{"id":85,"date":"2022-03-06T18:25:47","date_gmt":"2022-03-06T17:25:47","guid":{"rendered":"http:\/\/www.for5195.uni-wuerzburg.de\/?page_id=85"},"modified":"2026-06-10T10:58:13","modified_gmt":"2026-06-10T09:58:13","slug":"projects","status":"publish","type":"page","link":"https:\/\/www.for5195.uni-wuerzburg.de\/index.php\/projects\/","title":{"rendered":"Projects"},"content":{"rendered":"\n
P1: Jet Physics on Event Horizon Scales and Beyond<\/strong><\/summary>\n
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The Event Horizon Telescope (EHT) has repeatedly proven to be capable of imaging microarcsecond-scale structures near supermassive black holes. In the prominently featured case of Messier 87 and Sagittarius A*, this has led to famous depictions of black hole shadows and images of the mass accretion region with resolutions of roughly ten Schwarzschild radii. Using the same interferometric array and frequencies, other active galaxies at bigger distances or with smaller black-hole masses can be observed on scales of hundreds of Schwarzschild radii, probing the jet-launching, acceleration and collimation zone. This has recently been demonstrated for the case of 3C 279. In addition, only four other AGN were main targets in the 2017 and 2018 EHT observing sessions, three of which will be analysed in this project: NGC 1052, Centaurus A, and 4C+01.28. We combine: (I) imaging these AGN jets with highest-resolution VLBI observations at 1 mm (EHT) and 3 mm (GMVA), and (II) modelling these observations with state-of-the-art numerical general relativistic magnetohydrodynamic (GRMHD) simulations and general relativistic radiative transfer (GRRT) calculations.<\/p>\n<\/div>\n\n\n\n

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A 3D rendering of a general GRMHD simulation of jet launching from a fast spinning black hole with spin parameter a=0.94 and highly magnetised torus (magnetically arrested disk, MAD). Red and orange hues show the density of the torus and blue and white colours depict the jet.
Credit: Christian Fromm<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/details>\n\n\n\n
P2: Jet Lamp-Post Models for Radio-Loud Active Galactic Nuclei<\/strong><\/summary>\n

X-ray observations of accreting black holes routinely reveal relativistic reflection features from the innermost regions around the black hole, which allow us to measure its spin and to probe the location of the primary source of X-rays, the “corona”. Detailed observations of radio-quiet objects have consolidated our view of a compact corona close to the black hole. Its nature is still strongly debated. One possible interpretation connects it to the base and launching region of a sub-luminous jet. While most sources analysed to date have been Seyfert galaxies, recent high-quality data from radio-loud Active Galactic Nuclei (AGN) have also shown evidence for relativistic reflection.
We investigate the location and nature of the corona in AGN with relativistic jets. We develop a physically motivated relativistic reflection model by extending the RELXILL package and apply it to the observations. As a proof of concept, we apply the model to the reflection-dominated spectrum of radio-quiet AGN ESO033-G002 and find constraints on the corona geometry. Next, for the first time, we will analyse a sample of radio-loud AGN with this model, supported by system inclination constraints from VLBI measurements. This approach will allow us to measure the location and size of the corona, which will be compared to GRMHD simulations from P1 to address the long-standing question of whether the X-ray corona is the base of the relativistic jet.<\/p>\n\n\n\n

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A sketch of the sequence of steps required to put geometry constraints on the corona size and location. Top left: a broadband X-ray spectrum fit of ESO033-G002 with the new RELXILL model. Top right: artist’s impression of a black hole and spectrum formation in the system. Bottom right: corona geometry constraint found for ESO033-G002 in terms of height above the disk, and radius from the rotational axis. Bottom left: intermediate steps of the model construction, including general relativistic ray tracing, construction of disk irradiation profiles, and subsequent reprocessing in the disk, resulting in the reflection spectra available for fitting and finding system parameters.
Credit: Alexey Nekrasov<\/figcaption><\/figure>\n<\/details>\n\n\n\n
P3: Flaring and Time-Dependent Modelling of Blazars<\/strong><\/summary>\n
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One of the key characteristics of blazars is their strongly variable emission. Their flux varies by up to orders of magnitude throughout the entire electromagnetic spectrum down to time scales of minutes. The gamma-ray band is of particular interest as it often dominates the total energy output of blazars. Features imprinted in the light curves are likely related to changes in the underlying particle distributions and allow us to probe different physical mechanisms.
In this project we will study variability properties of blazars in the gamma-ray band and multi-frequency correlations. We will explore the constraints on phenomenological models of acceleration and substructure in blazars. The analysis and interpretation of blazar gamma-ray variability opens an unlimited discovery space challenging our theoretical understanding of the black-hole jet phenomenon. We specifically address the precision diagnosis of the fastest gamma-ray variability observed across a wide energy band up to very high energies (VHE, > 100 GeV) to test models of jet substructures and radiation mechanisms by applying novel methods. We also target the jet-disk connection by investigating statistical broadband variability properties.<\/p>\n<\/div>\n\n\n\n

