5xERC on Jets

Oct 22, 2025, 4:00 PM → Oct 25, 2025, 9:30 AM Europe/Berlin

Ringberg Schloss

Andrei P. Lobanov (MPI für Radioastronomie), Eduardo Ros, Ioannis Liodakis (FORTH), J. Anton Zensus (MPI für Radioastronomie), Minghui Xu (GFZ Potsdam), Talvikki Hovatta (Turku Univ.), Yuri Y. Kovalev (MPI für Radioastronomie)

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The workshop “5xERC on Jets” is a scientific meeting to be held at Ringberg Castle in Kreuth, south of Munich, from 22–24 October 2025. This event brings together the teams of five ERC-funded projects—M2FINDERS, ASTROGEODESY, BOOTES, PARTICLES, and MuSES—which, while distinct in methodology and focus, all converge on the study of relativistic jets in active galactic nuclei (AGN).

 

Conference group picture.  © Steinbach Industriephotographie

These projects tackle complementary aspects of jet physics, including magnetic field structures, time-domain optical polarization, multi-messenger particle composition, high-resolution VLBI imaging, and astrometric geodesy. The workshop provides a platform for sharing recent results, fostering collaboration, and addressing open challenges at the intersection of VLBI science, high-energy astrophysics, and jet composition studies.

The meeting also includes a select group of external experts and collaborators to broaden perspectives and deepen the scientific dialogue. Participation is by invitation only.

These projects have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme.  The details are:
ERC Advanced Grant - MuSES: Multi-messenger Studies of Energetic Sources, PI: Prof. Yuri Kovalev (Scientists, MPIfR), Grant Agreement No 101142396.
ERC Advanced Grant - M2FINDERS: Mapping Magnetic Fields with INterferometry Down to Event hoRizon Scales, PI: Prof. J. Anton Zensus (Director, MPIfR), Grant Agreement No 101018682.
ERC Consolidator Grant - PARTICLES: Particle Composition in Relativistic Jets, PI: Dr. Talvikki Hovatta (Aalto and Turku Universities, Finland, long-term collaborator with MPIfR through the MOJAVE and other projects), Grant Agreement No 101169986.
ERC Starting Grant - BOOTES: Black hOle Optical polarization TimE-domain Survey, PI: Prof. Ioannis Liodakis (Greek Foundation for Research and Technology, currently a Humboldt Fellow at MPIfR), Grant Agreement No 101076343.
ERC Starting Grant - ASTROGEODESY: Astro-geodesy by VLBI Global Observing System, PI: Dr. Ming Hui Xu (GFZ Potsdam, also guest scientist at MPIfR), Grant Agreement No 101076060.

poster_5xERC.pdf

Wed, October 22

6:30 PM → 8:00 PM Dinner

8:00 PM → 9:30 PM Preparation Science Working Game I

Thu, October 23

8:00 AM → 8:55 AM Breakfast

9:00 AM → 10:30 AM Session I

9:00 AM Welcome Remarks, Logistics

Speakers: Andrei Lobanov (Max-Planck-Institut für Radioastronomie, Bonn), Eduardo Ros, Yuri Kovalev (MPIfR)

9:10 AM Multi-Messenger Signatures from Accreting Black Holes and Relativistic Jets

We investigate the observational signatures of black hole accretion and jet formation using two-fluid GRMHD simulations that incorporate electron heating guided by kinetic (PIC) studies. Our radiative transfer calculations include anisotropic electron distributions, leading to strongly edge-brightened jets — a feature consistent with high-resolution VLBI observations of CenA and M87. By including resistivity, we investigate particle acceleration in the black hole magnetosphere and estimate the resulting neutrino emission. These simulations offer new insights into the link between black hole accretion, jet physics and multi-messenger signals. In this talk, I will present our latest results and outline future work within the DFG Research Unit "Relativistic Jets from AGNs."

Speaker: Christian Fromm

9:30 AM From ChatGPT to ChatGBT: What AI can do in radio interferometric data processing and what not.

Artificial intelligence is everywhere. It is expected to transform full disciplines and job sectors. Interestingly, among the first applications proposed for machine learning were pattern recognition and denoising tasks, and AI algorithms dominate the medical imaging sector, yet they have not yet enjoyed wide-spread application in radio-interferometry for various reasons. In this talk, I will discuss the current status of AI algorithms in radio interferometry, their potential and limitations, and present our latest efforts to understand and develop these approaches. Finally, I am going to discuss the question whether there will be a system such as ChatGBT (ChatGreenBankTelescope) or Artificial Intelligence Processing System (AIPS) in the next years, which replaces the traditional astronomers input during the calibration and imaging, and why that may remain a dream.

Speaker: Hendrik Mueller

9:50 AM FOR5195: A Network of German AGN-Jet Research

The DFG Research Unit FOR5195 has been founded in 2022 and is addressing open questions in AGN jet research as a coordinated network of 12 funded research groups across Germany and several associated groups: How are relativistic jets launched from the vicinity of supermassive black holes? How are they accelerated and collimated? Which radiative and dynamic processes govern the high-energy emission of AGN jets and what are AGN jets composed of? How do AGN jets interact with their host galaxy? How does AGN feedback heat the ICM and which are the observational signatures? To address these questions, we unite expert groups in theory, modeling and interpretation of observational data collected by the latest astronomical facilities across all scales and at all observational wavelengths.

