TY - JOUR A1 - Meyer, Dominique M.-A. A1 - Petrov, Mykola A1 - Pohl, Martin T1 - Wind nebulae and supernova remnants of very massive stars JF - Monthly notices of the Royal Astronomical Society N2 - A very small fraction of (runaway) massive stars have masses exceeding 60-70 M-circle dot and are predicted to evolve as luminous blue variable and Wolf-Rayet stars before ending their lives as core-collapse supernovae. Our 2D axisymmetric hydrodynamical simulations explore how a fast wind (2000 km s(-1)) and high mass-loss rate (10(-5)M(circle dot) yr(-1)) can impact the morphology of the circumstellar medium. It is shaped as 100 pc-scale wind nebula that can be pierced by the driving star when it supersonically moves with velocity 20-40 km s(-1) through the interstellar medium (ISM) in the Galactic plane. The motion of such runaway stars displaces the position of the supernova explosion out of their bow shock nebula, imposing asymmetries to the eventual shock wave expansion and engendering Cygnus-loop-like supernova remnants. We conclude that the size (up to more than 200 pc) of the filamentary wind cavity in which the chemically enriched supernova ejecta expand, mixing efficiently the wind and ISM materials by at least 10 per cent in number density, can be used as a tracer of the runaway nature of the very massive progenitors of such 0.1Myr old remnants. Our results motivate further observational campaigns devoted to the bow shock of the very massive stars BD+43 degrees 3654 and to the close surroundings of the synchrotron-emitting Wolf-Rayet shell G2.4+1.4. KW - shock waves KW - methods: numerical KW - circumstellar matter KW - stars: massive Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa554 SN - 0035-8711 SN - 1365-2966 VL - 493 IS - 3 SP - 3548 EP - 3564 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Wilhelm, Alina A1 - Telezhinsky, Igor A1 - Dwarkadas, Vikram V. A1 - Pohl, Martin T1 - Stochastic re-acceleration and magnetic-field damping in Tycho’s supernova remnant JF - Astronomy and astrophysics N2 - Context. Tycho's supernova remnant (SNR) is associated with the historical supernova (SN) event SN 1572 of Type Ia. The explosion occurred in a relatively clean environment, and was visually observed, providing an age estimate. This SNR therefore represents an ideal astrophysical test-bed for the study of cosmic-ray acceleration and related phenomena. A number of studies suggest that shock acceleration with particle feedback and very efficient magnetic-field amplification combined with Alfvenic drift are needed to explain the rather soft radio spectrum and the narrow rims observed in X-rays. Aims. We show that the broadband spectrum of Tycho's SNR can alternatively be well explained when accounting for stochastic acceleration as a secondary process. The re-acceleration of particles in the turbulent region immediately downstream of the shock should be efficient enough to impact particle spectra over several decades in energy. The so-called Alfvenic drift and particle feedback on the shock structure are not required in this scenario. Additionally, we investigate whether synchrotron losses or magnetic-field damping play a more profound role in the formation of the non-thermal filaments. Methods. We solved the full particle transport equation in test-particle mode using hydrodynamic simulations of the SNR plasma flow. The background magnetic field was either computed from the induction equation or follows analytic profiles, depending on the model considered. Fast-mode waves in the downstream region provide the diffusion of particles in momentum space. Results. We show that the broadband spectrum of Tycho can be well explained if magnetic-field damping and stochastic re-acceleration of particles are taken into account. Although not as efficient as standard diffusive shock acceleration, stochastic acceleration leaves its imprint on the particle spectra, which is especially notable in the emission at radio wavelengths. We find a lower limit for the post-shock magnetic-field strength similar to 330 mu G, implying efficient amplification even for the magnetic-field damping scenario. Magnetic-field damping is necessary for the formation of the filaments in the radio range, while the X-ray filaments are shaped by both the synchrotron losses and magnetic-field damping. KW - acceleration of particles KW - radiation mechanisms: non-thermal KW - ISM: supernova remnants KW - cosmic rays KW - ISM: individual objects: Tycho's SNR KW - shock waves Y1 - 2020 U6 - https://doi.org/10.1051/0004-6361/201936079 SN - 0004-6361 SN - 1432-0746 VL - 639 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Warren, Donald C. A1 - Ellison, Donald C. A1 - Barkov, Maxim V. A1 - Nagataki, Shigehiro T1 - Nonlinear Particle Acceleration and Thermal Particles in GRB Afterglows JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The standard model for GRB afterglow emission treats the accelerated electron population as a simple power law, N(E) proportional to E-p for p greater than or similar to 2. However, in standard Fermi shock acceleration, a substantial fraction of the swept-up particles do not enter the acceleration process at all. Additionally, if acceleration is efficient, then the nonlinear back-reaction of accelerated particles on the shock structure modifies the shape of the nonthermal tail of the particle spectra. Both of these modifications to the standard synchrotron afterglow impact the luminosity, spectra, and temporal variation of the afterglow. To examine the effects of including thermal particles and nonlinear particle acceleration on