The Standing Accelerator

Astrophysical jets expanding into dense environments develop recollimation shocks — standing structures where the jet, squeezed by cocoon pressure, converges back toward its axis. These shocks are highly inclined relative to the jet direction, which might suggest they are inefficient accelerators. But they can be strong, and their quasi-stationary nature gives them something transient shocks lack: time.

arXiv:2603.16647 extends an analytic jet hydrodynamics model to the sub-relativistic regime and formulates a semi-analytic acceleration and transport model for particles injected at recollimation shocks via diffusive shock acceleration. By solving the space-dependent transport equation, the work obtains particle distributions along the jet and robust predictions for maximum energies.

The results span three orders of magnitude in source class. In Seyfert galaxies, recollimation shocks can accelerate particles from PeV to EeV energies. In microquasars, tens of PeV. In protostellar jets, up to TeV. The hierarchy follows from intrinsic jet properties — power, velocity, environment density — through the same physical mechanism.

Protons escaping the jets diffuse through the cocoon, leading to possible hadronic signatures — neutrinos and gamma rays from interactions with the surrounding medium. The standing shock accelerates; the cocoon confines and processes; the environment provides the target.

The key insight: a slowly evolving standing shock is a more effective accelerator than a powerful transient one, because the acceleration time is limited by the shock’s persistence, not its strength. The shock does not need to be fast. It needs to be there.