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We show here that highly polarized X-ray synchrotron radiation from young supernova remnants (SNRs) can be modeled within the framework of diffusive shock acceleration (DSA) and nonlinear magnetic turbulence generation.Cosmic ray acceleration by SNR shocks to very high energies requires efficient magnetic turbulence amplification in the shock precursor.As the strong turbulence generated by Bell's instability far upstream from the viscous subshock convects through the subshock, nonlinear dynamical effects on the compressible fluctuations produce a downstream layer filled with strong anisotropic turbulence with predominantly radial magnetic fields.The synchrotron radiation from shock accelerated electrons in the turbulent downstream layer has a high degree of polarization shown to be consistent with recent observations of young SNRs by IXPE.In the case of Tycho's SNR, the measured X-ray radiation constrains the thickness of the energy containing interval and the amplitude of cosmic ray driven magnetic turbulence, as well as the maximal energy of accelerated protons.The preferential direction of the X-ray polarization depends sensitively on the SNR shock velocity and the ambient density.A unique feature of our model is the sensitive dependence of the degree and direction of X-ray polarization on the spatial overlap between regions of amplified magnetic turbulence and TeV electron populations.While this overlap occurs on scales orders of magnitude below the resolution of IXPE,its polarization measurement allows testing of turbulent plasma processes on unprecedented scales.The mechanism of formation of highly polarized X-ray synchrotron radiation in fast shocks with high level of anisotropic turbulent magnetic field preferentially directed along the shock normal may be applied to other systems like shocks produced by black hole jets., Comment: Physical Review in print, 14 pages, 6 figures

The results of an analysis of changes in the characteristics of atmospheric centers of action (ACAs) in the Northern (NH) and Southern (SH) hemispheres using results of simulations with the CMIP5 and CMIP6 ensembles of climate models are presented. The ability of models to simulate ACA features is estimated for the historical scenario in comparison with ERA5 reanalysis data. The projected changes are evaluated under RCP8.5 and SSP5-8.5 scenarios for CMIP5 and CMIP6 models, respectively. The ACA intensity is evaluated that defined as the difference in sea level pressure averaged over the ACA region and the entire hemisphere. In NH, reanalysis and models show greater intensity of subtropical oceanic anticyclonic ACAs in summer than in winter. The opposite is found for the intensity of NH subpolar oceanic cyclonic ACAs. The interannual variability of the ACA intensity in winter is generally greater than in summer. In SH, the season with greater intensity of oceanic anticyclonic and cyclonic ACAs and its interannual variability varies from ocean to ocean. CMIP5 and CMIP6 models show substantial changes of ACAs characteristics in the XXI century. More significant trends in the strengthening of ACAs in the 21st century appear in the SH, especially in the winter seasons. The most consistent weakening trends are found over continents for winter North American maximum and the summer Asian minimum. For the winter Siberian maximum, the weakening trend is found more pronounced in CMIP6 models than in CMIP5., Comment: 23 pages, 10 figures, 7 tables

Fast blue optical transients (FBOTs) represent a new class of highly energetic sources observed from radio to X-rays. High luminosity, light curves and spectra of the sources can be understood if they are associated with supernova-like or tidal disruption events. Radio observations of the transient sources revealed a mildly relativistic expansion of some of the remnants. The high power and mildly relativistic shock velocities are providing favorable conditions for very high energy particle acceleration. In this paper we present a model of particle acceleration in mildly relativistic magnetohydrodynamic (MHD) outflow of the transient source. To construct the non-thermal radiation and cosmic ray spectra in a broad range of energies we combined the microscopic particle-in-cell (PIC) simulations of electron and proton injection at mildly relativistic shock with Monte Carlo technique for high energy particle transport and acceleration. The kinetic PIC simulations provided the energy partition parameter $\epsilon_{e}$ used to fit the observed non-thermal radio emission using the magnetic field amplification mechanisms modelled with Monte Carlo simulations. The model allowed to describe the radio-spectrum of CSS161010 and it's X-ray luminosity. The high X-ray luminosity of AT2018 and AT2020mrf detected during the first weeks can be connected to the jet interaction with the stellar companion in a binary system. The model predicts that FBOTs can accelerate cosmic rays to energies above 10 PeV with a possible upper limit of maximum energy of 100 PeV. With the expected event rate of FBOTs they can contribute to the very high energy cosmic rays population in galaxies., Comment: 11 pages, 8 figures

