Short-Wavelength Radio Variability of High-Energy Emitting AGN Jets

• Applicant: Professor Dr. Matthias Kadler
• Subject Area: Astrophysics and Astronomy
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 443220636

The short-wavelength radio band is getting increasingly important in the context of astroparticle physics as it can probe dynamic processes near the base of AGN jets that are closely related to flares at the highest observable energies. In the past, such observations have always been fundamentally sensitivity limited. Thanks to recent technology upgrades and the addition of new telescopes to mm-VLBI arrays, our new observing programs can now probe the complete sample of AGN that have so far been detected by imaging air Cherenkov telescopes at TeV energies. Sensitive single-dish spectral and polarimetric monitoring can be combined with high-dynamic range imaging of the dynamic sub-milliarcsecond AGN-jet structure. The TELAMON program is a new long-term key-science project on the Effelsberg 100-m telescope that we have set up to monitor the radio spectra of AGN under scrutiny in astroparticle physics, namely TeV blazars and candidate neutrino-associated AGN. Since 2020, we have been performing high-frequency observations every 2−4 weeks at multiple frequencies up to 44 GHz both in total intensity and polarization. We are using these data to characterize the radio variability of very-high energy emitting AGN jets and trace dynamical processes in the pc-scale jets of blazars related to high-energy flares or neutrino detections. The sensitive Effelsberg dish with its versatile instrumentation system yields superior radio data over other programs for very-high-energy (VHE) emitting blazars, which are often faint radio sources. With the superior angular resolution and sensitivity of the upgraded GMVA (now including APEX, GLT and NOEMA) we will be able to probe the smallest-scale jet structure close to the mm-core of these sources and investigate their structural variability. Our new GMVA program targets the 17 radio-brightest TeV-detected AGN (Dec >-30deg), which are prime targets to study possible correlations between the dynamic jet structure and very-high-energy 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-gamma-ray emission sites. Moreover, we are coordinating our radio observations with AGN monitoring groups of the major TeV-gamma collaborations and other multiwavelength facilities.

• DFG Programme: Research Units
• Subproject of: FOR 5195: Relativistic Jets in Active Galaxies
• Co-Investigators: Professor Dr. Karl Mannheim, Ph.D.; Professor Dr. Eduardo Ros, Ph.D.