Speaker: Matthias Kadler (JMU Würzburg)

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:30 PM Session II

11:00 AM Black hOle Optical-polarization TimE-domain Survey -- BOOTES

Supermassive black holes form the most intriguing astrophysical systems offering countless opportunities to study fundamental physics in regimes not accessible to laboratories on Earth. Optical polarization can provide answers to long-standing black hole physics questions since polarized signatures can distinguish between competing theories. However, the polarimetric data necessary for such a task are missing. BOOTES is a unique observational program that can unify our understanding of transient and steady supermassive black hole systems using multiwavelength polarimetry. The project leverages on an unprecedented commitment of telescope time and a high-accuracy optical polarimeter from the Skinakas observatory to produce (1) the first comprehensive optical polarization monitoring of tidal disruption events; (2) the first systematic very-high-cadence optical polarization monitoring of relativistic jets. I will present the overall project as well as intriguing results from the two years of the project that started in January 2024.

Speaker: Ioannis Liodakis (Institute of Astrophysics - FORTH)

11:20 AM Looking at AGN through the prism of optical polarimetry

Optical data provide the longest and richest historical view of AGNs, but interpreting them is challenging because the emission combines multiple components: the accretion disk, scattering regions within the central parsec, the jet, and the light from the host galaxy. Optical polarization is a powerful tool to disentangle these contributions. Produced by several mechanisms, such as dust scattering or synchrotron emission, different mechanisms dominate depending on AGN type. Studying various AGN types, particularly non-blazars, allows us to probe gas kinematics near the supermassive black hole, AGN orientation, and accretion disk properties. In this talk, I will focus on what optical polarization can reveal in AGNs, beyond the synchrotron-dominated component, highlighting phenomena invisible in the radio.

Speaker: Dr Elena Shablovinskaia (Max-Planck-Institut für Radioastronomie)

11:35 AM Space-VLBI imaging and filament evolution of 3C 279

The space-VLBI mission RadioAstron at 22 GHz enables the study of the fine-scale structure of the most compact regions of AGN. With a resolution of 26 uas, we study the blazar 3C 279 who showed a filamentary structure in 2014. We study the evolution of this source regarding its morphology and polarization with new data from 2018 (the last one for this source from RadioAstron mission) using RML method eht-imaging. We also compare this epoch with near-in-time observations from monitoring program BEAM-ME and provide a brightness temperature study, showing a core consistent with near-equipartition.

Speaker: Teresa Toscano Domingo (Max-Planck-Institut für Radioastronomie)

11:50 AM Polarization variability in Tidal Disruption Events

Polarimetry of Tidal Disruption Events (TDEs) provides an invaluable diagnostic for the geometry and physical mechanisms driving the accretion flow, potentially distinguishing between competing emission models. Recent observations have revealed that even non-jetted TDEs can be highly polarized (e.g. AT2020mot), raising fundamental questions about emission mechanisms during the early stages of accretion. While stellar stream shocks offer an attractive interpretation, it is not yet clear whether this is a common underlying mechanism.
I will present results from the Black hOle Optical polarization TimE-domain Survey (BOOTES): the first systematic optical polarization survey of TDEs using the Nordic Optical Telescope, the Calar Alto 2.2m telescope, the Skinakas Observatory 1.3m telescope and the Very Large Telescope.
We find a variety of polarization behaviors across our sample. Combined with contemporaneous multi-wavelength data, these trends constrain the geometry and dynamics of the disrupted debris and the nascent accretion flow. In particular, we report time-variable polarization degree and position angle -an effect not previously reported in TDEs- pointing to more dynamic and anisotropic systems than commonly assumed.
Our results showcase how polarimetric monitoring opens a new window on the mechanisms powering supermassive-black-hole accretion and on the earliest stages of accretion-disk formation, and they provide testable predictions to distinguish shock-powered and reprocessing-dominated models.

Speaker: Alberto Floris (Institute of Astrophysics - FORTH)

12:05 PM Bayesian polarization calibration and imaging using resolve

Polarimetry using Very Long Baseline Interferometry (VLBI) observations of synchrotron emission from relativistic jets and accretion disks provides invaluable information about the magnetic field structure in active galactic nuclei (AGNs). However, calibration and imaging of polarimetric VLBI data are challenging due to the low signal-to-noise ratio (SNR) of polarimetric signals, instrumental leakage, and complex structure of the source. Recent progress in Bayesian imaging and computational power provides a strong foundation for overcoming the known limitations of the conventional calibration and imaging methods based on the CLEAN algorithm. This talk presents a novel approach for simultaneous gain and leakage calibration, and polarimetric image reconstruction using Bayesian imaging software "resolve". In Bayesian imaging, we can perform calibration and imaging simultaneously and obtain higher-resolution images with uncertainty estimation. Furthermore, the complex source structures are incorporated into the calibration process for robust polarization leakage calibration with less supervision. As examples, we reconstruct polarimetric images of GMVA+ALMA OJ287 at 86GHz and VLBA 3C273 at 15GHz using a-priori calibrated data. From posterior samples, we quantify the reliability of the calibration solutions and Stokes images. Our results clearly demonstrate that an automated Bayesian calibration and imaging approach can be used successfully for polarimetric image reconstruction and robust recovery of polarization information, such as electric vector position angle (EVPA) and rotation measure, which in turn helps to probe magnetic field configuration in AGNs.