For most elements, the isotopic ratios seen in cosmic rays (CRs) are similar to those in the solar wind. The most important exception to this is $^{22}$Ne/$^{20}$Ne where the CR value is $\sim 5$ times that of the solar wind. According to most recent models of nucleosynthesis, a large amount of $^{22}\mathrm{Ne}$ is generated in Wolf-Rayet (WR) stars. In the winds of carbon sequence WR stars, i.e., WC stars, the isotopic ratio $^{22}\mathrm{Ne}$/$^{20}\mathrm{Ne}$ can be much larger than in the solar wind. Here, we consider CRs produced by $^{22}\mathrm{Ne}$-enriched WR winds in compact clusters of young massive stars like Westerlund~1. We assume that efficient CR acceleration in clusters occurs over the WR lifetime in an ensemble of shock waves from multiple massive star winds. We estimate the fraction of all Galactic CRs such sources may produce for a given set of parameters., Comment: 5 pages, 1 figure, accepted for publication in IOP Journal of Physics: Conf. Series

It has been shown that supernova blast waves interacting with winds from massive stars in compact star clusters may be capable of producing cosmic-ray (CR) protons to above $10^{17}$ eV. We give a brief description of the colliding-shock-flows mechanism and look at generalizations of the diffusion of ~ 100 PeV CRs in the turbulent galactic magnetic field present in the galactic disk. We calculate the temporal evolution of the CR anisotropy from a possible distribution of young compact massive star clusters assuming the sources are intermittent on time scales of a few million years, i.e., comparable to their residence time in the Milky Way. Within the confines of our model, we determine the galactic/extra-galactic fraction of high-energy CRs resulting in anisotropies consistent with observed values. We find that galactic star clusters may contribute a substantial fraction of ~ 100 PeV CRs without producing anisotropies above observed limits., Comment: Submitted to Advances in Space Research, 14 pages, 6 figures

Relativistic particle acceleration in collisionless shocks of supernova remnants is accompanied by magnetic field amplification from cosmic ray (CR) driven plasma instabilities. Bell's fast CR-current instability is predicted to produce turbulence with a non-zero mean electric field in the shock precursor. We present a Monte Carlo model of Fermi shock acceleration explicitly taking into account an effective mean upstream electric field. Our model is nonlinear and includes the backreaction effects of efficient Fermi acceleration on the shock structure., Comment: 6 pages, 2 figures, accepted in Journal of Physics: Conference Series (in press, 2019)

We consider acceleration of leptons up to GeV-TeV energies in the bow shock wind nebula of PSR J0437-4715 and their subsequent diffusion through the interstellar magnetic fields. The leptons accelerated at the pulsar wind termination surface are injected into re-acceleration in colliding shock flows. Modelled spectra of synchrotron emission from the accelerated electrons and positrons are consistent with the far-ultraviolet and X-ray observations of the nebula carried out with the Hubble Space Telescope and Chandra X-ray Observatory. These observations are employed to constrain the absolute fluxes of relativistic leptons, which are escaping from the nebula and eventually reaching the Solar System after energy-dependent diffusion through the local interstellar medium accompanied by synchrotron and Compton losses. It is shown that accelerated leptons from the nebula of PSR J0437-4715 can be responsible both for the enhancement of the positron fraction above a few GeV detected by PAMELA and AMS-02 spectrometers and for the TeV range lepton fluxes observed with H.E.S.S., VERITAS, Fermi, CALET, and DAMPE., Comment: 16 pages, 3 figures, Astrophysical Journal Letters, in print