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An artist’s depiction of gravitational lensing. According to Einstein’s general relativity theory, massive or compact objects (like a galaxy in this sketch) can bend light rays from a distant object (here a background galaxy hosting a luminous active galactic nucleus). Consequently, the detector (here a space-borne satellite orbiting planet Earth) observes two or even more images of the distant active galaxy. If the active galaxy, the lensing galaxy and the detector are not exactly aligned on a straight line, both light paths have different path lengths and the light-travel times along these paths will be different, too. In this case flares and any features of the variable emission of the active galaxy will be detectable with a time delay.
Copyright: ESA\/ATG medialab<\/a><\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/details>\n\n\n\n
P4: Millimetre-VLBI Studies of Gamma-Ray Bright Radio Galaxies<\/strong><\/summary>\n

Nearby radio galaxies are ideal laboratories to study the physical processes occurring in the vicinity of supermassive black holes. In radio galaxies, unlike in blazars, projection and relativistic effects have a mild impact on the observed jet emission, which enables us to more easily infer the intrinsic jet properties and to probe emitting regions which would be, otherwise, hidden. In this project we will conduct millimetre-VLBI studies of radio galaxies detected by the Fermi-LAT telescope at gamma-ray energies, with the aim of characterizing the jet internal structure, pinpointing the high-energy emission sites, and modelling the broadband emission. The novelty of our approach resides in the possibility to directly constrain the fundamental parameters of the relativistic plasma, on the exact scales where the gamma-ray emission is thought to originate. While previous analyses have been limited to a few case studies, our goal is to obtain a broad overview of the high-energy emission processes in relation to the accretion properties and the jet power.<\/p>\n\n\n\n

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\"Multi-wavelength
The image shows multi-wavelength light curves (left panels) and the position r<\/em> of the jet components detected with radio VLBI techniques vs. time (right) for a radiatively efficient radio galaxy (3C 111, top panels) and an inefficient one (3C 371, bottom). The radiatively efficient radio galaxy shows gamma-ray flares and moving features while the radiatively inefficient one has a stable gamma-ray activity and stationary features. We ascribe these differences to the different accretion modes in the central engine. Credit: Vieri Bartolini<\/figcaption><\/figure>\n<\/details>\n\n\n\n
P5: Jet Composition under Scrutiny<\/strong><\/summary>\n

The goal of this project is to shed light on the plasma composition in AGN jets that is a long- standing question since the seminal work by Blandford and Znajek as early as 1977. Although a basic framework for describing extragalactic jets exists, their unknown plasma composition has always rendered the interpretation of observational data and the understanding of their launching mechanism difficult.
This project will combine two distinct, but complementary approaches to the problem, that are in fact highly interrelated. That is (i) modelling the jet plasma dynamics by relativistic MHD simulations, and (ii) modelling of the spectral energy distribution from high energy particles in the jet. Sub-project (ii) will benefit from the realistic physical modelling of the jet dynamics from its launching area to (almost) pc scales including resistive effects as e.g. reconnection flares, and sub-project (i) may apply more realistic cooling of the jet plasma.<\/p>\n\n\n\n

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A volume rendering of the velocity field from a simulation of a relativistic jet. The forward moving jet (in red) is enveloped by the back-flowing material (in blue).
Credit: Ravi Pratab Dubey<\/a><\/figcaption><\/figure>\n<\/div>\n<\/details>\n\n\n\n
P6: Large-Scale Blazar Jets: Clues on High-Energy Emission from Low-Frequency Radio Observations<\/strong><\/summary>\n