Speaker: Jongseo Kim (MPIfR)

12:20 PM resolve-ing Cygnus-A - a multifrequency view

We report multi-frequency Very Long Baseline Interferometry (VLBI) observations at 12, 15, 22, and 43 GHz with the VLBA and at 86 GHz with the GMVA. Image reconstruction was performed with the Bayesian algorithm resolve, which achieves intrinsic resolution, which is better relative to the CLEAN algorithm. Moreover, resolve enables simultaneous, joint imaging across multiple frequencies. We test this multi-frequency imaging framework and present the first results. The improved-resolution spectral products enabled by resolve will allow us to trace spectral and magnetic-field structures closer to the vicinity of the black hole.

Speaker: Aleksei Nikonov (MPIFR)

12:30 PM → 2:00 PM Lunch

2:00 PM → 3:30 PM Session III

2:00 PM MuSES: introduction

I will introduce the advance ERC project MuSES (grant No 101142396) "Multi-messenger Studies of Extragalactic Super-colliders".
We are taking a further step in AGN studies by using a multimessenger approach aimed at probing jet formation and collimation, particle acceleration, and neutrino production mechanisms in blazars.
See details in https://www.mpifr-bonn.mpg.de/muses

Speaker: Yuri Kovalev (MPIfR)

2:20 PM Probing the Statistical Correlation of Tidal Disruption Events with High Energy Neutrinos

High energy (HE) neutrinos have been observed by neutrino observatories for over a decade. Nevertheless, their origin and mechanisms responsible for their production still remain a mystery. Tidal disruption events (TDEs) have been proposed as candidate HE neutrino emitters, however a statistical association between the two has yet to be established. I will discuss results from the statistical association of a large sample of optical TDEs with neutrinos using TDECat and HE neutrino events observed by IceCube. While our analysis disfavors the association between neutrinos and the TDE population, we still find interesting associations with individual events including the jetted TDE Sw J2058+05. Our results highlight the need of upcoming surveys like LSST and next-generation neutrino observatories(e.g. KM3NET, IceCube-Gen2) to further test this potential correlation.

Speaker: Dimitrios Alkinoos Langis (Institute of Astrophysics FORTH)

2:35 PM MOJAVE ejections and gamma-ray flares: are they related?

The production mechanism and site of gamma-ray emission in AGN have been under debate for decades. In the framework of leptonic models, constraining the location of gamma-ray radiation production would favor one source of seed photons among the accretion disk, the broad line region, the dusty torus and the jet itself via Synchrotron self-Compton. With this goal, we investigate the gamma-ray $-$ radio association in a sample of about 50 AGN.
We make use of Fermi/LAT adaptively binned light curves and the appearance times of new jet features as traced by the MOJAVE project, which are the result of the unparalleled observational cadence over the last $\sim$30 years to study AGN radio jets. We investigate the correlation between gamma-ray flares and jet knots ejection epochs considering variable time lags.
I will present early results of the gamma-ray $-$ VLBI analysis and discuss constraints on the origin of gamma-ray emission from blazar jets.

Speaker: Cecilia Degli Agosti (MPIfR)

2:50 PM Search for blazar counterparts to the high-energy neutrino event IceCube-201130A: a multi- frequency VLBI study

To this day, the mechanisms behind the ejection, acceleration, and collimation of jets in active galactic nuclei (AGN) are not fully understood. In recent years, AGN, including blazars, have been shown to be promising candidate sources of cosmic high-energy neutrinos with energies above 100TeV. The regions where proton acceleration is expected to occur, in order to produce such neutrinos, lie very close to the central engine, at scales of just a few parsecs. Investigating possible neutrino production in blazars can provide major insight into the processes taking place at the centers of the most luminous objects in the universe.
So far, ten VLBA follow-up trigger observations at three frequencies are available in our data set, targeting blazars associated with high-energy neutrino alerts. The present work constitutes the first detailed study of one of them. Specifically, the event IceCube-201130A presents a valuable test case for individual associations. It is a track-like event with a high probability of astrophysical origin, detected on 30 November 2020 by the IceCube observatory. The blazars 0159−117 and 0203−120 both lie within its 90% localization region, making them candidate sources for its production. Using six epochs of VLBA data, we examine their jet kinematics, spectral and polarization properties, variability, and other physical characteristics that might point to changes in the jet around the time of the neutrino event.