afterglow emission, we follow a hydrodynamical model for an afterglow jet and simulate acceleration at numerous points during the evolution. When thermal particles are included, we find that the electron population is at no time well fitted by a single power law, though the highest-energy electrons are; if the acceleration is efficient, then the power-law region is even smaller. Our model predicts hard-soft-hard spectral evolution at X-ray energies, as well as an uncoupled X-ray and optical light curve. Additionally, we show that including emission from thermal particles has drastic effects (increases by factors of 100 and 30, respectively) on the observed flux at optical and GeV energies. This enhancement of GeV emission makes afterglow detections by future gamma-ray observatories, such as CTA, very likely. KW - acceleration of particles KW - cosmic rays KW - gamma-ray burst: general KW - shock waves KW - turbulence Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa56c3 SN - 0004-637X SN - 1538-4357 VL - 835 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Petruk, Oleh A1 - Kuzyo, T. A1 - Orlando, S. A1 - Pohl, Martin A1 - Miceli, M. A1 - Bocchino, F. A1 - Beshley, V. A1 - Brose, Robert T1 - Post-adiabatic supernova remnants in an interstellar magnetic field BT - oblique shocks and non-uniform environment JF - Monthly notices of the Royal Astronomical Society N2 - We present very-high-resolution 1D MHD simulations of the late-stage supernova remnants (SNRs). In the post-adiabatic stage, the magnetic field has an important and significant dynamical effect on the shock dynamics, the flow structure, and hence the acceleration and emission of cosmic rays. We find that the tangential component of the magnetic field provides pressure support that to a fair degree prevents the collapse of the radiative shell and thus limits the total compression ratio of the partially or fully radiative forward shock. A consequence is that the spectra of cosmic rays would not be as hard as in hydrodynamic simulations. We also investigated the effect on the flow profiles of the magnetic-field inclination and a large-scale gradient in the gas density and/or the magnetic field. A positive density gradient shortens the evolutionary stages, whereas a shock obliquity lowers the shock compression. The compression of the tangential component of the magnetic field leads to its dominance in the downstream region of post-adiabatic shocks for a wide range of orientation of the upstream field, which may explain why one preferentially observes tangential radio polarization in old SNRs. As most cosmic rays are produced at late stages of SNR evolution, the post-adiabatic phase and the influence of the magnetic field during it are most important for modeling the cosmic-ray acceleration at old SNRs and the gamma-ray emission from late-stage SNRs interacting with clouds. KW - shock waves KW - ISM: magnetic fields KW - ISM: supernova remnants Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1750 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 4253 EP - 4270 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Sushch, Iurii A1 - Brose, Robert A1 - Pohl, Martin T1 - Modeling of the spatially resolved nonthermal emission from the Vela Jr. supernova remnant JF - Astronomy and astrophysics : an international weekly journal N2 - Vela Jr. (RX J0852.0-4622) is one of just a few known supernova remnants (SNRs) with a resolved shell across the whole electromagnetic spectrum from radio to very-high-energy (>100 GeV; VHE) gamma-rays. Its proximity and large size allow for detailed spatially resolved observations of the source, making Vela Jr. one of the primary sources used for the study of particle acceleration and emission mechanisms in SNRs. High-resolution X-ray observations reveal a steepening of the spectrum toward the interior of the remnant. In this study we aim for a self-consistent radiation model of Vela Jr. which at the same time would explain the broadband emission from the source and its intensity distribution. We solve the full particle transport equation combined with the high-resolution one-dimensional (1D) hydrodynamic simulations (using Pluto code) and subsequently calculate the radiation from the remnant. The equations are solved in the test particle regime. We test two models for the magnetic field profile downstream of the shock: damped magnetic field, which accounts for the damping of strong magnetic turbulence downstream, and transported magnetic field. Neither of these scenarios can fully explain the observed radial dependence of the X-ray spectrum under spherical symmetry. We show, however, that the softening of the spectrum and the X-ray intensity profile can be explained under the assumption that the emission is enhanced within a cone. KW - radiation mechanisms: non-thermal KW - acceleration of particles KW - cosmic rays KW - ISM: supernova remnants KW - X-rays: individuals: Vela Jr (RX J08520-4622) KW - shock waves Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201832879 SN - 1432-0746 SN - 0004-6361 VL - 618 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Petruk, Oleh A1 - Kuzyo, T. A1 - Orlando, S. A1 - Pohl, Martin A1 - Miceli, M. A1 - Bocchino, F. A1 - Beshley, V. A1 - Brose, Robert T1 - Erratum: Post-adiabatic supernova remnants in an interstellar magnetic field: oblique shocks and non-uniform environment. - (Monthly notices of the Royal Astronomical Society. - 479, (2018), pg. 4253 - 4270) T2 - Monthly notices of the Royal Astronomical Society N2 - This is a correction notice for ‘Post-adiabatic supernova remnants in an interstellar