Prof. Dr. J. Anton Zensus

Director and Head of the Research Department
"Radio Astronomy/VLBI"

Phone: +49 228 525-298 (Secretary)

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Radio Astronomy / VLBI

Radio Astronomy / VLBI

By employing radio-interferometry, extragalactic objects and their centres are investigated in great detail. The Very Long Base Line Interferometry (VLBI) technique is applied by correlating data from telescopes distributed worldwide and using them as a “giant“ combined telescope within the framework of coordinated arrays as the the European VLBI network (EVN). In addition, global VLBI experiments are conducted in cooperation with telescopes in the USA.

Compact radio sources are the astronomical 'target' in our studies.  Thanks to the expertise over decades our group in radio interferometric techniques our department counts among the world leading groups in this area.

Our main research topics focus in the investigation of Active Galactic Nuclei (AGN) and their emission.  The emission has a non-thermal nature and is rapidly variable.  AGN show strong plasma outflows originating near a central, massive black hole.  These so-called jets, emit synchrotron radio light.  AGN have intrinsically two-sided jets, but usually only one of them is seen, due to relativistic effects (Doppler boosting).  AGN jets also display the intriguing phenomenon of superluminal motion.  Blazars are the small fraction of those AGN with jets pointing towards the observer.  The key physical concepts involve jet launching, opacity effects in the ‘core’ near the jet base, and the propagation of shocks in the jets and energy dissipation.

<div class="hide_in_print">Stacked image of the quasar CTA102, from the MOJAVE project</div>
Stacked image of the quasar CTA102, from the MOJAVE project

Main Research Topics

The group research can be summarised in three main areas: very-high-resolution imaging of compact radio sources, VLBI monitoring of milliarcsecond-scale structural changes, and spectral and polarisation monitoring of radio sources.  A series of additional projects and initatives rounds the scientific portfolio of the VLBI department.

Very-high-resolution imaging

The MPIfR leads efforts at high resolutions in two directions: using the shortest possible wavelengths to overcome overcome further the opacity barrier of synchrotron self-absorption in AGN, and extending VLBI to baseline lengths larger than the Earth size with radio telescopes in space.

<div class="hide_in_print">Space-VLBI Image of the BL Lac Object 0716+714</div>
Space-VLBI Image of the BL Lac Object 0716+714

Space-VLBI: Ground-space VLBI provides an alternative way of increasing resolution in radio interferometry.  The Russian RadioAstron  project successfully launched a 10-m radio telescope, Spektr-R, in July 2011.  With a perigee of 10,000 km and an apogee of 399,000 km, angular resolution down to a few microarcseconds are possible.  Available wavelengths are 92, 18, 6, and 1.3 cm.  Our group is involved on several Key Science Programs of this collaboration, leading 3 (out of 8) of these.  The VLBI Technology Division is involved in the correlation of ground-RadioAstron data with the MPIfR DiFX software correlator.

<div class="hide_in_print">Simulation of onset of jet near black hole</div>
Simulation of onset of jet near black hole

mm-VLBI: The MPIfR leads the operation of the Global mm-VLBI Array (GMVA), which combines European mm-telescopes (Effelsberg, Pico Veleta, Plateau de Bure, Onsala, Metsähovi) with the Very Long Baseline Array in the USA to provide a 3-mm imaging capability with 50 microarcsecond resolution.  The network has prospects of future participation with additional mm-telescopes such as the ones of the Korean VLBI Network. 

VLBI at 1.3mm constitutes the next logical (and challenging) step for higher angular resolution and to overcome further the opacity barrier of synchrotron self-absorption in AGN and to open a direct view into sub-parsec scale regions not previously accessible.  The so-called Event Horizon Telescope (EHT) has the goal of imaging the shadow of the black hole in the Galactic Centre at this wavelength.  The high technological approach requires the participation of the VLBI Technology division.

VLBI monitoring of milliarsecond-scale structural changes

TANAMI view of Centaurus A
TANAMI view of Centaurus A

MOJAVE: The project MOJAVE (Monitoring of Jets in AGN with VLBA Experiments) is an extensive VLBA monitoring survey aimed at studying the evolution and magnetic field structure of parsec-scale jets in blazars at 15 GHz.  MPIfR scientists are major participants in this NRAO key science program. Most of the jets in the MOJAVE project have been monitored since the mid-1990s providing a unique opportunity to study their long-term behaviour including accelerations, bending, and development of instabilities in jets.  Several multi-frequency experiments yield Faraday rotation measures, frequency-dependent core-shifts and spectral index maps for a subset of sources.

TANAMI:  The project TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) is a younger brother of the MOJAVE program, to provide dual-frequency (8 and 22 GHz) monitoring of extragalactic jets south of −30° declination.  The array comprises the Australian Long Baseline Array and telescopes in South Africa (Hartebeesthoek), Antarctica (O’Higgins) and Chile (TIGO), and new sites in New Zealand are joining the array.

Spectral and Polarisation Monitoring


The F-GAMMA program (Fermi-Gamma-ray space telescope AGN Multi-frequency Monitoring Alliance) is run by a MPIfR-lead consortium of scientific groups and observatories, to collect high-precision broad-band flux density and polarisation data for a large number of gamma-ray loud AGN in the ‘low-energy’ synchrotron part of blazar spectral energy distributions.  The alliance includes the Effelsberg 100-m, Pico Veleta 30-m and the APEX 12-m telescope for 0.8 mm band observations.


ROBOPOL is a project to build an optical polarimeter for a systematic study of the large swings of optical polarisation angle, often seen during high-energy outbursts of blazars.  Those give clues of the magnetic field strength, jet composition and physical processes.  The project is run by a consortium including MPIfR, Caltech, IUCAA, and Toruń Observatory.

Other projects

The research portfolio of the MPIfR VLBI department is complemented by several research projects, several of them addressing AGN jet phenomenology and the underlying processes, but also addressing Galactic objects, the Galactic Centre source SgrA*, X-ray binaries, supernovae, and supernova remnants.


Our department leads RadioNet4, a project supported by the European Commission under the Horizon 2020 Framework Programme. RadioNet4 builds on the success of two preceding RadioNet projects and takes a leap forward towards the facilities of the future (such as ALMA and the SKA).

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