Speaker: Georgios Kalaitzidakis (Max Planck Institute for Radio Astronomy)

3:05 PM High-Frequency Radio Studies of High-Energy Emitting AGN Jets

High-frequency radio astronomy plays a key role in the new era in multi-messenger astronomy, particularly for studying the physics of high-energy neutrino-emission and TeV gamma-rays from blazar jets. In the context of the DFG Research Unit FOR5195, we conduct high-frequency single-dish and VLBI radio observations to identify flares and probe the jet structures of these high-energy sources at sub-milliarcsecond scales. The TELAMON program at the Effelsberg 100-m telescope is designed to monitor TeV-blazars and neutrino-candidate AGN at high radio frequencies up to 44 GHz with the goal of characterizing their variability and enabling rapid VLBI follow-up. This is complemented by a new long-term mm-VLBI monitoring program on the upgraded GMVA. In this talk, we put a special focus on new results on the neutrino-candidate blazars PKS 0215+015 and TXS 0506+056, combining TELAMON monitoring with dedicated VLBI observations, and additional data from the MOJAVE program.

Speaker: Florian Eppel (JMU Würzburg)

3:30 PM → 4:00 PM Coffee Break

4:00 PM → 5:15 PM Session IV

4:00 PM Seeking time- and frequency-stable reference points for geodesy and astrometry

AGNs serve as the reference on the sky for geodesy and astrometry for realizing the fundamental terrestrial and celestial reference frames since they are far distant. AGNs in these geodetic and astrometric observations typically have the core and jet structure that is variable over time and frequency on the spatial scales from mas to sub-mas. It is now understood that seeking time- and frequency-stable reference points is very essential. Project Astrogeodesy aims to model source structure of hundreds of AGNs to obtain the stable referecence points for the new generation geodetic VLBI system named VGOS, with the MOJAVE data as a pilot testbed. This talk will introduce the Astrogeodesy project.

Speaker: Minghui Xu (German Research Center for Geosciences (GFZ))

4:20 PM Upgraded Astrometry for the Study of Relativistic Jets in AGNs, in ngVLBI era.

Bona Fide Astrometry is a tool for the study of relativistic jets in AGNs. It eliminates perilous assumptions and is used for robust alignment of time and frequency sequencing of images.
The exquisite resolution power inherent to VLBI results in the highest precision astrometry and resolution images, making it a unique tool for studies of the cosmological distant objects, such as AGNs.
Next-generation astrometrical methods (SFPR, MultiView) developed in the last decade have expanded the application of astrometry to new research fields, and can address key open scientific questions, with high precision measurements at a wider range of frequencies and for many more objects, compared with standard Phase Referencing techniques.
M2FINDERS WP2 is concerned with SFPR. Here we present a scientific driver to expand the toolset for the direct study of AGN jets , by incorporating innovative absolute and relative astrometric measurements and thereby discriminate between the proposed scenarios for jet formation mechanisms and characterise the powering BH.

Speaker: Dr Maria J. Rioja (ICRAR-UWA / IGN, OAN)

4:40 PM Interferometric Closure-Based Analysis of Core-Shift and Faraday Rotation from VGOS Observations

Understanding the physical conditions and structure of compact jets in active galactic nuclei (AGN) is a key objective of geodetic very long baseline interferometry (VLBI). These jets, typically associated with blazars and radio-loud quasars, are commonly used in geodetic VLBI scheduling. Accurate characterization of features like magnetic field structure or core-shift has important implications for both jet physics and the precision of VLBI-based astrometry and geodesy.

The VLBI Global Observing System (VGOS), with its broad, simultaneous
frequency coverage from 3 to 11 GHz, offers a unique opportunity for high-fidelity, multi-frequency polarimetric studies of compact AGN jets. However, to properly analyze VLBI data, a thoughtful calibration is required, as atmospheric and instrumental effects can contaminate the observed visibilities, modulating signal phase and amplitude.

In this talk, we present a new method for characterizing AGN jet structures using closure traces, a novel set of observables immune to atmospheric and instrumental effects. By applying a physically motivated, parameterized model of compact jet components to VGOS observations of the blazar 1803+784, we perform a robust, calibration-independent analysis of its physical properties.

Speaker: Ezequiel Albentosa Ruiz (University of Valencia)

4:55 PM Collection of 130,000 AGN images

Over a course of 30 years, over 40,000 sources were observed
in a number of all-sky programs for determination of their
positions and brightness distributions. More than 1/2 of
them have been detected and imaged. I will present the summary
of these programs and characterize the image collection as
a whole.

Speaker: Leonid Petrov

5:15 PM → 6:15 PM Preparation Science Working Game II

6:30 PM → 8:00 PM Dinner

8:00 PM → 9:30 PM Preparation Science Working Game III

Fri, October 24

8:00 AM → 8:55 AM Breakfast

9:00 AM → 10:30 AM Session V

9:00 AM The M2FINDERS Project - An Update

Recent breakthroughs from e.g., LIGO/VIRGO, GRAVITY and the Event Horizon Telescope (EHT) privde compelling evidence for the existence of black holes and enabled imaging of structures at event-horizon scales. However, we are still making progress on one of the most crucial aspects of understanding black hole physics: the strength and geometry of the magnetic field near the event horizon. This field governs jet formation and accretion dynamics, but it cannot be fully investigated through imaging alone.