magnetic field: oblique shocks and non-uniform environment’ (DOI: https://doi.org/10.1093/mnras/sty1750), which was published in MNRAS 479, 4253–4270 (2018). The publisher regrets to inform that the colour was missing from the colour scales in Figs 8(a)–(d) and Figs 9(a) and (b). This has now been corrected online. The publisher apologizes for this error. KW - errata KW - addenda KW - shock waves KW - ISM: magnetic fields KW - ISM: supernova remnants Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty2861 SN - 0035-8711 SN - 1365-2966 VL - 482 IS - 2 SP - 1979 EP - 1980 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Matsumoto, Yosuke A1 - Amano, Takanobu A1 - Hoshino, Masahiro T1 - Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants BT - I. Electron Shock-surfing Acceleration JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Electron injection at high Mach number nonrelativistic perpendicular shocks is studied here for parameters that are applicable to young SNR shocks. Using high-resolution large-scale two-dimensional fully kinetic particle-in-cell simulations and tracing individual particles, we in detail analyze the shock-surfing acceleration (SSA) of electrons at the leading edge of the shock foot. The central question is to what degree the process can be captured in 2D3V simulations. We find that the energy gain in SSA always arises from the electrostatic field of a Buneman wave. Electron energization is more efficient in the out-of-plane orientation of the large-scale magnetic field because both the phase speed and the amplitude of the waves are higher than for the in-plane scenario. Also, a larger number of electrons is trapped by the waves compared to the in-plane configuration. We conclude that significant modifications of the simulation parameters are needed to reach the same level of SSA efficiency as in simulations with out-of-plane magnetic field or 3D simulations. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock waves Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab1b6d SN - 0004-637X SN - 1538-4357 VL - 878 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kobzar, Oleh A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Bohdan, Artem T1 - Spatio-temporal evolution of the non-resonant instability in shock precursors of young supernova remnants JF - Monthly notices of the Royal Astronomical Society N2 - A non-resonant cosmic ray (CR) 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 CRs 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 non-resonant instability inelastically scatters CRs, modifying and anisotropizing their energy distribution. Spatial CR scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads to significant non-adiabatic 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. KW - acceleration of particles KW - shock waves KW - turbulence KW - methods: numerical KW - cosmic rays KW - ISM: supernova remnants Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx1201 SN - 0035-8711 SN - 1365-2966 VL - 469 SP - 4985 EP - 4998 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Fraschetti, F. A1 - Pohl, Martin T1 - Particle acceleration model for the broad-band baseline spectrum of the Crab nebula JF - Monthly notices of the Royal Astronomical Society N2 - We develop a simple one-zone model of the steady-state Crab nebula spectrum encompassing both the radio/soft X-ray and the GeV/multi-TeV observations. By solving the transport equation for GeV-TeV electrons injected at the wind termination shock as a log-parabola momentum distribution and evolved via energy losses, we determine analytically the resulting differential energy spectrum of photons. We find an impressive agreement with the observed spectrum of synchrotron emission, and the synchrotron self-Compton component reproduces the previously unexplained broad 200-GeV peak that matches the Fermi/Large Area Telescope (LAT) data beyond 1 GeV with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) data. We determine the parameters of the single log-parabola electron injection distribution, in contrast with multiple broken power-law electron spectra proposed in the literature. The resulting photon differential spectrum provides a natural interpretation of the deviation from power law customarily fitted with empirical multiple broken power laws. Our model can be applied to the radio-to-multi-TeV spectrum of a variety of astrophysical outflows, including pulsar wind nebulae and supernova remnants, as well as to interplanetary shocks. KW - acceleration of particles KW - shock waves KW - cosmic rays KW - ISM: supernova remnants Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx1833 SN - 0035-8711 SN - 1365-2966 VL - 471 SP - 4866 EP - 4874 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Mizuno, Yosuke A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Zhang, Bing A1 - Nishikawa, Ken-Ichi A1 - Hardee, Philip E. T1 - Magnetic field amplification and saturation in turbulence behind a relativistic shock JF - Monthly notices of the Royal Astronomical Society N2 - We have investigated via 2D relativistic magnetohydrodynamic simulations the long-term evolution of turbulence created by a relativistic shock propagating through an inhomogeneous medium. In the post-shock region, magnetic field is strongly amplified by turbulent motions triggered by pre-shock density inhomogeneities. Using a long-simulation box we have followed the magnetic field amplification until it is fully developed and saturated. The turbulent velocity is subrelativistic even for a strong shock. Magnetic field amplification is controlled