More than 100 AGN jets have been imaged with arcsecond resolution in the optical and X- ray band. In the case of low-power FR I sources, the X-ray jet emission can generally be interpreted as an extension of the radio synchrotron spectrum. In contrast to this, the radio, optical and X-ray broadband spectra of FR II jets clearly show that other emission processes are at play. Originally, inverse-Comptonisation of CMB photons was suggested. This model requires bulk relativistic motion on kiloparsec scales (which is difficult to observe) and an electron energy distribution extending down to Lorentz factors smaller than 100, which is lower than the energy regimes traced by GHz instruments like the VLA. Very recently, first results of high-resolution LOFAR observations of AGN jets involving international baselines at hundreds of MHz found radio flux densities in one quasar well below the values estimated by extrapolating the GHz spectra and thus falling short of explaining the observed X-ray knot emission in terms of the IC\/CMB model. In this project, we will perform a systematic LOFAR study of the long-wavelength radio emission of quasar jets on kiloparsec scales and combine this with shorter-wavelength radio and higher-energy optical and X-ray broadband spectral data to test the IC\/CMB and alternative emission models. We will further develop and optimize high-resolution imaging techniques at the low and ultra-low radio frequencies in total and linearly-polarized intensity with LOFAR-VLBI techniques to measure magnetic-field geometries and constrain jet-composition and emission models.<\/p>\n<\/details>\n\n\n\n

P7: Jet Feedback on Groups and Galaxy Clusters<\/strong><\/summary>\n

The cosmological concordance model provides an astonishing match to data of large-scale cosmological surveys, but on smaller scales – where baryonic effects start to become dominant – the situation is less clear. In particular, the halo mass and galaxy luminosity distributions differ significantly so that the star conversion efficiency has to be a strong function of halo mass. Gas is less efficiently converted to stars towards the scale of galaxy groups and clusters. The interplay of cooling gas, subsequent star formation, and nuclear activity appears to be tightly coupled to a self-regulated feedback loop that may explain this cosmological conundrum. The main goal of this project aims at studying the non-thermal aspect of the active galactic nucleus (AGN) feedback loop in the vicinity of the jet and compares observed and synthetic radio emission to calibrate feedback. To this end, this project uses LOFAR data (in particular newly available high-resolution long-baseline data) and state-of-the-art cosmological magneto-hydrodynamical simulations with cosmic ray and AGN jet physics of galaxy groups and clusters to explore how jets transfer energy and momentum to the intra-cluster medium (ICM) and regulate cooling and star formation. Results from this project serve as large-scale constraints to small-scale relativistic jet models that will be pursued in this proposed research unit in particular by the Mercator fellow Manel Perucho. Our main research questions are:
(1) How is feedback energy transferred from the AGN jets to the cooling ICM and which observational signatures demonstrate this unambiguously?
(2) Can cosmic ray heating from AGN jets reproduce the observed bimodality of the distribution of cool core and non-cool core clusters?
(3) Which parameters determine the morphology of AGN jets (luminosity, environment, or composition) and how can we relate this to feedback?<\/p>\n\n\n\n

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This figure shows radio emission (reddish hues, obtained by the Very Large Array) from Active Galactic Nuclei (AGN) jets and X-ray emission (bluish hues, detected by the Chandra satellite telescope) from the galaxy cluster MS 0735.6+7421. We can see that the radio emitting jet bubbles are coincident with regions of low X-ray emission. This implies that the jet is carving out regions of the galaxy cluster gas, giving us insight into how AGN jets heat up the cold gas in galaxy clusters (so called intra-cluster medium) and prevent star formation. One of the aims of this project is to produce synthetic radio observations of AGN jets using purely simulation data to compare them to real radio observations, such as these ones. Credit: NASA, ESA, CXC\/NRAO\/STScl, B. McNamara<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/details>\n\n\n\n
P8: Jet Physics Across the Multimessenger Spectrum<\/strong><\/summary>\n

Blazars are among the leading candidates for cosmic-ray and high-energy neutrino production. They are capable of accelerating particles to extremely high energies, potentially producing both gamma-rays and neutrinos through hadronic processes, and are the overwhelming population of sources detected at with the Fermi-LAT.
The goal of this project is to identify the sources of both neutrinos and cosmic rays. While cosmic rays are deflected by magnetic fields, neutrinos travel vast distances without deflection or significant absorption, making them powerful messengers for unravelling the sources of cosmic rays. Purpose is to investigate the electromagnetic properties of candidate astrophysical counterparts of detected neutrinos.
To achieve this, the project aims to leverage the synergy between complementary neutrino observatories (IceCube and KM3NeT) and the Fermi-LAT follow-up capabilities. It involves the use of both archival data and newly reported high-energy neutrino events detected in real time by IceCube and KM3NeT as well as Fermi-LAT real-time multimessenger activities.
In the final phase, the project will investigate the sample of blazars spatially coincident with high-energy neutrino events, comparing their properties to the general gamma-ray blazar population observed by Fermi-LAT. Furthermore, blazar sub-classes, such as BL Lac objects and flat-spectrum radio quasars, will be analysed separately to investigate potential patterns between their gamma-ray and neutrino properties.<\/p>\n\n\n\n

