Radio Astronomy / VLBI

Radio Astronomy / VLBI

mm-VLBI in our scientific and technology divisions

Radio Astronomy/VLBI Department: Overview

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.

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.

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 operated a 10-m radio telescope, Spektr-R, between July 2011 and January 2019.  With a perigee of 10,000 km and an apogee of 399,000 km, angular resolution down to a few microarcseconds was made possible.  Available wavelengths were 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.  After the termination of the space mission in mid 2019, the data post-processing and analysis will continue over several years.

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.

Millimetre very-long-baseline interferometry at the Max Planck Institute for Radio Astronomy

The Max Planck Institute for Radio Astronomy in Bonn (Germany) is one of the key players in the Event Horizon Telescope (EHT) collaboration. The team led by J. Anton Zensus played a crucial role in the discovery of the Black Hole in M87. The movie describes the work and achievements of the group in the field of mm VLBI: from observational and technological challenges until the break through scientific discoveries.

The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. In coordinated press conferences across the globe, EHT researchers revealed that they succeeded, unveiling the first direct visual evidence of the supermassive black hole in the centre of Messier 87 and its shadow.

The shadow of a black hole seen here is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across. While this may sound large, this ring is only about 40 microarcseconds across — equivalent to measuring the length of a credit card on the surface of the Moon.

Although the telescopes making up the EHT are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm during a 2017 global campaign. Each telescope of the EHT produced enormous amounts of data – roughly 350 terabytes per day – which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.

VLBI monitoring of milliarsecond-scale structural changes

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.

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.

Opticon RadioNet Pilot Project

Until now, Europe has had two major collaborative networks for ground-based astronomy, one in the optical domain and the other in the radio-wave domain. OPTICON and RadioNet came together in March 2021 to form Europe’s largest ground-based astronomy collaborative network. Launched with funding to the tune of €15 million under the Horizon 2020 programme, the Opticon RadioNet Pilot project aims to harmonise observational methods and tools, and provide access to a wider range of astronomy facilities. The CNRS will coordinate the project, together with the University of Cambridge and the Max Planck Institute for Radio Astronomy.

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