We give a brief review of the origin and acceleration of cosmic rays (CRs), emphasizing the production of CRs at different stages of supernova evolution by the first-order Fermi shock acceleration mechanism. We suggest that supernovae with trans-relativistic outflows, despite being rather rare, may accelerate CRs to energies above 10$^{18}$ eV over the first year of their evolution. Supernovae in young compact clusters of massive stars, and interaction powered superluminous supernovae, may accelerate CRs well above the PeV regime. We discuss the acceleration of the bulk of the galactic CRs in isolated supernova remnants and re-acceleration of escaped CRs by the multiple shocks present in superbubbles produced by associations of OB stars. The effects of magnetic field amplification by CR driven instabilities, as well as superdiffusive CR transport, are discussed for nonthermal radiation produced by nonlinear shocks of all speeds including trans-relativistic ones., Comment: Accepted for publication in Space Science Reviews January 2018

We discuss a specific population of galactic PeVatrons which may be the main source of the galactic cosmic-ray (CR) component well above PeV energies. Supernovae in compact clusters of massive stars are proposed as powerful sources of CRs, neutrinos, and gamma-ray emission. Numerical simulations of non-linear Fermi acceleration at converging shock flows have revealed that these accelerators can provide very hard spectra of protons up to $10^{16}-10^{17}$ eV which is well above the "knee" in the all-particle CR spectrum at about $3\times10^{15}$ eV. We suggest that known supernova remnants interacting with stellar winds in the compact clusters of young massive stars Westerlund I and Cl*1806-20 can be associated with the sources of the TeV gamma-ray emission detected by H.E.S.S. and may be responsible for a fraction of the high-energy neutrinos detected with the IceCube observatory. A recent CR composition measurement with the LOFAR array has revealed a light-mass component possibly dominating the all-particle spectrum at energies around $10^{17}$ eV. Such a strong light component (mainly protons and helium) may require specific galactic CR sources such as supernovae interacting with compact clusters of massive stars in addition to isolated supernova remnants., Comment: Accepted for publication in Advances in Space Research May 2017

Fast collisionless shocks in cosmic plasmas convert their kinetic energy flow into the hot downstream thermal plasma with a substantial fraction of energy going into a broad spectrum of superthermal charged particles and magnetic fluctuations. The superthermal particles can penetrate into the shock upstream region producing an extended shock precursor. The cold upstream plasma flow is decelerated by the force provided by the superthermal particle pressure gradient. In high Mach number collisionless shocks, efficient particle acceleration is likely coupled with turbulent magnetic field amplification (MFA) generated by the anisotropic distribution of accelerated particles. This anisotropy is determined by the fast particle transport making the problem strongly nonlinear and multi-scale. Here, we present a nonlinear Monte Carlo model of collisionless shock structure with super-diffusive propagation of high-energy Fermi accelerated particles coupled to particle acceleration and MFA which affords a consistent description of strong shocks. A distinctive feature of the Monte Carlo technique is that it includes the full angular anisotropy of the particle distribution at all precursor positions. The model reveals that the super-diffusive transport of energetic particles (i.e., Levy-walk propagation) generates a strong quadruple anisotropy in the precursor particle distribution. The resultant pressure anisotropy of the high-energy particles produces a non-resonant mirror-type instability which amplifies compressible wave modes with wavelengths longer than the gyroradii of the highest energy protons produced by the shock., Comment: Published in Physical Review E, Vol. 95, p. 033207 (2017)

Starburst regions with multiple powerful winds of young massive stars and supernova remnants are favorable sites for high-energy cosmic ray acceleration. A supernova shock colliding with a fast wind from a compact cluster of young stars allows the acceleration of protons to energies well above the standard limits of diffusive shock acceleration in an isolated SN. The proton spectrum in such a wind-supernova PeV accelerator is hard with a large flux in the high-energy-end of the spectrum producing copious gamma-rays and neutrinos in inelastic nuclear collisions. We argue that SN shocks in the Westerlund 1 cluster in the Milky Way may accelerate protons to about 40 PeV. Once accelerated, these CRs will diffuse into surrounding dense clouds and produce neutrinos with fluxes sufficient to explain a fraction of the events detected by IceCube Observatory from the inner Galaxy., Comment: 10 pages, 7 figures, MNRAS v.453, p.113-121, 2015