M2FINDERS aims to address this issue by combining multi-frequency polarimetric VLBI imaging, opacity and rotation-measure tomography, and advanced modelling of relativistic flows. M2FINDERS is designed to deliver the most stringent constraints yet on the strength, topology and evolution of magnetic fields around supermassive black holes.

Recent EHT polarisation studies of M87* have revealed dynamic magnetic structures, including horizon-scale field reversals and evidence of magnetically arrested accretion. These discoveries highlight the need for high-fidelity, multi-frequency and time-resolved polarimetry — the core focus of M2FINDERS. The project will therefore provide magnetographic insights that complement EHT imaging, offering new tests of jet-launching models and event-horizon physics and advancing our empirical understanding of how black holes power relativistic outflows.

Speaker: Eduardo Ros

9:20 AM JetOrigin project

In the currently favoured theoretical picture, the relativistic jets from supermassive black holes are powered by magnetic fields extracting rotational energy of the spinning black hole and/or the accretion disk and the acceleration and collimation of the flow is also ascribed to a magnetohydrodynamic (MHD) process. Rapid advancement in numerical simulations using general relativistic MHD codes has provided support for this picture by demonstrating the launching of fast jets from spinning black holes. Testing the magnetic jet formation model through observations is, however, difficult, and there remains a significant gap between the simulations and observations, which is due to shortcomings on both sides. In the JetOrigin project, we aim to bridge this gap from the observational side, taking advantage of the developments in the interferometric capabilities at the short mm-wavelengths. We are using the superb polarimetric capability offered by the ALMA array to measure Faraday rotation to probe the magneto-ionic medium in and around the jet base. Faraday rotation measure (RM) is an observable that can be relatively directly compared to the MHD simulations since it depends on the magnetic field and the density of the cold electrons, both available from the GRMHD output. Polarimetric VLBI observations are then used to map the RM distribution and constrain the 3D magnetic field structure on scales relevant for testing the simulation models. For this, we have already obtained Event Horizon Telescope (EHT) and Global mm-VLBI Array observations of 3C273 that will be analysed in the project. These observations will be compared with a suite of GRMHD jet formation simulations. We will also use multi-band VLBA observations of a sample of 25 sources to derive jet velocity fields, collimation profiles, and magnetic field profiles that will be compared against the predictions of magnetic jet acceleration models.

Speaker: Dr Tuomas Savolainen (Aalto University)

9:40 AM Magnetic Field Structure of AGN Jet with ALMA Polarimetry

Magnetic fields are known to be involved in the launching of relativistic jets from supermassive black holes. While there is lack of knowledge on the precise jet launching mechanisms, both observations and simulations have recently provided us new information on the expected magnetic field structure at the jet launching region. Our work focuses on the polarization observations in the millimeter regime which depict presence (or lack) of wavelength-dependence in the jet polarization properties, and thus give us information to map magnetic field structures near the jet base.

We present our work on ALMA observations of 3C120 for which polarimetric observations were conducted at 2mm and 1.3mm wavelengths. The (preliminary) results tell us that there is a high Faraday Rotation Measure (RM) in the millimeter-wavelengths. For example, we have detected an RM of -10 000 radians per square meter for 3C120 in band 4 which is a result that matches expectations for a helical magnetic field. These data are used as input in models which aim to identify both the Faraday dispersion mechanism(s) and the number of Faraday components causing the dispersion on the source.

Speaker: Mikael Turkki (FINCA, University of Turku)

9:55 AM Twisting Paths of Supermassive Black Hole Jets

Jets launched by supermassive black holes can appear remarkably straight, yet some reveal surprising bends even on the smallest observable scales. Using over 120,000 VLBI brightness distributions from nearly 18,000 AGN, we compiled one of the largest samples of curved jets to date. We find that about 7% of these jets show clear curvature. Such patterns can be explained by plasma instabilities, accretion disk precession or by supermassive black hole binaries – a scenario of particular interest, as the search for close black hole pairs is one of the most active and promising fields in astrophysics today. Our results thus provide new clues to the physics behind jet bending and contribute to the broader effort of uncovering binary black holes in the Universe.

Speaker: Vladislav Makeev

10:10 AM Aligning Multifrequency VLBI Observations of AGN Jets Using Super-Resolved Images

The acceleration of relativistic jets launched by active galactic nuclei (AGN) is expected to be driven by magnetic fields, however, the exact acceleration mechanism is still uncertain. Several magnetic jet acceleration models, which include predictions on the evolution of the magnetic field strength and other jet properties along the jet axis, have been proposed. In order to potentially constrain such models, multifrequency very long baseline interferometry (VLBI) observations of AGN jets are essential. With the angular resolution provided by VLBI, multifrequency observations can be used to extract synchrotron emission spectra from specific emission regions in a jet, allowing us to probe the strength of the magnetic field at the corresponding locations and thus study its spatial variation.

