Research Highlights
Here we show recent research results from the Radio Astronomy/Very-Long-Baseline Interferometry department.
Monitoring of radio galaxy M87 confirms black hole rotation
26 September 2023
The nearby radio galaxy M87, located 55 million light-years from the Earth and harboring a black hole 6.5 billion times more massive than the Sun, exhibits an oscillating jet that swings up and down with an amplitude of about 10 degrees, confirming the black hole's spin.
The study, which was headed by Chinese researcher Dr. Cui Yuzhu and published in Nature on Sept. 27, was conducted by an international team using a global network of radio telescopes, including RuSen Lu and Jae-Young Kim, affiliated to the MPI für Radioastronomie.
Through extensive analysis of telescope data from 2000–2022, the research team revealed a recurring 11-year cycle in the precessional motion of the jet base, as predicted by Einstein's General Theory of Relativity. The study links the dynamics of the jet with the central supermassive black hole, offering evidence that M87's black hole spins. More information can be found at the Nature paper cited above.
A further success of the space-VLBI RadioAstron mission: discovery of a mini-cocoon around the restarted parsec-scale jet in 3C 84
17 August 2023
In the current issue publication in The Astrophysical Journal, an international team researchers led by Tuomas K. Savolainen from Aalto University, also affiliated to the MPIfR, unveiled a remarkable finding in the galaxy 3C 84. Utilizing space-VLBI observations, they detected a mini-cocoon surrounding a reactivated parsec-scale jet. These observations, conducted with a global array of radio telescopes, provided unprecedented resolution, revealing intricate sub-structures within the jet. Most notably, the 5 GHz image revealed low-intensity emissions from the cocoon-like structure, suggesting that the jet's increased power inflates a hot plasma bubble as it traverses the galaxy's central region. The study estimates the mini-cocoon's energy, pressure, volume, and its impact on the interstellar medium. The findings indicate that a significant portion of the jet's energy is transferred to this mini-cocoon, and its quasi-spherical shape influences the energy distribution in the surrounding medium, shedding new light on the dynamics of radio galaxies. This discovery has the potential to reshape our understanding of cosmic processes and will undoubtedly stimulate further exploration and discussion in the field of astrophysics. More information, at the original publication here.
Space-VLBI to probe the compactness of the central region of M87 at 13 mm wavelength
14 July 2023
RadioAstron has observed the central region of the well-known elliptical galaxy M 87, by using state-of-the-art 22 GHz space-VLBI in the framework of the RadioAstron mission. As part of the Nearby AGN Key Science Program, the galaxy was observed in February 2014. These observations, involving 21 ground stations and extending to Earth-sized projections, resulted in spatial resolutions as fine as 20 Schwarzschild radii, equivalent to a mere 150 microarcseconds. This probed details of the core region previously concealed. The most remarkable revelation is the identification of an extraordinarily high brightness temperature, surpassing 1012 K. This unexpected finding challenges existing theoretical frameworks and prompts astronomers to explore unconventional explanations, such as extreme Doppler boosting, unique jet perspectives, or novel particle acceleration mechanisms. This work, led by Jae-Young Kim from Kyungpook National University and also affiliated to the MPIfR are presented in the current issue of The Astrophysical Journal. More information can be found in the original publication here.
MOEA/D: A Novel Approach to Radio Astronomy Imaging
30 June 2023
In the vast realm of radio astronomy, image reconstruction remains an intricate puzzle. Despite the rising sensitivity and capabilities of telescopes, the challenge of solving this ill-posed inverse problem persists. Recent breakthroughs have introduced innovative algorithms, featuring constrained nonlinear optimization and Bayesian inference, as potential solutions. The Event Horizon Telescope (EHT) Collaboration's endeavors in scrutinizing image reconstructions have offered valuable insights. Yet, when dealing with active galactic nuclei, existing methods encounter a time-consuming obstacle: the need for large and expensive surveys, each with different optimization parameters. In a new publication led by the MPIfR researcher Hendrik Müller, together with Alejandro Mus from the Universitat de València in Spain and Andrei Lobanov from the MPIfR, they present a groundbreaking solution – a nonconvex, multiobjective optimization approach that opens new avenues. Their methodology leverages a multiobjective version of the genetic algorithm, known as MOEA/D. This genetic algorithm explores the objective function through evolutionary operations, identifying various local minima while avoiding the pitfalls of saddle points. These experiments, utilizing synthetic data based on the 2017 EHT array and a potential future EHT configuration, demonstrate the power of MOEA/D. We successfully unveil a complete Pareto front of nondominated solutions, showcasing the diverse image morphologies. The publication discuss strategies for identifying the most natural guess among these solutions and put it to the test using synthetic data. Finally, they apply MOEA/D to observe the black hole shadow in Messier 87 with the 2017 EHT data. In conclusion, MOEA/D proves to be a flexible and efficient tool, outperforming Bayesian methods in terms of speed and solution exploration. This research comprises the first of two papers, with the first explaining the fundamentals of multiobjective optimization and MOEA/D while demonstrating its capabilities in recovering static images. A further publication will extends the algorithm's utility, enabling dynamic and polarimetric reconstructions, both static and dynamic. More details about the present publication, at the current issue of Astronomy & Astrophysics, is available here.
