Active Galactic Nuclei Jets and Multiple Oblique Shock Acceleration: Starved Spectra

Abstract

Shocks in jets and hot spots of Active Galactic Nuclei (AGN) are one prominent class of possible sources of very high energy cosmic ray particles (above 10^18eV). Extrapolating their spectrum to their plausible injection energy from some shock, implies an enormous hidden energy for a spectrum of index ~-2. Some analyzes suggest the particles' injection spectrum at source to be as steep as -2.4 to -2.7, making the problem much worse, by a factor of order 10^6. Nevertheless, it seems implausible that more than at the very best 1/3 of the jet energy, goes into the required flux of energetic particles thus, one would need to allow for the possibility that there is an energy problem, which we would like to address in this work. Sequences of consecutive oblique shock features, or conical shocks, have been theorized and eventually observed in many AGN jets. Based on that, we use by analogy the 'Comptonisation' effect and we propose a scenario of a single injection of particles which are accelerated consecutively by several oblique shocks along the axis of an AGN jet. We use detailed test-particle approximation Monte Carlo simulations in order to calculate particle spectra by acceleration at such a shock pattern while monitoring the efficiency of acceleration, calculating differential spectra. We find that the first shock of a sequence of oblique shocks, establishes a low energy power-law spectrum with ~E^-2.7. The consecutive shocks push the spectrum up in energy, rendering flatter distributions with steep cut-offs and characteristic depletion at low energies, an effect which could explain the puzzling apparent extra source power as well as the flat or inverted spectra from distant flaring sources.