Mechanism of relativistic massive-particle acceleration in AGN jets

Determine the physical mechanism that accelerates massive particles to relativistic energies in systems powered by supermassive black holes, with particular attention to powerful extragalactic jets (e.g., blazar jets such as PKS 1424+240) as candidate sites of acceleration responsible for very-high-energy photon and neutrino production.

Background

Active galactic nuclei are prominent sources of very-high-energy photons and likely produce high-energy neutrinos, implicating efficient acceleration of massive particles to relativistic energies. Despite decades of observation and modeling, the specific acceleration mechanism (e.g., magnetic reconnection, shock acceleration, shear acceleration) operating in the environments of supermassive black holes remains unidentified.

In this study, VLBA observations of the blazar PKS 1424+240 reveal a jet viewed within its cone and a clear toroidal magnetic-field component, supporting extreme relativistic beaming and consistent Doppler factors for multi-messenger emission. While these geometric and magnetic-field insights help reconcile radio and VHE inferences, they do not establish the underlying particle acceleration process, which the authors explicitly note remains an open question in modern astrophysics.

References

The mechanism responsible for accelerating massive particles to relativistic energies remains an open question in modern astrophysics, with powerful extragalactic jets being a key candidate.

— Looking into the Jet Cone of the Neutrino-Associated Very High Energy Blazar PKS 1424+240  (2504.09287 - Kovalev et al., 12 Apr 2025) in Section 6 (Summary)

While the observed emission is largely interpreted as high-energy protons interacting with ambient radiation fields and gas in the vicinity of the supermassive black hole [27–29], the exact interaction channels and acceleration mechanisms remain uncertain [30].

— Astrophysical Sensitivity Projections for the IceCube Upgrade  (2608.28395 - Abbasi et al., 28 Aug 2026) in Section I, Introduction, p. 4

AGNs are an important type of CR accelerator, though it is unknown if they primarily accelerate protons or electrons.

— TeV Gamma Rays and Neutrinos from Winds Driven by Radiatively Inefficient Accretion Flows of Low-Luminosity Active Galactic Nuclei  (2608.30596 - Sakai et al., 31 Aug 2026) in Section 1, Introduction