We present a nonlinear Monte Carlo model of efficient diffusive shock acceleration (DSA) where the magnetic turbulence responsible for particle diffusion is calculated self-consistently from the resonant cosmic-ray (CR) streaming instability, together with non-resonant short- and long-wavelength CR-current-driven instabilities. We include the backpressure from CRs interacting with the strongly amplified magnetic turbulence which decelerates and heats the super-alfvenic flow in the extended shock precursor. Uniquely, in our plane-parallel, steady-state, multi-scale model, the full range of particles, from thermal (~eV) injected at the viscous subshock, to the escape of the highest energy CRs (~PeV) from the shock precursor, are calculated consistently with the shock structure, precursor heating, magnetic field amplification (MFA), and scattering center drift relative to the background plasma. In addition, we show how the cascade of turbulence to shorter wavelengths influences the total shock compression, the downstream proton temperature, the magnetic fluctuation spectra, and accelerated particle spectra. A parameter survey is included where we vary shock parameters, the mode of magnetic turbulence generation, and turbulence cascading. From our survey results, we obtain scaling relations for the maximum particle momentum and amplified magnetic field as functions of shock speed, ambient density, and shock size., Comment: Accepted for publication in ApJ

Energetic nonthermal particles (cosmic rays, CRs) are accelerated in supernova remnants, relativistic jets and other astrophysical objects. The CR energy density is typically comparable with that of the thermal components and magnetic fields. In this review we discuss mechanisms of magnetic field amplification due to instabilities induced by CRs. We derive CR kinetic and magnetohydrodynamic equations that govern cosmic plasma systems comprising the thermal background plasma, comic rays and fluctuating magnetic fields to study CR-driven instabilities. Both resonant and non-resonant instabilities are reviewed, including the Bell short-wavelength instability, and the firehose instability. Special attention is paid to the longwavelength instabilities driven by the CR current and pressure gradient. The helicity production by the CR current-driven instabilities is discussed in connection with the dynamo mechanisms of cosmic magnetic field amplification., Comment: 37 pages, 7 figures, Space Science Reviews (in press)

We discuss processes in galactic cosmic ray (GCR) acceleration sites - supernova remnants, compact associations of young massive stars, and superbubbles. Mechanisms of efficient conversion of the mechanical power of the outflows driven by supernova shocks and fast stellar winds of young stars into magnetic fields and relativistic particles are discussed. The high efficiency of particle acceleration in the sources implies the importance of nonlinear feedback effects in a symbiotic relationship where the magnetic turbulence required to accelerate the CRs is created by the accelerated CRs themselves. Non-thermal emission produced by relativistic particles (both those confined in and those that escape from the cosmic accelerators) can be used to constrain the basic physical models of the GCR sources. High resolution X-ray synchrotron imaging, combined with GeV-TeV gamma ray spectra, is a powerful tool to probe the maximum energies of accelerated particles. Future MeV regime spectroscopy will provide unique information on the composition of accelerated particles., Comment: 10 pages, 5 figures, invited talk at "High Energy Gamma-Ray Astronomy", eds. F.Aharonian, W.Hofmann, F.Rieger

