On possible `cosmic ray cocoons' of relativistic jets

We consider effects on an (ultra-) relativistic jet and its ambient medium caused by high energy cosmic rays accelerated at the jet side boundary. As illustrated by simple models, during the acceleration process a flat cosmic ray distribution can be created, with gyroradia for highest particles' energies reaching the scales comparable to the jet radius or the energy density comparable to the ambient medium pressure. In the case of efficient radiative losses a high energy bump in the spectrum can dominate the cosmic ray pressure. In extreme cases the cosmic rays are able to push the ambient medium off, providing a `cosmic ray cocoon' separating the jet from the surrounding medium. The considered cosmic rays provide an additional jet breaking force and lead to a number of consequences for the jet structure and its radiative output. In particular the involved dynamic and acceleration time scales are in the range observed in variable AGNs.Comment: LaTeX (7 pages, 3 figures, uses mn.sty); MNRAS, accepte

Cosmic-Ray Acceleration at Ultrarelativistic Shock Waves: Effects of Downstream Short-Wave Turbulence

The present paper is the last of a series studying the first-order Fermi acceleration processes at relativistic shock waves with the method of Monte Carlo simulations applied to shocks propagating in realistically modeled turbulent magnetic fields. The model of the background magnetic field structure of Niemiec & Ostrowski (2004, 2006) has been augmented here by a large-amplitude short-wave downstream component, imitating that generated by plasma instabilities at the shock front. Following Niemiec & Ostrowski (2006), we have considered ultrarelativistic shocks with the mean magnetic field oriented both oblique and parallel to the shock normal. For both cases simulations have been performed for different choices of magnetic field perturbations, represented by various wave power spectra within a wide wavevector range. The results show that the introduction of the short-wave component downstream of the shock is not sufficient to produce power-law particle spectra with the "universal" spectral index 4.2. On the contrary, concave spectra with cutoffs are preferentially formed, the curvature and cutoff energy being dependent on the properties of turbulence. Our results suggest that the electromagnetic emission observed from astrophysical sites with relativistic jets, e.g. AGN and GRBs, is likely generated by particles accelerated in processes other than the widely invoked first-order Fermi mechanism.Comment: 9 pages, 8 figures, submitted to Ap

Acceleration time scale for the first-order Fermi acceleration in relativistic shock waves

The acceleration time scale for the process of first-order Fermi acceleration in relativistic shock waves with oblique magnetic field configurations is investigated by the method of Monte Carlo particle simulations. We demonstrate the presence of correlation between the particle energy gain at interaction with the shock and the respective time elapsed since the previous interaction. Because of that any derivation of the acceleration time scale can not use the distribution of energy gains and the distribution of times separately. The time scale discussed in the present paper, $T_{acc}^{(c)}$, is the one describing the rate of change of the particle spectrum cut-off energy in the time dependent evolution. It is derived using a simplified method involving small amplitude particle momentum scattering and intended to model the situations with anisotropic cosmic ray distributions. We consider shocks with parallel, as well as oblique, sub- and super-luminal magnetic field configurations with finite amplitude perturbations, $\delta B$. We got some interesting results like non-monotonic changes of $T_{acc}^{(c)}$ with $\delta B$, which arises due to the particle cross-field diffusion.Comment: 10 pages, TeX type, 11 Encapsulated PostScript figures, psbox.tex included, uses mn.sty (MN.sty - 11 pages, [hbt] above figures, footer on each page), accepted for publication in MNRAS (July 1996

Cosmic Ray Acceleration at Relativistic Shock Waves with a "Realistic" Magnetic Field Structure

The process of cosmic ray first-order Fermi acceleration at relativistic shock waves is studied with the method of Monte Carlo simulations. The simulations are based on numerical integration of particle equations of motion in a turbulent magnetic field near the shock. In comparison to earlier studies, a few "realistic" features of the magnetic field structure are included. The upstream field consists of a mean field component inclined at some angle to the shock normal with finite-amplitude sinusoidal perturbations imposed upon it. The perturbations are assumed to be static in the local plasma rest frame. Their flat or Kolmogorov spectra are constructed with randomly drawn wave vectors from a wide range $(k_{min}, k_{max})$. The downstream field structure is derived from the upstream one as compressed at the shock. We present particle spectra and angular distributions obtained at mildly relativistic sub- and superluminal shocks and also parallel shocks. We show that particle spectra diverge from a simple power-law, the exact shape of the spectrum depends on both the amplitude of the magnetic field perturbations and the wave power spectrum. Features such as spectrum hardening before the cut-off at oblique subluminal shocks and formation of power-law tails at superluminal ones are presented and discussed. At parallel shocks, the presence of finite-amplitude magnetic field perturbations leads to the formation of locally oblique field configurations at the shock and the respective magnetic field compressions. This results in the modification of the particle acceleration process, introducing some features present in oblique shocks, e.g., particle reflections from the shock. We demonstrate for parallel shocks a (nonmonotonic) variation of the particle spectral index with the turbulence amplitude.Comment: revised version (37 pages, 13 figures