Stochastic acceleration in the relativistic jets of BL Lacertae objects

Abstract

BL Lacertae objects (BL Lacs) constitute a rare class of active galactic nuclei (AGNs) with the extreme observational features attributed to the Doppler-boosted emission from the relativistic jets, closely aligned along our line-of-sight. The spectral energy distribution (SED), extending over 17–19 orders of the frequency from the radio to the TeV energy range, is of nonthermal origin and shows a typical two-component structure. The lower-energy component, which ranges from the radio to the X-ray frequencies, is widely accepted to be a synchrotron radiation emitted by relativistic electrons. The latter can be accelerated and collimated via the Blandford-Znajek mechanism or magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, the accelerated particles should loose the energy, sufficient for the emission of the KeV-TeV photons, very quickly and they require an additional acceleration via the mechanisms operating locally in the jet. According to the different studies and simulations, particles can undergo a strong stochastic (second-order Fermi) acceleration due to the resonant scattering of the magnetohydrodynamic waves, representing the turbulent magnetic irregularities. On the other hand, the jet turbulence can be strongly amplified due to the interaction between the shock front and the density inhomogeneities in the pre-shock relativistic jet fluid. Our intense X-ray spectral study of TeV-detected, bright BL Lacs (Mrk 421, 1ES 1959 + 650, Mrk 501) often demonstrate the signatures of the efficient stochastic acceleration (a low spectral curvature and different correlations expected within the stochastic process). We have also detected the cases when these sources show the relation between the synchrotron SED peak location Ep and peak height Sp in the form Sp∝Epα with the values of the exponent close to 0.6. The latter is expected when the momentum-diffusion coefficient and the turbulence spectrum are variable during the stochastic acceleration process: there is a transition from the Kraichnan-type into the hard-sphere turbulence.