DoG-HiT: A Breakthrough in High-Resolution Imaging
24 May 2023
A substantial change forward is underway in the field of Very Long Baseline Interferometry (VLBI). With a limited number of antennas and constrained observing time, imaging cosmic phenomena with precision has always been a challenge. However, recent innovations in multiscalar imaging, such as the DoG-HiT method, have changed the game. Known for their speed, accuracy and unbiased performance, these techniques address the sparsity problem in Fourier domain sampling. In a publication written by the MPIfR scientists Hendrik Müller and Andrei P. Lobanov, they introduce a multiscalar approach that extends to polarimetric imaging, dynamically evolving sources, and dynamic polarimetric reconstructions, aptly named "mr-support imaging". By using wavelet transforms and a set of statistically significant coefficients as priors, we've achieved remarkable results. Synthetic data tests with the Event Horizon Telescope (EHT) demonstrate the power of mr-support imaging, providing a rich regularisation for complex dynamics at the event horizon scale. Looking ahead, the ngEHT extension promises even more exciting opportunities for dynamic polarimetric reconstructions. As instruments continue to evolve, the potential to observe dynamically evolving patterns in unprecedented detail grows. This work introduces a simpler but effective regulariser, multiresolution support, to the arsenal of dynamic reconstruction methods. More details can be found at the present issue of Astronomy & Astrophysics, here.
First image of a black hole expelling a powerful jet
Selected press coverage
Articles derived from the Nature paper and the related press releases
A Fresh View of an Increasingly Familiar Black Hole (Dennis Overbye), The New York Times, 26 April 2023
Black-hole image reveals details of turmoil around the abyss (Davide Castelvecchi), Nature, 26 April 2023
Testing Gravity with Black Holes: Can we tell them apart?
16 March 2023
A team of researchers led by the PhD candidate Jan Röder at the MPIfR has used computer simulations to test their ability to distinguish between two theories of gravity under different accretion and emission scenarios. They carried out 3D simulations of two black holes, one based on the Kerr solution to general relativity and the other on ton a branch of solutions to Einstein-Maxwell-dilaton-axion gravity, the dilaton black hole. Further, they modelled thermal synchrotron emission and applied a non-thermal electron distribution function to compare with multi-wavelength observations. The study found that differences between the two black hole spacetimes are not always distinguishable with current observational technology, and that the choice of emission model has a greater effect on the spectra than the accretion model or the choice of spacetime. More information is available directly at the publication in the last issue of Astronomy and Astrophysics, here.
Radio dynamics of the binary black hole candidate OJ 287 from Effelsberg monitoring
23 February 2023
In a new study from the MOMO program, addressing the multifrequency radio variability of the blazar OJ 287 from 2015 to 2022, led by Stefanie Komossa from the MPIfR, data covering a wide range of activity states and employs data spanning from 2015 to 2022, complemented by Fermi γ-ray observations, are presented. The team of researchers employ techniques such as discrete correlation functions to analyse the data. The connection between the radio emissions and the multiwavelength radiation is explored in great detail. Fascinatingly, they discover deep fades in both radio and optical-UV fluxes that recur every 1-2 years. However, one of the most intriguing findings is the absence of a precursor flare of thermal bremsstrahlung, as predicted by one of the binary supermassive black hole models for OJ 287. This raises questions about the prevailing understanding of this blazar's behavior. The study also focuses on the remarkable 2016/2017 nonthermal outburst initially detected with Swift, shedding light on its nature. The authors propose that this outburst represents the latest occurrence of the famous optical double-peaked outbursts of OJ 287, favoring binary scenarios that do not require a highly precessing secondary supermassive black hole. For more details on this research, check out the full publication in the Astrophysical Journal here.
General relativity and the twinkling of Sgr A* in Infrared and X-Ray light
18 January 2023
An international team of astronomers led by Sebastiano von Fellenberg at the MPI für Radioastronomie has studied the supermassive black hole at the center of the Milky Way, called Sagittarius A*. Astronomers have observed occasional bright flares of near-infrared and X-ray light. These flares are thought to come from the black hole's innermost accretion flow. By analyzing data from the Spitzer and Chandra observatories, the researchers identified 25 near-infrared and 24 X-ray flares. Using a computer program that takes into account the effects of general relativity, the researchers modeled the trajectories of "hot spots" and examined the light curves of the flares for signs of these effects. They found that, despite their varying shapes, all flares share a common, exponential impulse response. This impulse response is symmetric, meaning that the rise and fall times are the same, and has an exponential time constant of about 15 minutes. The researchers determined that the characteristic flare shape is not consistent with hot-spot orbits viewed edge-on, and were able to estimate the inclination of the orbital plane of the hot spots with respect to the observer (about 30 degrees, but less than 75 degrees) and the characteristic timescale of the intrinsic variability (a few tens of minutes).
More information, at the original publication in the Astronomy & Astrophysics journal, here.
Journey to the Heart of a Cosmic Beast: Uncovering the Connection Between Radio Waves and Gamma Rays in 3C 84
03 January 2023
A team of radio astronomers, led by the young scientist Georgios F. Paraschos at the MPI für Radioastronomie, has studied the conection between the high-energy and the radio emission in the nearby quasar 3C 84. The team compared the radio and gamma-ray light curves of 3C 84 (aka NGC 1275) to understand how its jets are formed. By analyzing the time differences between the flares seen in these light curves, they found that the energy of the particles and magnetic fields in the jets are balanced. They also determined the location of the "jet apex" and found that the gamma-ray emission is related to the radio emission. Additionally, the team led by Dr. Paraschos calculated two parameters that describe the properties of the jet, and the results are consistent with a mechanism proposed in 1977 by Blandford and Znajek for jet formation. These findings provide new insights into the mysterious and complex processes that drive the formation and evolution of cosmic jets.
More information at the original publication in the Astronomy & Astrophysics journal, here.