However, imaging VLBI observations of AGN jets with resolved jet structure using the common CLEAN-imaging process, generally requires phase self-calibration of the observed visibilities, leading to the loss of absolute astrometry in the observations. In order to extract synchrotron spectra from such images, the observations at different frequencies must be realigned by comparing observed emission from optically thin regions of the jet. We study the use of a 2D cross-correlation algorithm for aligning multifrequency images of AGN jets, which have been observed using the Very Long Baseline Array (VLBA) as part of the MOJAVE survey at 15, 24 and 43 GHz. With simulated observations, we show that super-resolving the images used for cross-correlation, may allow us to recover more accurate image alignments.

Speaker: Joonas Suortti (Aalto University)

10:30 AM → 10:40 AM Group Picture

10:40 AM → 11:00 AM Coffee Break

11:00 AM → 12:40 PM Session VI

11:00 AM Particle composition in relativistic jets - PARTICLES

Relativistic jets launched by supermassive black holes are the most extreme and energetic persistent phenomena in the Universe. They are capable of accelerating particles to energies inaccessible by particle accelerators on Earth, and deposit enormous amount of energy in their surroundings during their lifetime. Yet, despite decades of studies, one of the most fundamental properties, the composition of these jets is still unknown. The jets can consist of either electron-positron pairs, electron-proton plasma where also protons are present and accelerated, or an admixture of the two. Circular polarization is the only direct way to determine the particle composition of jets, and we have designed a project that aims to resolve the particle composition of relativistic jets. We will utilize a new triple-band receiver to be installed at Aalto University Metsähovi Radio Observatory to conduct full-Stokes observations of a sample of bright relativistic jets both in single-dish and in very long baseline interferometry modes. I will describe the main goals of the project and the current status of the work.

Speaker: Talvikki Hovatta (Aalto University Metsähovi Radio Observatory)

11:20 AM The clockwork outbursts of ASASSN-14ko:polarimetry of a repeating partial disruption event.

ASASSN-14ko is a very intriguing repeating nuclear transient, characterized by bright optical flares every ~115 days, best explained as a repeating partial tidal disruption event (TDE). TDEs usually prefer dormant supermassive black holes (SMBHs). However, ASASSN-14ko is found in a system with not one, but two active galactic nuclei (AGN) separated by 1.7".

So far, the observational properties of the TDE population are far from homogeneous and what their differences reveal about the process of accretion disc formation are unknown. The two main competing theories produce almost identical total intensity signatures preventing progress through conventional methods. Instead, polarization, and its extra capacity of measuring the direction of oscillation of the electromagnetic field, not only offers new paths for understanding the dynamics of the accretion flow. Moreover, the temporal evolution of the polarization parameters can put constraints on the orientation and geometry of the accretion disc.



In this talk, I will present the first photo- and spectro- polarimetry observations of ASASSN-14ko with FORS2@VLT following two consecutive flares of ASASSN-14ko, and also during quiescence. The polarization variability revealed by our observations provides invaluable insight into accretion properties around supermassive black holes, and serves as a test-bed for state-of-the-art models, while comparing the behaviour of both nuclei, and probing the effect of a merger on the TDE rate.

Speaker: Beatriz Agís González (IA-FORTH)

11:35 AM Probing horizon scale magnetic fields and strong-field gravity with polarized VLBI observations

Magnetic fields play a pivotal role in dynamics of black hole accretion flows and formation of relativistic jets.
They can also be used effectively for discriminating between different
physical settings on the horizon scales near supermassive black holes. Observations by the Event Horizon Telescope (EHT) provided unprecedented insights into accretion structures near black holes. Interpreting these observations requires a theoretical framework linking polarized emission to underlying system properties and magnetic field geometries.
In the talk, I will talk about new EHT polarization results of M87* including 2018 and 2021 observations, and describe how polarized emission can be modeled and interpreted to probe both astrophysics and gravity. The approach emphasizes on a semi-analytic framework for polarized imaging, focusing on magnetic field configurations and show how these models can capture spacetime signatures as well. This also connects to general relativistic magentohydrodynamics (GRMHD) simulations and their role in interpreting the polarization images.

Speaker: Saurabh Saurabh

11:50 AM Using the Highest Resolutions to Understand Relativistic Jets

A new era of global VLBI now permits us to study relativistic jets at extreme resolutions, probing AGN down to the event horizon itself. In this talk I will briefly describe the goals and prospects for the multi-frequency analysis of the quasar 3C273 using 2021 observations from the EHT and GMVA. The wide spectral windows of the EHT observations will allow us to construct robust rotation measure and spectral index maps of the source at ~10 muas resolutions, and the inclusion of the nearly coincident GMVA observations at 86 GHz will provide critical information about the source structure at larger scales.

