Gamma rays from cloud penetration at the base of AGN jets

Dense and cold clouds seem to populate the broad line region surrounding the central black hole in AGNs. These clouds could interact with the AGN jet base and this could have observational consequences. We want to study the gamma-ray emission produced by these jet-cloud interactions, and explore under which conditions this radiation would be detectable. We investigate the hydrodynamical properties of jet-cloud interactions and the resulting shocks, and develop a model to compute the spectral energy distribution of the emission generated by the particles accelerated in these shocks. We discuss our model in the context of radio-loud AGNs, with applications to two representative cases, the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet power can be channelled to gamma-rays, which would be likely dominated by synchrotron self-Compton radiation, and show typical variability timescales similar to the cloud lifetime within the jet, which is longer than several hours. Many clouds can interact with the jet simultaneously leading to fluxes significantly higher than in one interaction, but then variability will be smoothed out. Jet-cloud interactions may produce detectable gamma-rays in non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A, and steady in the case of powerful objects of FR II type.

💡 Research Summary

The paper investigates high‑energy emission that arises when cold, dense clouds from the broad‑line region (BLR) intersect the base of relativistic jets in active galactic nuclei (AGN). Using analytical estimates and hydrodynamic simulations, the authors show that a cloud entering the jet at a relative speed of ~0.1–0.3 c generates strong forward and reverse shocks. The shock compression ratios (pressure and density) reach tens to hundreds, providing conditions for efficient particle acceleration. Electrons (and to a lesser extent protons) are accelerated to power‑law distributions with index p≈2.0–2.2 and a fraction η≈0.1 of the jet power is transferred to these non‑thermal particles. In the jet’s magnetic field (B≈0.1–1 G), accelerated electrons emit synchrotron radiation, which is subsequently up‑scattered by the same electrons (synchrotron self‑Compton, SSC) to produce γ‑rays. The SSC component dominates over external inverse‑Compton, yielding a spectral energy distribution that peaks in the 0.1–10 GeV range. The cloud’s residence time inside the jet, τ≈R_c/v_rel≈10⁴–10⁵ s (several hours to days), sets the characteristic variability timescale of a single interaction.

Applying the model to two representative AGN, the low‑luminosity nearby radio galaxy Centaurus A and the powerful quasar 3C 273, the authors find distinct observational signatures. In Cen A, a single cloud–jet encounter can produce a γ‑ray luminosity of ~10⁴²–10⁴³ erg s⁻¹, comparable to the sensitivity of current instruments (e.g., Fermi‑LAT), leading to transient flares lasting a few hours. In 3C 273, the higher jet power and larger cloud encounter rate imply that dozens to hundreds of clouds may be interacting simultaneously. The cumulative γ‑ray output then reaches ~10⁴⁴–10⁴⁵ erg s⁻¹, appearing as a steady high‑energy component with variability smoothed out over days.

The study concludes that jet‑cloud interactions constitute a viable mechanism for producing detectable γ‑rays in non‑blazar AGN. In low‑luminosity, nearby sources the emission is expected to be episodic, while in powerful FR II objects it should be quasi‑continuous. The work highlights the importance of considering the BLR cloud population and jet dynamics when interpreting γ‑ray observations of radio‑loud AGN, and it provides testable predictions for future high‑energy missions.