Special Colloquium
Ciera Sargent	SCHEDULED

Durham University

Red quasars exhibit a higher incidence of compact (galaxy-scale or smaller) radio emission than blue quasars, arising from systems near the radio-loud/radio-quiet threshold. This result cannot be fully explained by the standard orientation model, instead favouring red quasars as a distinct phase in a quasar’s lifecycle, possibly an obscured-to-unobscured transition where low-power jets and/or AGN-driven winds drive away gas and dust. I will show there is an excess of steep-slope radio emission (alpha ~-1) from red quasars with compact radio morphologies over 144 MHz, 1.4 GHz, and 3 GHz. This excess steep radio emission signature is not seen in normal blue quasars (radio compact or extended) or red quasars with extended low-frequency radio emission, which instead show a broad range of radio spectral slopes consistent with a range of different physical processes. I will show that the strength of this excess steep-slope radio emission increases with increasing dust extinction, along with an overall increase in the radio-detection fraction. I argue that this excess steep-slope radio emission is due to shocks between quasar-driven winds/jets and the dusty nuclear-host galaxy environment. The majority (~86%) of the dustiest quasars (E(B-V)>0.4) with steep slopes have radio luminosities consistent with the prediction from a wind-shock model with a wind efficiency of 7%. This agrees with the scenario where these compact red quasars are undergoing a “dusty blow-out” phase, where a compact jet and/or AGN-driven winds interact with a dusty ISM, causing shocks, leading to steep spectral slopes and enhanced radio detection rates.

	Special Colloquium
Dr. Stamatios Stathopoulos	SCHEDULED

DESY

Active galactic nuclei are multi-scale and multi-messenger systems in which different observables probe different emitting zones. The emission from these objects can be from the vicinity of the central engine (black-hole magnetosphere) to the parsec-scale jet. In this talk I will discuss recent work on time-dependent modeling of particle acceleration and radiation in AGN, with an emphasis on how high-energy and radio signatures can be connected. I will first present a model for magnetospheric current sheets in M87*, motivated by kinetic simulations, and discuss their role in pair enrichment, MeV/X-ray flaring, and proton acceleration. I will then show how time-dependent lepto-hadronic modeling can be used to interpret delayed radio flares in neutrino-associated blazars, focusing on TXS 0506+056. In that case, I will argue that a simple expanding neutrino-emitting blob is insufficient to explain the observed radio behavior, pointing instead to downstream re-acceleration and changes in beaming.

	Main Colloquium
Professor Anna Nelles	SCHEDULED

Erlangen Centre for Astroparticle Physics

Cosmic particles such as nuclei and neutrinos populate the Universe from highly abundant solar wind particles to rare cosmic rays of energies exceeding what we can create on Earth. These latter particles are so rare that one needs detectors of several square kilometers in size to measure a meaningful number of them on Earth; a challenging experimental effort. When interacting at Earth, cosmic rays are typically detected with dedicated particle detectors or Cherenkov telescopes. However, it was been know for several decades that they also create measurable radio pulses. I will elaborate on our quest to detect cosmic particles with the new generation of array based radio telescopes such as LOFAR and SKA-Low, as well as dedicated radio neutrino experiments in the ice of Greenland and Antarctica.

	Promotionskolloquium
Vieri Bartolini	SCHEDULED

MPIfR

This thesis investigates the physical connection between accretion processes and relativistic jet properties in radio-loud active galactic nuclei (AGN), with emphasis on misaligned AGN (MAGN). By combining multi-frequency VLBI observations up to 88 GHz with long-term γ-ray monitoring from the Fermi Large Area Telescope and ancillary multiwavelength data, this work explores how different accretion regimes influence jet magnetization, internal structure, and high-energy variability. Growing observational evidence indicates that the excitation class and accretion mode play a fundamental role in shaping jet dynamics and radiative behaviour. Therefore, we explore the dichotomy between High-Excitation Galaxies (HEGs), associated with radiatively efficient thin disks, and Low-Excitation Galaxies (LEGs), typically powered by radiatively inefficient accretion flows. MAGN provide an ideal laboratory for this investigation because relativistic and projection effects are reduced, allowing VLBI observations to probe the compact jet base where high-energy emission is expected to originate. A detailed polarimetric analysis of the HEG radio galaxy 3C 111 reveals a complex magnetized parsec-scale jet. Spectral and rotation-measure mapping identify an optically thick feature located approximately 1–2 parsecs from the core that is co-spatial with extremely high Faraday rotation, suggesting an interaction between the jet and a dense clumpy torus cloud. Farther downstream, a significant transverse RM gradient provides strong evidence for a helical magnetic field. These results indicate that the jet in 3C 111 is still strongly magnetized on parsec scales and propagates in a dense environment, possibly providing dense external photon fields for γ-ray production through inverse Compton. The role of the accretion regime is further explored through a comparative multi-epoch and multi-wavelength analysis of the HEG 3C 111 and the LEG 3C 371, two MAGN with similar global properties but markedly different Eddington ratios. The HEG 3C 111 is dominated by superluminal moving knots and is detected in γ-rays primarily during major flares associated with the ejection of new jet features. In contrast, the LEG 3C 371 exhibits predominantly stationary structures that may act as persistent particle acceleration sites, producing steadier high-energy emission. The observed RM evolution and jet kinematics suggest that stronger disk winds in HEGs may stabilize a relativistic spine through mass loading of an outer sheath, whereas weaker winds in LEGs favor the formation of standing recollimation shocks. We propose that the link between accretion mode and jet properties is driven by differences in disk winds: radiatively efficient HEGs produce strong winds that stabilize the spine–sheath jet, enabling extended acceleration and superluminal features, while weaker winds in LEGs leave the spine more unstable, favoring stationary recollimation shocks. Extending the analysis to a sample of nine nearby Fermi-LAT-detected MAGN reveals differences between HEGs and LEGs in core brightness temperatures and variability patterns. HEG cores tend to approach the inverse-Compton limit and exhibit a higher probability of strong γ-ray flaring, while LEG cores remain closer to equipartition and display more persistent, lower-amplitude emission. Limb-brightening, possibly indicative of a spine-sheath velocity stratification, is observed in three of the four LEGs, as well as in the closest HEG, suggesting that the less frequent detection of this feature in HEGs may be due to their average larger distances, resulting in insufficient spatial resolution in VLBI imaging. The results of this thesis support a unified scenario in which the accretion mode is a primary driver of jet internal structure, stability, and high-energy dissipation. The work provides a framework for future large-sample radio–γ studies and offers observational constraints for relativistic magnetohydrodynamic simulations of disk–jet coupling in radio-loud AGN.

	Special Colloquium
Paloma Thevenet	SCHEDULED

Observatoire de Paris

The blazar 3C 66A is known for its optical flux periodicity and complex jet kinematics. Using 22/43 GHz KaVa (KVN and VERA array) observations and 43 GHz VLBA (Very Long Baseline Array) archival data, we have found that its pc-scale jet has a twisted structure and that the inner jet undergoes periodic swings every 11 years. In this talk, we will describe the peculiar characteristics of 3C 66A and delve into possible interpretation scenarios. The multiwavelength flux variability and jet orientation changes hint at a supermassive black hole binary (SMBHB) in which orbital motion and disk-orbit misalignment lead to jet precession. However, combinations of other mechanisms, such as Lense-Thirring disk precession and jet instabilities, could also account for the properties of 3C 66A, underscoring the challenge in robust SMBHB candidate identification.