Electron positron pairs in blazar jets and gamma-ray loud radio-galaxies
The matter content of extragalactic relativistic jets is still an unsolved issue. There are strong arguments against pure electron-positron pair jets, but pairs could outnumber the electrons associated with protons by a factor 10-20. This impacts on the estimate of the jet kinetic power, by reducing it by the same factor, and on the total energy delivered to leptons by the particle acceleration mechanism. Pairs cannot be created in the same jet-zone responsible for the high energy gamma-ray emission we see in blazars, because the reprocessing of the created pairs would overproduce the X-ray flux. Copious pair creation could occur in the inner zone of the still accelerating jet, where the bulk Lorentz factor is small. It is found that the inner zone can produce a sufficient number of pairs to replenish the zone of the jet where most of the luminosity is emitted, but only if the gamma-ray luminosity of the inner jet is above 1e44 erg/s at ~1 MeV. Since the beaming is modest, this emission can be observed at large viewing angles, and detected in radio-galaxies and lobe dominated quasars at the flux level of 1e-12 - 1e-11 erg/cm2/s for a source at a redshift z=0.1.
💡 Research Summary
The paper tackles the long‑standing problem of the matter composition of relativistic jets in active galactic nuclei, focusing on the possible dominance of electron‑positron (e⁺e⁻) pairs over the electrons that accompany protons. The authors first review the arguments against a pure pair plasma: if pairs were produced in the same region that emits the observed high‑energy γ‑rays (the “γ‑ray zone”), the subsequent re‑processing of the pairs would generate an X‑ray flux far exceeding what is actually observed. Consequently, any substantial pair creation must occur elsewhere in the jet.
They propose that the inner part of the jet, still in the acceleration phase and characterized by a modest bulk Lorentz factor (Γ ≲ 5), provides the right conditions. In this region the jet is relatively compact, magnetic fields are strong, and the particle distribution can produce a luminous MeV component. Using standard γ‑γ absorption cross‑sections and radiative transfer equations, they calculate that if the inner jet emits a luminosity L₁MeV ≳ 10⁴⁴ erg s⁻¹ around 1 MeV, the pair production rate is sufficient to replenish the outer γ‑ray zone with enough leptons to explain the observed radiation without violating X‑ray constraints.
Because the inner zone’s emission is only mildly beamed, its flux does not depend strongly on the viewing angle. Therefore, even observers at large angles (typical of radio galaxies and lobe‑dominated quasars) should detect a steady MeV component with an observed flux of order 10⁻¹²–10⁻¹¹ erg cm⁻² s⁻¹ for a source at redshift z≈0.1. This prediction is compatible with existing detections of faint MeV excesses in several mis‑aligned AGN.
The authors discuss the broader implications. If pairs outnumber the proton‑associated electrons by a factor of 10–20, the kinetic power inferred for the jet is reduced by the same factor, altering estimates of jet feedback on the host galaxy and the intergalactic medium. Moreover, the presence of abundant pairs changes the efficiency and spectral shape of particle acceleration mechanisms, since the energy budget per lepton is lower.
Finally, the paper outlines observational tests. Upcoming MeV‑range missions such as AMEGO or e‑ASTROGAM, with improved sensitivity and angular resolution, could directly measure the predicted MeV flux from mis‑aligned sources, providing a decisive test of the inner‑jet pair‑production scenario. Confirmation would reshape our understanding of jet composition, energetics, and the role of relativistic outflows in galaxy evolution.