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The properties of relativistic jets, their interaction with the environment, and their emission of radiation can be self-consistently studied by using particle-in-cell (PIC) numerical simulations. Using three-dimensional (3D), relativistic PIC simulations, we present the first self-consistently calculated synthetic spectra of head-on and off-axis emission from electrons accelerated in cylindrical, relativistic plasma jets containing an initial toroidal magnetic field. The jet particles are initially accelerated during the linear stage of growing plasma instabilities, which are the Weibel instability (WI), kinetic Kelvin-Helmholtz instability (kKHI), and mushroom instability (MI). In the nonlinear stage, these instabilities are dissipated and generate turbulent magnetic fields which accelerate particles further. We calculate the synthetic spectra by tracing a large number of jet electrons in the nonlinear stage, near the jet head where the magnetic fields are turbulent. Our results show the basic properties of jitter-like radiation emitted by relativistic electrons when they travel through a magnetized plasma with the plasma waves driven by kinetic instabilities (WI, kKHI, and MI) growing into the nonlinear regime. At low frequencies, the slope of the spectrum is ~ 0.94, which is similar to that of the jitter radiation. The results are relevant to active galactic nuclei/blazars and gamma-ray burst jet emission and set the ground for future studies on synthetic spectra from relativistic jets., Comment: 9 pages, 8 figures, submitted to MNRAS

Using large-scale fully-kinetic two-dimensional particle-in-cell simulations, we investigate the effects of shock rippling on electron acceleration at low-Mach-number shocks propagating in high-$\beta$ plasmas, in application to merger shocks in galaxy clusters. We find that the electron acceleration rate increases considerably when the rippling modes appear. The main acceleration mechanism is stochastic shock-drift acceleration, in which electrons are confined at the shock by pitch-angle scattering off turbulence and gain energy from the motional electric field. The presence of multi-scale magnetic turbulence at the shock transition and the region immediately behind the main shock overshoot is essential for electron energization. Wide-energy non-thermal electron distributions are formed both upstream and downstream of the shock. The maximum energy of the electrons is sufficient for their injection into diffusive shock acceleration. We show for the first time that the downstream electron spectrum has a~power-law form with index $p\approx 2.5$, in agreement with observations., Comment: 15 pages, 14 figures, to be published in ApJ

Particle acceleration and heating at mildly relativistic magnetized shocks in electron-ion plasma are investigated with unprecedentedly high-resolution two-dimensional particle-in-cell simulations that include ion-scale shock rippling. Electrons are super-adiabatically heated at the shock, and most of the energy transfer from protons to electrons takes place at or downstream of the shock. We are the first to demonstrate that shock rippling is crucial for the energization of electrons at the shock. They remain well below equipartition with the protons. The downstream electron spectra are approximately thermal with a limited supra-thermal power-law component. Our results are discussed in the context of wakefield acceleration and the modelling of electromagnetic radiation from blazar cores., Comment: 11 pages, 11 figures; This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

Mildly relativistic shocks in magnetized electron-ion plasmas are investigated with 2D kinetic particle-in-cell simulations of unprecedentedly high resolution and large scale for conditions that may be found at internal shocks in blazar cores. Ion-scale effects cause corrugations along the shock surface whose properties somewhat depend on the configuration of the mean perpendicular magnetic field, that is either in or out of the simulation plane. We show that the synchrotron maser instability persists to operate in mildly relativistic shocks in agreement with theoretical predictions and produces coherent emission of upstream-propagating electromagnetic waves. Shock front ripples are excited in both mean-field configurations and they engender effective wave amplification. The interaction of these waves with upstream plasma generates electrostatic wakefields., Comment: 14 pages, 19 figures; This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

We investigated electromagnetic precursor wave emission in relativistic shocks by using two-dimensional particle-in-cell simulations. We found that the wave amplitude is significantly enhanced by a positive feedback process associated with ion-electron coupling through the wakefields for high magnetization. The wakefields collapse during the nonlinear process of the parametric decay instability in the near-upstream region, where nonthermal electrons and ions are generated. The intense coherent emission and the particle acceleration may opperate in high-energy astrophysical objects., Comment: 7 pages, 4 figures, accepted to ApJL

