnetic pressure, shows a rhomb-like figure. When the plasma is in a configuration outside the rhomb, the instabilities generate fluctuations which quickly relax the plasma distribution into the stable regime within the rhomb shape. It is interesting to investigate these plasma instabilities further. While for large plasma beta the temperature anisotropies are bounded by the mirror and the firehose instability exact reproductions of the borders for small plasma beta are lacking. Our on-going resarch aims for explanation of these borders.
Related recent Publications:

H.E.S.S – The High Energy Steroscopic System

Das "Large Cherenkov Telescope" des H.E.S.S.-Projekts.

The H.E.S.S collaboration is running the most successful air Cherenkov telescope in the world. The underlying physical goal of the H.E.S.S. project is the investigation of production and evolution of high energetic particles in the universe. Since charged particles are deflected by cosmic magnetic fields, their origins cannot be inferred directly. The H.E.S.S. telescopes observe gamma radiation, which is unaffected by magnetic fields and, thus, gives direct evidence of its source. Non-thermal particle populations appear for example in supernovae, pulsars and active galaxies. The gamma ray production mechanism may be leptonic processes like synchrotron radiation and inverse-Compton scattering or hadronic processes like the decay of neutral pinos. The measured spectra of gamma-rays are closely related to the particle populations in the respective sources. The telescopes observation technique is the following: Fast elementary particles are created by scattering processes of high energetic gamma rays in the upper atmosphere. Being subject to similar processes themselves, the particles produce secondary particles giving rise to a so-called air shower. Since these particles exceed the specific speed of light in air, they emit Cerenkov light. These faint blue light pulses, which last only a few nanoseconds, can be detected by the H.E.S.S. telescopes. The five telescopes are used in stereoscopic mode. Thus, by measuring the magnitude and geometry of the incoming pulse, the energy and the direction of the initial gamma photon can be inferred. The chair for theoretical Space- and Astrophysics from the Ruhr-University is a foundation member of the H.E.S.S. collaboration. Universities and research departments from twelve countries are involved in the project. Main tasks of the Bochum group lie in the field of theoretical accompaniment of the experiment, interpretation of the data, maintaining the contact with the astrophysical community as well as taking part in the shifts at the telescopes in Namibia.