Active galactic nuclei jets and multiple oblique shock acceleration: starved spectra

Abstract

Context. 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 18 eV). Extrapolating their spectrum to their plausible injection energy from some shock implies an enormous hidden energy for a spectrum of index ∼−2. Some analyses suggest the particles' injection spectrum at source to be as steep as −2.4 to −2.7, which exacerbates the problem, by a factor of 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. Aims. Sequences of consecutive oblique shock features, or conical shocks, have been theoretically predicted and eventually observed in many AGN jets. Based on that, we use by analogy the Comptonization effect and propose a scenario of a single injection of particles consecutively accelerated by several oblique shocks along the axis of an AGN jet. Methods. We developed a test-particle approximation Monte Carlo simulations to calculate particle spectra by acceleration at such a shock pattern while monitoring the efficiency of acceleration by calculating differential spectra. Results. We find that the first shock of a sequence of oblique shocks establishes a low-energy power-law spectrum with ∼E −2.7 . The following consecutive shocks push the spectrum up in energy, rendering flatter distributions with steep cut-offs, and characteristic depletion at low energies, which could explain the puzzling apparent extra source power. Conclusions. Our numerical calculations show a variation of spectral indices, a general spectral flattening, and starved spectra, which connect to the relativistic nature of the shocks, the multiple shock acceleration conditions, and the steepness of the magnetic field to the shock normal. This helps in understanding the jet-magnetic field geometry and the irregular or flat spectra observed in many AGN jets (e.g., CenA, 3C 279, PKS 1510-089). Furthermore, the E −2.4 − E −2.7 ultra-high-energy cosmic-ray injected source spectra claimed by many authors might be explained by the superposition of several, perhaps many, emission sources, all of which end their particle shock-acceleration sequence with flatter, starved spectra produced by two or more consecutive oblique shocks along their jets. It might also imply a mixed component of the accelerated particles above 10 19 eV. Moreover, the present acceleration model can explain the variability of inverted gamma-ray spectra observed in high redshifted flaring extragalactic sources.

Exceptional AGN-driven turbulence inhibits star formation in the 3C 326N radio galaxy

“…This is a plausible explanation of the high fraction of warm molecular gas in some galaxies hosting AGN inferred from H 2 and CO spectroscopy (Ferland et al 2008;Ogle et al 2010;Mittal et al 2011). Some studies suggest that flaring of radio jets can enhance the output of cosmic rays in AGN (e.g., de Gasperin et al 2012;Laing & Bridle 2013;Meli & Biermann 2013) and that ≈10 % of the jet kinetic power may be converted into cosmic ray luminosity (Gopal-Krishna et al 2010;Biermann & de Souza 2012). However, we lack observations that quantify the interaction of cosmic rays with the gas that depends on their propagation.…”

“…7) that it is not possible to conclusively rule out the cosmic ray heating, provided that such a high ionization rate can be maintained on the large scale of the galaxy's interstellar medium. We do not have strong constraints on the diffuse cosmic ray intensity in radio galaxies, which may be enhanced if the jets flare (Biermann & de Souza 2012;Laing & Bridle 2013;Meli & Biermann 2013) but could also be low in the absence of star formation as is observed in 3C 326N.…”

Constraining the Ism Properties of the Cloverleaf Quasar Host Galaxy With Herschel Spectroscopy

“…We also point out that, in addition to starburst-related phenomena discussed above, cosmic rays can be produced in the magnetic fields in AGN (Biermann & de Souza 2012;Laing & Bridle 2013;Meli & Biermann 2013), though we do not presently examine the effects of AGN-supplied cosmic rays on the molecular gas in the Cloverleaf.…”

CONSTRAINING THE PHYSICAL CONDITIONS IN THE JETS OF Γ-Ray FLARING BLAZARS USING CENTIMETER-BAND POLARIMETRY AND RADIATIVE TRANSFER SIMULATIONS. II. EXPLORING PARAMETER SPACE AND IMPLICATIONS

“…(see Eilek & Hughes (1991) for a review of this, but note that multiple phases of acceleration, for example, at a sequence of oblique or conical shocks, can result in a flat spectrum with a depleted low-energy region, as modeled by Meli & Biermann 2013. ) One would therefore expect the distribution to extend down to the reservoir of low-energy particles from which the radiating ones derive.…”