mm-VLBI in our scientific and technology divisions
EHT Image of the Black Hole in SgrA* - MPIfR scientists tell the story
Special episode of RadioViews! This time we have insight information, emotional thoughts, and exciting scientific news collected by the Event Horizon Telescope (EHT) Collaboration members at the Max Planck Institute for Radio Astronomy in Bonn, Germany. It has been a great, devoting journey since Sagittarius A* was observed by the EHT in 2017. Our colleagues tell their side of the story.
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.
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.
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: 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.
VLBI monitoring of milliarsecond-scale structural changes
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.
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.