Powerful stellar winds and supernova explosions with intense energy release in the form of strong shock waves can convert a sizeable part of the kinetic energy release into energetic particles. The starforming regions are argued as a favorable site of energetic particle acceleration and could be efficient sources of nonthermal emission. We present here a non-linear time-dependent model of particle acceleration in the vicinity of two closely approaching fast magnetohydrodynamic (MHD) shocks. Such MHD flows are expected to occur in rich young stellar cluster where a supernova is exploding in the vicinity of a strong stellar wind of a nearby massive star. We find that the spectrum of the high energy particles accelerated at the stage of two closely approaching shocks can be harder than that formed at a forward shock of an isolated supernova remnant. The presented method can be applied to model particle acceleration in a variety of systems with colliding MHD flows., Comment: 9 pages, 5 figures, MNRAS in press

We briefly discuss models of energetic particle acceleration by supernova shock in active starforming regions at different stages of their evolution. Strong shocks may strongly amplify magnetic fields due to cosmic ray driven instabilities. We discuss the magnetic field amplification emphasizing the role of the long-wavelength instabilities. Supernova shock propagating in the vicinity of a powerful stellar wind in a young stellar cluster is argued to increase the maximal CR energies at a given evolution stage of supernova remnant (SNR) and can convert a sizeable fraction of the kinetic energy release into energetic particles., Comment: Invited talk at CRISM Montpellier conference, to be published in Memorie della Societ\`a Astronomica Italiana 2011, 6 pages

High-resolution Chandra observations of Tycho's SNR have revealed several sets of quasi-steady, high-emissivity, nearly-parallel X-ray stripes in some localized regions of the SNR. These stripes are most likely the result of cosmic-ray (CR) generated magnetic turbulence at the SNR blast wave. However, for the amazingly regular pattern of these stripes to appear requires the simultaneous action of a number of shock-plasma phenomena and is not predicted by most models of magnetic field amplification. A consistent explanation of these stripes yields information on the complex nonlinear plasma processes connecting efficient CR acceleration and magnetic field fluctuations in strong collisionless shocks. The nonlinear diffusive shock acceleration (NL-DSA) model described here, which includes magnetic field amplification from a cosmic-ray current driven instability, does predict stripes consistent with the synchrotron emission observations of Tycho's SNR. We argue that the local ambient mean magnetic field geometry determines the orientation of the stripes and therefore it can be reconstructed with the high resolution X-ray imaging. The estimated maximum energy of the CR protons responsible for the stripes is $\sim 10^{15}$\,eV. Furthermore, the model predicts that a specific X-ray polarization pattern, with a polarized fraction ~ 50%, accompanies the stripes, which can be tested with future X-ray polarimeter missions., Comment: Astrophysical J. Letter in press, 5 pages, 2 Figures

In order for diffusive shock acceleration (DSA) to accelerate particles to high energies, the energetic particles must be able to interact with magnetic turbulence over a broad wavelength range. The weakly anisotropic distribution of accelerated particles, i.e., cosmic rays (CRs), is believed capable of producing this turbulence in a symbiotic relationship where the magnetic turbulence required to accelerate the CRs is created by the accelerated CRs themselves. In efficient DSA, this wave-particle interaction can be strongly nonlinear where CRs modify the plasma flow and the specific mechanisms of magnetic field amplification. Resonant interactions have long been known to amplify magnetic fluctuations on the scale of the CR gyroradius, and Bell (2004) showed that the CR current can efficiently amplify magnetic fluctuations with scales smaller than the CR gyroradius. Here, we show with a multi-scale, quasi-linear analysis that the presence of turbulence with scales shorter than the CR gyroradius enhances the growth of modes with scales longer than the gyroradius, at least for particular polarizations. We use a mean-field approach to average the equation of motion and the induction equation over the ensemble of magnetic field oscillations accounting for the anisotropy of relativistic particles on the background plasma. We derive the response of the magnetized CR current on magnetic field fluctuations and show that, in the presence of short-scale, Bell-type turbulence, long wavelength modes are amplified. The polarization, helicity, and angular dependence of the growth rates are calculated for obliquely propagating modes for wavelengths both below and above the CR mean free path. The long wavelength growth rates we estimate for typical supernova remnant parameters are sufficiently fast to suggest a fundamental increase in the maximum CR energy a given shock can produce., Comment: MNRAS in press