Blazars & AGN jets
Active galactic nuclei (AGN) are powered by supermassive black holes of 10⁶–10¹⁰ M☉ accreting gas at the centres of galaxies. A fraction of them launch relativistic jets — collimated outflows with bulk Lorentz factors Γ ≈ 10–30 that remain coherent over hundreds of kiloparsecs. When a jet happens to point at Earth, the object appears as a blazar: Doppler boosting amplifies its emission and produces the rapid variability seen from radio to TeV gamma rays. Viewed from the side, the same objects appear as radio galaxies — the misaligned parent population.
See the 2D animation or 3D visualization of this scene.
Where in the jet are UHECRs accelerated?
Several regions of jetted AGN satisfy the Hillas criterion:
- Jet knots — compact (R ~ 0.01–0.1 pc), strongly magnetized (B ~ 0.1–1 G) shocks moving along the jet, where diffusive shock acceleration (Fermi-I) can proceed rapidly. Individual knots in M87’s jet are resolved by VLBI and HST.
- Hotspots — the termination shocks of powerful (FR II) jets, with R ~ kpc and B ~ 100 µG.
- Radio lobes — enormous (R ~ 10–100 kpc) magnetized reservoirs where even slow, stochastic acceleration integrated over megayears can reach ultra-high energies, especially for heavier nuclei (E_max ∝ Z).
Observational status
- Centaurus A, the nearest radio galaxy (~3.8 Mpc), lies near an intermediate-scale excess in the Auger arrival-direction data — one of the most-discussed anisotropy hints.
- The Auger correlation of arrival directions with catalogs of starburst galaxies and gamma-ray AGN reaches ~4σ post-trial significance, but no association has crossed the discovery threshold.
- The blazar TXS 0506+056 was associated with the IceCube neutrino IC-170922A (~3σ), the first evidence that blazar jets accelerate hadrons — though at energies far below the UHECR regime.
Challenges
Blazars beam their emission — and possibly their cosmic rays — into narrow cones, so the local density of aligned sources is low. Magnetic deflections must then explain why we do not see sharp point sources. For the most luminous blazars (FSRQs), dense radiation fields may photodisintegrate nuclei before escape, favouring lower-luminosity BL Lacs and radio galaxies as survivable UHECR accelerators.
Related
- Tidal disruption events — transient jets from dormant black holes
- Gamma-ray bursts — the most extreme relativistic jets known
- Open questions