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\"Comparison<\/figure>\n<\/div>\n\n\n\n
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Comparison of candidate neutrino blazars with all blazars in the 4LAC AGN sample. The 2D distribution shows gamma-ray energy fluxes (in the energy range from 100 MeV to 100 GeV) and redshifts for all the blazars in 4LAC as grey dots. Star-shaped markers indicate candidate neutrino blazars suggested in the literature as neutrino-emitting blazars on the basis of their multiwavelength properties in addition to their coincidence with high-energy neutrinos. Black circles show additional 4LAC sources coincident with well-reconstructed neutrino alerts, which have identified counterparts and measured redshifts. In the side plots, the inverse cumulative distribution function of the gamma-ray energy fluxes (right) and the normalised counts distribution of the redshifts (top) are depicted.
Credit: S. Garrappa, S. Buson, J. Sinapius, et al. “Fermi-LAT follow-up observations in seven years of real-time high-energy neutrino alerts”<\/a>. A&A 687, A59 (2024), A59<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/details>\n\n\n\n
P9: Supermassive Black Holes and their Jets through Cosmic Time<\/strong><\/summary>\n
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Timeline of the universe, from the Big Bang to today. The majority of stars, galaxies and non-jetted AGN exists around four billion years after the Big Bang (“cosmic noon”). Adversely, recent works on blazar evolution confirms that the majority of blazar jets is present already when the universe was two billion years old. Since these jets are powered by supermassive black holes, they are also tracing the growth and evolution of these black holes through time. Credit: NAOJ<\/figcaption><\/figure>\n\n\n\n

The question of supermassive black-hole formation and evolution in the early universe is an open issue. The existence of numerous active supermassive black holes which host powerful relativistic jets within the first billion years of the universe poses a challenge to our understanding of black-hole seeding and growth and to its connection to jet launching and regulation.
In particular, there are two fundamental questions, that we aim to solve with this project:
(1) How many jets and supermassive black holes exist within the first billion years?
(2) Can we build a successful sub-grid blazar model that matches the observed evolution of both X-ray blazar jets and radio galaxies in cosmological simulations?
To address these open issues, we will focus our attention on the most extreme blazar jets. They are tracers of the general population and their detection shows that supermassive black holes exist in large numbers within the universe\u2019s first billion years.
From an observational standpoint, we aim to construct a comprehensive catalogue of the most powerful and distant blazars (so-called MeV blazars) with complete black-hole masses and jet-properties measurements as well as trace their evolution with a variety of X-ray surveys.
From a simulation point of view, we will develop a blazar sub-grid model prescription that aims at reproducing the observed evolution of blazar jets as manifested in the X-ray\/gamma-ray blazar appearance and in radio galaxies.<\/p>\n\n\n\n

\"Schematic
Schematic of the cosmological-simulations-based work for this project. Credit: Lea Marcotulli<\/figcaption><\/figure>\n<\/details>\n\n\n\n
P10: Short-Wavelength Radio Variability of High-Energy-Emitting AGN Jets<\/strong><\/summary>\n

Current Cherenkov telescopes (e.g. MAGIC, H.E.S.S. and VERITAS) have already detected ~ 90 AGN at TeV energies and in the near future the upcoming CTAO will significantly enhance very-high-energy (VHE) astronomy. Simultaneously, the short-wavelength radio band is getting increasingly important in the context of astro-particle physics as it can probe dynamic processes near the base of AGN jets (pc-scale jets of blazars) that are closely related to flares at the highest observable energies or to VHE-neutrino emission.
In this project, we are studying the cm- and mm-radio variability of TeV-emitting and neutrino-candidate AGN jets using novel observational data from two large and long-term monitoring programmes with the Global Millimeter VLBI Array (GMVA) and the Effelsberg 100-m telescope.
The TELAMON programme is a key-science project at the sensitive Effelsberg telescope that we have set up to monitor (spectral and polarimetric monitoring) the radio spectra of AGN under scrutiny in astro-particle physics, namely TeV blazars and candidate neutrino-associated AGN. Since 2020, we have been performing observations every few weeks at frequencies up to 44 GHz both in total intensity and polarisation.
With the superior angular resolution and sensitivity of the upgraded GMVA (including APEX, GLT and NOEMA) we will be able to probe the smallest-scale jet structures close to the mm-core and to investigate their morphological variability (high-dynamic-range imaging of the dynamic sub-milliarcsecond AGN-jet structure). Our new GMVA programme targets the 17 radio-brightest TeV-detected AGN (dec > – 30\u00b0), which are prime targets to study possible correlations between the dynamic jet structure and VHE-flaring activity.
Using our single-dish monitoring data in combination with the VLBI data, we will be able to constrain the location of the TeV-emission sites. Moreover, we are coordinating our radio observations with AGN monitoring groups of the major TeV collaborations and other multiwavelength facilities.<\/p>\n\n\n\n