One of the key questions in the study of relativistic jets is how magnetic reconnection occurs and whether it can effectively accelerate electrons in the jet. We performed 3D particle-in-cell (PIC) simulations of a relativistic electron-proton jet of relatively large radius that carries a helical magnetic field. We focussed our investigation on the interaction between the jet and the ambient plasma and explore how the helical magnetic field affects the excitation of kinetic instabilities such as the Weibel instability (WI), the kinetic Kelvin-Helmholtz instability (kKHI), and the mushroom instability (MI). In our simulations these kinetic instabilities are indeed excited, and particles are accelerated. At the linear stage we observe recollimation shocks near the center of the jet. As the electron-proton jet evolves into the deep nonlinear stage, the helical magnetic field becomes untangled due to reconnection-like phenomena, and electrons are repeatedly accelerated as they encounter magnetic-reconnection events in the turbulent magnetic field., Comment: 8 pages, 5 figures, submitted to ApJL, movie will be added (dBtotByz11MF_011.mp4 5.8MB), significantly revised with corrected figures. After rejected by ApJL, MNRAS, in press, 8 pages, 6 figures, https://doi.org/10.1093/mnras/staa421

Active galactic nuclei (AGNs) are considered to be one of the most appropriate sources of ultra high energy cosmic rays (UHECRs, $E \gtrsim 10^{18} \mathrm{~eV}$). Radiogalaxy Virgo A (M87) in the centre of a cluster of galaxies Virgo Cluster (VC) can be a prominent source of UHECRs. We investigate the possible contribution of Virgo A and the VC to the flux of events with trans-GZK energies - extremely high energy cosmic rays (EHECRs) - from the recent Auger and Telescope Array (TA) data sets ($E > 52 \mathrm{~EeV}$ and $E > 57 \mathrm{~EeV}$, respectively). We simulate EHECR propagation from Virgo A and the VC taking into account their deflections in galactic (GMF) and extragalactic (EGMF) magnetic fields and show that there is no excess of EHECR arrival directions from images of Virgo A/VC at different EHECR rigidities. By means of event-by-event analysis we recover the extragalactic arrival directions of EHECR events detected by Auger and TA for representative set of nuclei H(p), He, N, Si, Fe, and find evidences of enhanced fluxes of N-Si-Fe EHECRs from the Local Filament and Hot/Cold Spot regions. The Local Filament with its enhanced magnetic field is an expected contributor to the UHECR flux as the closest to the Earth last scattering centre, whereas Hot/Cold Spot region is a part of a larger arc-like spot, possibly created by diffusively spreading jet of UHECRs, accelerated in the relativistic jet of Virgo A during a prominent nuclear outburst about 10 - 12 Myr ago., Comment: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)

In this study we investigate jet interaction at a microscopic level in a cosmological environment, which responds to a key open question in the study of relativistic jets. Using small simulation systems during prior research, we initially studied the evolution of both electron-proton and electron-positron relativistic jets containing helical magnetic fields, by focusing on their interactions with an ambient plasma. Here, using larger jet radii, we have performed simulations of global jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). We found that the evolution of global jets strongly depends on the size of the jet radius. For example, phase bunching of jet electrons, in particular in the electron-proton jet, is mixed with larger jet radius due to the more complicated structures of magnetic fields with excited kinetic instabilities. In our simulation study these kinetic instabilities lead to new types of instabilities in global jets. In the electron-proton jet simulation a modified recollimation occurs and jet electrons are strongly perturbed. In the electron-positron jet simulation mixed kinetic instabilities occur at early times followed by a turbulence-like structure. Simulations using much larger (and longer) systems are further required in order to thoroughly investigate the evolution of global jets containing helical magnetic fields., Comment: 7 pages, 4 figures, submitted to special issue "Polarised Emission from Astrophysical Jets", June 12-16, 2017, Ierapetra, Greece. arXiv admin note: substantial text overlap with arXiv:1609.09363

We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta $\beta_{\rm p}=5\cdot 10^{-4}$ and $\beta_{\rm p}=0.5$), we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane while keeping it at $90^\circ$ to the shock normal. Ion reflection off the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration (SSA) mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field the fraction of supra-thermal electrons is much higher than for other configurations, because only in this case the Buneman modes are fully captured by the 2D simulation grid. Shocks in plasma with moderate $\beta_{\rm p}$ provide more efficient pre-acceleration. The relevance of our results to the physics of fully three-dimensional systems is discussed.