Speaker: Britton Jeter (Finnish Centre for Astronomy with ESO)

12:05 PM Aligned with the jet: a polarimetric multiwavelength population study of blazars

Polarimetry offers a powerful window into the physical conditions and particle acceleration processes within blazar jets.
In this study, we investigate the multiwavelength polarimetric behaviour of High Synchrotron Peaked (HSP) and Low Synchrotron Peaked (LSP) blazars to explore differences in their emission mechanisms and magnetic field structure near the acceleration region.
We take advantage of several X-ray polarisation observations of HSP by the Imaging X-ray Polarimetry Explorer, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and other monitoring programs. We find that the polarisation degree (PD) of HSPs in X-rays is systematically higher than in the optical and mm-radio bands, consistent with earlier IXPE findings. Moreover, the electric vector position angles (PA) in the X-rays are predominantly aligned with the jet axis, with most measurements showing minimal offset within uncertainties.
When comparing the offset of the polarisation angle from the jet axis between the LSP and HSP populations—measured in optical and X-rays respectively—we find a consistent distribution. This points toward a shared magnetic field structure near the particle acceleration region and lends strong support to the emerging picture of energy-stratified acceleration followed by radiative energy loss in blazar jets.

Speaker: Sara Capecchiacci (IA-FORTH)

12:20 PM Active Galactic Nuclei (AGNs) structure evolution and core shift variability with continuous (15 days) VLBI observations

We analyze multi-epoch, multi-frequency Very Long Baseline Interferometry (VLBI) observations from the CONT17 campaign, which combines data from both VLBA and IVS antennas. The dataset spans continuous 15-day bservations at two frequencies—2.3 GHz (S-band) and 8.6 GHz (X-band)—each recorded in a single polarization. Up to 80 extragalactic radio sources were observed, although not all provide adequate spatial or temporal uv-coverage. For this study, we selected sources with both sufficient spatial resolution uv-coverage patterns, as these are best suited for our imaging and variability analyses. One of the key questions in the Astrogeodesy project is to understand and quantify the structural evolution and core shift variability of AGN jets on timescales of about one week. A central objective of this study is to measure the core shift between the S and X bands in sources with extended radio emission. The core shift refers to the frequency-dependent displacement of the apparent radio core position, caused by gradients in particle density and magnetic field strength along the jet. We investigate whether such variations occur on timescales of days and whether they correlate with source activity. Preliminary results, based on null-hypothesis testing for variability, show that within a 15-day span the blazar 0133+476 exhibits significant core shift variability, with a median of 0.86 mas. 1803+784, a BL Lac object, exhibits also significant core shift variability, with a median of 0.75 mas. Ongoing work extends this analysis of correlations of core-shift variability with source activity as well as source structure and core shift variability to a broader sample of AGNs.

Speaker: Wara Chamani (GFZ Helmholtz Centre for Geosciences)

12:40 PM → 2:00 PM Lunch

2:00 PM → 3:30 PM Session VII

2:00 PM Automating the detection of polarization angle rotations in blazars Re-analysis of RoboPol data reveals 27 new rotations.

We present a fully automated pipeline for detecting electric vector position angle (EVPA) rotations in blazar polarimetric time series. Our method combines robust EVPA unwrapping with Bayesian Blocks segmentation and dual statistical validation (two-sample $t$-test and binomial direction test) to identify rotation events while controlling false positives. Applied to the complete RoboPol 2013–2017 dataset, the pipeline identifies 27 statistically significant rotations with amplitudes $\geq 90^\circ$, including 10 previously unpublished events from the 2016–2017 observing season. Compared to the results of Blinov et al. (2015, 2016a,b), our automated amplitudes show excellent agreement, while the measured durations are systematically longer (by $\sim$93.5% on average), reflecting improved sensitivity to gradual, extended rotations. Correlation analysis reveals that slower rotations (longer periods) are associated with enhanced gamma-ray flux ($\rho = 0.688$, $p = 7.3 \times 10^{-5}$). We release the detection code and catalog to enable systematic studies of jet magnetic-field evolution and their connection to high-energy activity.

Speaker: Anastasia Glykopoulou (Institute of Astrophysics, FORTH)

2:05 PM Core Jet Study of BL Lac with Multi-Frequency Phase Referencing

We describe an approach for mm VLBI astrometry that provides bona-fide astrometric alignment of the mm-wavelength images from a single source, for the measurement of frequency-dependent effects, such as “core-shifts” near the black hole of active galactic nucleus jets. MFPR, other than the intial demonstration, has not been widely used, unlike it's older brother SFPR, but it represents another approach for Frequency Phase Transfer by solving for the total atmospheric contributions.

We used this to measure the core-shift between 22 and 43 GHz for the jet in BL Lacertae to be -8 ± 5, 20 ± 6 μas, in R.A. and decl., respectively. By comparison to conventional phase referencing at centimeter-wavelengths we are able to show that this core shift at millimeter-wavelengths is significantly less than what would be predicted by extrapolating the low-frequency result, which closely followed the predictions of the Blandford & Königl conical jet model. As such it would be the first demonstration for the association of the VLBI core with a recollimation shock, normally hidden at low frequencies due to the optical depth, which could be responsible for the γ-ray production in blazar jets.