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Light curve of S2 0109+224 from Effelsberg\/TELAMON (averaged over all sub-bands). The source exhibited multiple fast flares in the last two years, which are highlighted by red ellipses and labelled with letters. The time interval actively monitored with VLBA is indicated by the red lines.<\/figcaption><\/figure>\n<\/div>\n\n\n\n
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Preliminary VLBI image of S2 0109+224 at Q-band from our first observation on 29 April 2022. In this epoch, the source shows a very compact structure dominated by emission from the central core region.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/details>\n\n\n\n
Associated Junior Research Group: Blazars as Probes of Fundamental Physics and Intergalactic Magnetic Fields<\/strong><\/summary>\n

Electromagnetic AGN emission provides a unique test bed to study the propagation of photons over cosmological distances. In particular, high-energy gamma rays produced in jets can interact with intergalactic radiation fields such as the extragalactic background light (EBL) and produce electron-positron pairs. The EBL is composed of star light and light reprocessed to infrared wavelengths through the interaction with dust in the interstellar medium integrated over the age of the Universe. Due to strong foreground emission in the solar system, it is extremely difficult to directly measure the EBL. Instead, the amount of gamma-ray absorption can reveal the EBL intensity as a function of cosmic time and wavelength. Gamma-ray observations of blazar jets can therefore be used to constrain the cosmic star formation history.

Additionally, the electron-positron pairs can inverse-Compton scatter with photons of the EBL and cosmic microwave background, with the scattered photon energy ending up in the gamma-ray band. This process can then trigger an electromagnetic cascade as the up-scattered photons again produce pairs. The spectral, temporal and spatial signature of this cascade emission will depend on the intergalactic magnetic field (IGMF) since it deflects the electrons and positrons. Gamma-ray observations of the cascade could therefore be used to infer the strength of the IGMF in cosmic voids or to probe whether the IGMF is of astrophysical origin (through the galactic plasma outflows \u201cpolluting\u201d the intergalactic medium) or instead produced in primordial phase transitions.

Gamma-ray propagation could also be affected through the interaction with yet undiscovered fundamental particles. In particular, gamma rays could oscillate into axions or axion-like particles (ALPs) in the presence of external magnetic fields. These types of particles are well motivated dark matter candidates and in the case of the axion additionally explain the non-observation of the electric dipole moment of the neutron (the so-called strong CP problem). The photon-axion\/ALP interaction would lead to unique observational signatures in gamma-ray spectra of AGN such as energy-dependent spectral distortions or the reduction of the gamma-ray opacity caused by EBL interactions.<\/p>\n\n\n\n

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The Virgo cluster (upper right) is a gravitationally bound galaxy cluster hosting thousands of galaxies, among others the galaxies Messier 87 and Messier 84, each of which harbours an AGN with a relativistic jet. The electromagnetic radiation from the AGN jets shines through the surrounding universe and is finally be detectable in the Milky Way Galaxy. However, according to the Primakoff effect (upper left), photons could be converted to ALPs and vice versa in the background of magnetic fields. On the journey from the emitting AGN jet to our detectors, the photons inevitably traverse the IGMF and might thus be prone to the Primakoff effect. If photon-ALP conversion happens, it will affect the survival probability of the photons (lower right). Consequently, the observed spectra of the jets will have very distinct, energy-dependent imprints. Evidence for such signatures in the very high energy spectra of AGN jets would therefore mean an indirect detection of ALPs and give us clues about the nature of the elusive dark matter. Additionally these spectral imprints could help us to probe the IGMF and the EBL. Credit: Rahul Joseph Cecil<\/figcaption><\/figure>\n<\/details>\n\n\n\n
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DFG Research Unit (Forschungsgruppe) FOR 5195 \u2013 Relativistic Jets in Active Galaxies<\/a><\/h1>\n\n

Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)<\/p><\/div>\n\n\n\n

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