by the turbulent motion and saturation occurs when the magnetic energy is comparable to the turbulent kinetic energy. Magnetic field amplification and saturation depend on the initial strength and direction of the magnetic field in the pre-shock medium, and on the shock strength. If the initial magnetic field is perpendicular to the shock normal, the magnetic field is first compressed at the shock and then can be amplified by turbulent motion in the post-shock region. Saturation occurs when the magnetic energy becomes comparable to the turbulent kinetic energy in the post-shock region. If the initial magnetic field in the pre-shock medium is strong, the post-shock region becomes turbulent but significant field amplification does not occur. If the magnetic energy after shock compression is larger than the turbulent kinetic energy in the post-shock region, significant field amplification does not occur. We discuss possible applications of our results to gamma-ray bursts and active galactic nuclei. KW - MHD KW - relativistic processes KW - shock waves KW - turbulence KW - methods: numerical KW - gamma-ray burst: general Y1 - 2014 U6 - https://doi.org/10.1093/mnras/stu196 SN - 0035-8711 SN - 1365-2966 VL - 439 IS - 4 SP - 3490 EP - 3503 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Mizuno, Yosuke A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Zhang, Bing A1 - Nishikawa, Ken-Ichi A1 - Hardee, Philip E. T1 - Magnetic-field amplification by turbulence in a relativistic shockpropagating through an inhomogeneous medium JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We perform two-dimensional relativistic magnetohydrodynamic simulations of a mildly relativistic shock propagating through an inhomogeneous medium. We show that the postshock region becomes turbulent owing to preshock density inhomogeneity, and the magnetic field is strongly amplified due to the stretching and folding of field lines in the turbulent velocity field. The amplified magnetic field evolves into a filamentary structure in two-dimensional simulations. The magnetic energy spectrum is flatter than the Kolmogorov spectrum and indicates that a so-called small-scale dynamo is occurring in the postshock region. We also find that the amount of magnetic-field amplification depends on the direction of the mean preshock magnetic field, and the timescale of magnetic-field growth depends on the shock strength. KW - gamma-ray burst: general KW - magnetohydrodynamics (MHD) KW - methods: numerical KW - relativistic processes KW - shock waves KW - turbulence Y1 - 2011 U6 - https://doi.org/10.1088/0004-637X/726/2/62 SN - 0004-637X VL - 726 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Stroman, Thomas A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Bret, Antoine T1 - Could cosmic rays affect instabilities in the Transition layer of nonrealativistic collisionless shocks? JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - There is an observational correlation between astrophysical shocks and nonthermal particle distributions extending to high energies. As a first step toward investigating the possible feedback of these particles on the shock at the microscopic level, we perform particle-in-cell (PIC) simulations of a simplified environment consisting of uniform, interpenetrating plasmas, both with and without an additional population of cosmic rays. We vary the relative density of the counterstreaming plasmas, the strength of a homogeneous parallel magnetic field, and the energy density in cosmic rays. We compare the early development of the unstable spectrum for selected configurations without cosmic rays to the growth rates predicted from linear theory, for assurance that the system is well represented by the PIC technique. Within the parameter space explored, we do not detect an unambiguous signature of any cosmic-ray-induced effects on the microscopic instabilities that govern the formation of a shock. We demonstrate that an overly coarse distribution of energetic particles can artificially alter the statistical noise that produces the perturbative seeds of instabilities, and that such effects can be mitigated by increasing the density of computational particles. KW - cosmic rays KW - instabilities KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/746/1/24 SN - 0004-637X VL - 746 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Bret, Antoine A1 - Wieland, Volkmar T1 - Nonrelativistic parallel shocks in unmagnetized and weakly magnetized plasmas JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present results of 2D3V particle-in-cell simulations of nonrelativistic plasma collisions with absent or parallel large-scale magnetic field for parameters applicable to the conditions at young supernova remnants. We study the collision of plasma slabs of different density, leading to two different shocks and a contact discontinuity. Electron dynamics play an important role in the development of the system. While nonrelativistic shocks in both unmagnetized and magnetized plasmas can be mediated by Weibel-type instabilities, the efficiency of shock-formation processes is higher when a large-scale magnetic field is present. The electron distributions downstream of the forward and reverse shocks are generally isotropic, whereas that is not always the case for the ions. We do not see any significant evidence of pre-acceleration, neither in the electron population nor in the ion distribution. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/759/1/73 SN - 0004-637X SN - 1538-4357 VL - 759 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER -