Large-Scale Blazar Jets: Clues on High-Energy Emission from Low-Frequency Radio Observations

• Applicant: Professor Dr. Matthias Kadler
• Subject Area: Astrophysics and Astronomy
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 443220636

About 200 AGN jets have been imaged with arcsec resolution in the 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-Comptonization 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. High-resolution LOFAR observations of AGN jets involving international baselines at hundreds of MHz found radio flux densities in quasars 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. Moreover, a prevalence of positional offsets between radio and X-ray knots in recent systematic studies of quasar jets and a trend of declining flux ratios with increasing distance from the jet core strengthen the need for high-resolution radio data at the longest wavelengths in order to better constrain multi-zone emission models. In this project, we will continue a systematic LOFAR study of the long-wavelength radio emission of quasar jets on kiloparsec scales. We are combining our LOFAR results 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 put a special focus on quasar jets at different redshift ranges to address cosmological effects such as strong interactions between the jets and the denser intergalactic medium during early evolutionary phases of the universe. We will further develop and optimize high-resolution imaging techniques at the low and ultralow radio frequencies in total and linearly-polarized intensity with LOFAR-VLBI techniques to measure magnetic-field geometries and constrain jet-composition and emission models.

• DFG Programme: Research Units
• Subproject of: FOR 5195: Relativistic Jets in Active Galaxies
• Co-Investigator: Professor Marcus Brüggen, Ph.D.