Speaker: Richard Dodson (ICRAR/UWA)

2:25 PM Demonstration of frequency phase transfer from 86 to 258 GHz on the APEX - IRAM 30m baseline

The receiver N3AR operating at a frequency range between 67 and 116 GHz has been commissioned at the APEX telescope in October 2024. This adds a new low-frequency band for APEX, with the capability of simultaneous dual-frequency observations using a dichroic beamsplitter. The 3 mm receiver also allows APEX to join the existing 3 mm global VLBI network. One of our commissioning goals was to perform simultaneous dual-band VLBI observations at 86 and 258 GHz using receivers with shared-optical-paths (SOP) to correct the atmospheric phase fluctuations using the frequency phase transfer (FPT) technique. This was possible together with the IRAM 30 m telescope, which has already developed such a capability. We aimed to verify the expected phase coherence and sensitivity improvement at the higher frequency achievable by applying FPT. With the dual-band, single baseline data, we applied the FPT method, which uses the lower frequency data to correct the simultaneously observed higher-frequency data. We evaluate the improvement compared to the conventional single-band observing mode by analyzing the coherence factor in the higher frequency data. Our results show that the phase fluctuations at the two bands are well correlated. After applying FPT, the interferometric phases at the higher frequency vary much more slowly, and the coherence factor is significantly improved. Our analysis confirms the feasibility of applying FPT to frequencies above 250 GHz with SOP receivers. Future observations in this mode could dramatically improve the sensitivity and imaging fidelity of high-frequency VLBI.

Speaker: Guang-Yao Zhao (Max Planck Institute for Radio Astronomy)

2:40 PM Astronomical calibration and imaging of geodetic VLBI data

The VLBI Global Observing System (VGOS) is a world-wide network of 13-m radio antennas which carry out simultaneous broad-band (3 to 10 GHz) geodetic VLBI observations of AGN, which are used to determine the orientation of the Earth and the position of the radio antennas which provide the data. In order to improve the accuracy of these measurements, we need to take into account the structure of the jets, which are highly variable in time. Our group aims to improve the accuracy of geodetic products by quantifying temporal and frequency-dependent changes of AGN, yielding a rich astronomical database spanning decades. In this talk, I detail our efforts regarding the
astronomical calibration and imaging of geodetic data, as well as the next steps in analyzing the jet structure.

Speaker: Petra Benke

2:55 PM Probing AGN jet launching from different perspectives: Blazars versus radio galaxies

Relativistic jets in active galactic nuclei (AGN) are essential to probe the physics of accreting supermassive black holes and their environments, yet the mechanisms driving their formation remain elusive.
Multi-epoch, multi-frequency very long baseline interferometry observations
offer the highest angular resolution to study the jet structure and launching processes in the immediate vicinity of the central engine.
In this study we investigate the launching process and kinematics of AGN jets from two contrasting perspectives:
NRAO$\,150$ is a bright blazar with high variability and complex morphology due to its small angle towards our line of sight.
3C$\,84$, a compact and nearby radio jet in the Perseus cluster, is pointing with a larger angle towards us, and is a prime target for testing jet launching scenarios.

Our analysis reveals a correlation between structural changes and simultaneous $\gamma$-ray flares in NRAO$\,150$ and 3C$\,84$.
Both sources show evidence for a toroidal magnetic field configuration, and in each case the high-energy emission region is located downstream of the VLBI core, providing new constraints on particle acceleration and energy dissipation in blazars and radio galaxies.
In this talk, I will present how the viewing angle variation, the jet interaction with the ambient medium, and the magnetic field orientation shape the observed jet properties, and outline future observational tests.

Speaker: Ms Lena Carolin Debbrecht (MPIfR)

3:10 PM Automated Modeling with AAP-Imfit: Astrometry and Photometry via CASA

Very Long Baseline Interferometry (VLBI) provides the highest-resolution radio intensity maps, crucial for detailed studies of compact sources like active galactic nuclei (AGN) and their relativistic jets. Analyzing jet components in these maps traditionally involves manual Gaussian fitting, a time- consuming bottleneck for large datasets. To address this, we present an automated batch-processing tool, based on the Gaussian fitting capabilities of CASA, designed to streamline VLBI jet component characterization (AAP-Imfit). Our algorithm sets a detection limit, performs automatic 2D Gaussian fitting, and removes model artifacts, efficiently extracting component flux densities and positions. This method enables systematic and reproducible analysis, significantly reducing the time required for fitting extensive VLBI datasets. We validated AAP-Imfit by using VLBI observations of the blazars 3C 279 and 3C 454.3, comparing our results with published fits. The close agreement in residual root mean square (RMS) values and model/residual-to-map RMS ratios confirms the accuracy of our automated approach in reproducing original flux distributions. While visual inspection remains important for complex or faint features, this routine significantly accelerates VLBI component fitting, paving the way for large-scale statistical studies of jet dynamics.

Speaker: Víctor Manuel Patiño Álvarez (Instituto Nacional de Astrofísica, Óptica y Electrónica)

3:30 PM → 4:00 PM Coffee Break

4:00 PM → 6:00 PM Session VIII

6:30 PM → 8:00 PM Dinner

Sat, October 25

8:00 AM → 8:55 AM Breakfasst & Checkout