A nonresonant cosmic-ray-current-driven instability may operate in the shock precursors of young supernova remnants and be responsible for magnetic-field amplification, plasma heating and turbulence. Earlier simulations demonstrated magnetic-field amplification, and in kinetic studies a reduction of the relative drift between cosmic rays and thermal plasma was observed as backreaction. However, all published simulations used periodic boundary conditions, which do not account for mass conservation in decelerating flows and only allow the temporal development to be studied. Here we report results of fully kinetic Particle-In-Cell simulations with open boundaries that permit inflow of plasma on one side of the simulation box and outflow at the other end, hence allowing an investigation of both the temporal and the spatial development of the instability. Magnetic-field amplification proceeds as in studies with periodic boundaries and, observed here for the first time, the reduction of relative drifts causes the formation of a shock-like compression structure at which a fraction of the plasma ions are reflected. Turbulent electric field generated by the nonresonant instability inelastically scatters cosmic rays, modifying and anisotropizing their energy distribution. Spatial CR scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads to significant nonadiabatic heating of the background plasma maintaining bulk equipartition between ions and electrons. The highest temperatures are reached at sites of large-amplitude electrostatic fields. Ion spectra show supra-thermal tails resulting from stochastic scattering in the turbulent electric field. Together, these modifications in the plasma flow will affect the properties of the shock and particle acceleration there., Comment: Accepted for publication in MNRAS. 16 pages, 15 figures

We study the interaction of relativistic jets with their environment, using 3-dimensional relativistic particle-in-cell simulations for two cases of jet composition: (i) electron-proton ($e^{-}-p^{+}$) and (ii) electron-positron ($e^{\pm}$) plasmas containing helical magnetic fields. We have performed simulations of "global" jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability and the Mushroom instability. We have found that these kinetic instabilities are suppressed and new types of instabilities can grow. For the $e^{-}-p^{+}$ jet, a recollimation-like instability occurs and jet electrons are strongly perturbed, whereas for the $e^{\pm}$ jet, a recollimation-like instability occurs at early times followed by kinetic instability and the general structure is similar to a simulation without a helical magnetic field. We plan to perform further simulations using much larger systems to confirm these new findings., Comment: 4 pages, 1 figure, New Frontiers in Black Hole Astrophysics, Proceedings of the IAU Symposium No. 324, A. Gomboc, ed

In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron$-$proton ($e^{-}-p^{+}$) and electron$-$positron ($e^{\pm}$) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of "global" jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the $e^{-}-p^{+}$ jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the $e^{\pm}$ jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields., Comment: 9 pages, 6 figures, Blazars through Sharp Multi-Wavelength Eyes. Malaga (Spain). 30 May - 3 June 2016, movie is available at http://www.mpdi.com/link, Video S1

Active galactic nuclei (AGN) are considered as one of the most appropriate sources of cosmic rays with energy exceeding $\sim 10^{18}$ eV. Virgo A (M87 or NGC 4486) is the second closest to the Milky Way active galaxy. According to existing estimations it can be a prominent source of ultra high energy cosmic rays (UHECR). However not many events have been registered in the sky region near Virgo A, possibly due to magnetic field influence. In the present work we check UHECR events from the recent sets of data (AUGER, Telescope Array etc.) for possibility of their origination in this AGN. We carried out the simulation of UHECR motion from Virgo A taking into account their deflections in galactic (GMF) as well as extragalactic (EGMF) magnetic fields according to the several latest models. The maps of expected UHECR arrival directions were obtained as a result. It has been found the following: 1) UHECR deflection caused by EGMF is comparable with GMF one, moreover the influence of EGMF sometimes is dominating; 2) effect of EGMF demonstrates obvious asymmetry in the final distribution of expected UHECR arrival directions; 3) the results of simulations depend on chosen GMF model and are still open for the discussion., Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands

The propagation of ultra high energy cosmic rays in Galactic and extragalactic magnetic fields is investigated in the present paper. The motion of charged particles of different energies and chemical composition is simulated using different Galactic magnetic field models. Positions for the real sources of events registered at the Auger observatory are calculated taking into account the influence of both Galactic and extragalactic turbulent fields. The possibility of their correlation with the Centaurus A radio galaxy is analyzed., Comment: 17 pages, 3 figures, 3 tables, submitted to Kinematics and Physics of Celestial Bodies