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Jacek Niemiec

Publications and source records attributed to Jacek Niemiec.

45 records · Page 3Linked to original sources
arXiv2007

Magnetic turbulence production by streaming cosmic rays upstream of SNR shocks

We present preliminary results of Particle-In-Cell simulations of magnetic turbulence production by isotropic cosmic-ray ions streaming upstream of supernova remnant shocks. The studies aim at testing the MHD predictions by Bell (2004, 2005) of a strong amplification of short-wavelength nonresonant wave modes and at studying the subsequent evolution of the magnetic turbulence and its backreaction on cosmic ray trajectories. The detailed knowledge of the upstream turbulence properties is crucial to ascertain all aspects of the shock acceleration process - the transport properties of cosmic rays, the shock structure, thermal particle injection and heating processes. An amplification of magnetic field would also facilitate the acceleration of particles beyond the "knee" in the cosmic-ray spectrum. Our kinetic approach is particularly suited to address the backreaction on the cosmic rays, and it allows us to test Bell's prediction of the eventual formation of extended filamentary structure in the cosmic-ray distribution and also to arrive at a reliable estimate of the total saturation magnetic-field level. The parameters chosen for the simulations are favorable for the rapid excitation of purely growing modes. We confirm the generation of the turbulent magnetic field due to the drift of cosmic-ray ions in the upstream plasma, but show that the growth rate of the field perturbations is much slower than estimated using the MHD approach and the amplitude of the turbulence saturates at about dB/B~1. The magnetic field also remains below equipartition with the upstream plasma.

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arXiv2006

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.

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arXiv2005

Cosmic-ray Acceleration at Ultrarelativistic Shock Waves: Effects of a "Realistic" Magnetic Field Structure

First-order Fermi acceleration processes at ultrarelativistic shocks are studied with Monte Carlo simulations. The accelerated particle spectra are derived by integrating the exact particle trajectories in a turbulent magnetic field near the shock. ''Realistic'' features of the field structure are included. We show that the main acceleration process at superluminal shocks is the particle compression at the shock. Formation of energetic spectral tails is possible in a limited energy range only for highly perturbed magnetic fields, with cutoffs occuring at low energies within the resonance energy range considered. These spectral features result from the anisotropic character of particle transport in the downstream magnetic field, where field compression produces effectively 2D perturbations. Because of the downstream field compression, the acceleration process is inefficient in parallel shocks for larger turbulence amplitudes, and features observed in oblique shocks are recovered. For small-amplitude turbulence, wide-energy range particle spectra are formed and modifications of the process due to the existence of long-wave perturbations are observed. In both sub- and superluminal shocks, an increase of γleads to steeper spectra with lower cut-off energies. The spectra obtained for the ``realistic'' background conditions assumed here do not converge to the ``universal'' spectral index claimed in the literature. Thus the role of the first-order Fermi process in astrophysical sources hosting relativistic shocks requires serious reanalysis.

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arXiv2005

Particle Acceleration at High-$γ$ Shock Waves

First-order Fermi acceleration processes at ultrarelativistic shocks are studied with Monte Carlo simulations. The accelerated particle spectra are obtained by integrating the exact particle trajectories in a turbulent magnetic field near the shock, with a few ``realistic'' features of the field structure included. We show that the main acceleration process at oblique shocks is the particle compression at the shock. Formation of energetic spectral tails is possible in a limited energy range for highly perturbed magnetic fields. Cut-offs in the spectra occur at low energies in the resonance range considered. We relate this feature to the structure of the magnetic field downstream of the shock, where field compression produces effectively 2D turbulence in which cross-field diffusion is very small. Because of the field compression downstream, the acceleration process is inefficient also in parallel high-$γ$ shocks for larger turbulence amplitudes, and features observed in oblique shocks are recovered. For small-amplitude perturbations, particle spectra are formed in a wide energy range and modifications of the acceleration process due to the existence of long-wave perturbations are observed. The critical turbulence amplitude for efficient acceleration at parallel shocks decreases with shock Lorentz factor. We also study the influence of strong short-wave perturbations downstream of the shock on the particle acceleration processes. The spectral indices obtained do not converge to the ``universal'' value . Our results indicate inefficiency of the first-order Fermi process to generate high-energy cosmic rays at ultrarelativistic shocks with the considered perturbed magnetic field structures.

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arXiv2005

Radiation spectra and polarization in magnetar bursts

We present Monte Carlo simulations of radiative transfer in magnetar atmospheres. We include the effects of vacuum polarization, electron and proton scattering, and free-free absorption. Simulations are performed for the atmosphere model with the magnetic field perpendicular and also tilted with respect to the neutron star surface, and we show that the average spectrum does not strongly depend on the orientation of the magnetic field. We investigate the region of the parameter space where the vacuum absorption-like feature appears in the spectrum and we analyze the shape of the proton cyclotron line. Our results indicate that the existence of the vacuum polarization feature should be a general attribute of soft gamma-ray repeaters burst spectra, provided that the energy release takes place at the sufficiently dense region, and the atmosphere scaleheight is large enough. We discuss the existence of such a feature in recent observational data on these sources.

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arXiv2004

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.

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arXiv2003

First-order Fermi Particle Acceleration at Relativistic Shock Waves with a 'Realistic' Magnetic Field Turbulence Model

First-order Fermi acceleration process at a relativistic shock wave is investigated by means of Monte Carlo simulations involving 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 and finite-amplitude perturbations imposed upon it. The perturbations are assumed to be static in the local plasma rest frame. We apply an analytic model for the turbulence with a flat or a Kolmogorov spectrum within a finite (wide) wave vector range. The magnetic field is continuous across the shock -- the downstream field structure is derived from the upstream one from the hydrodynamical shock jump conditions. We present and discuss the obtained particle spectra and angular distributions at mildly relativistic sub- and superluminal shocks. We show that particle spectra diverge from a simple power-law, an exact shape of the spectrum depends on both an amplitude of the magnetic field perturbations and the considered wave power spectrum.

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arXiv2003

Cosmic ray acceleration at parallel relativistic shocks in the presence of finite-amplitude magnetic field perturbations

We study the first-order Fermi acceleration process at parallel shock waves by means of Monte Carlo simulations. A 'realistic' model of the magnetic field turbulence is applied involving sinusoidal waves imposed on the background mean field parallel to the shock normal. The finite-amplitude magnetic field perturbations lead to locally oblique field configurations at the shock and the respective magnetic field compression. It results in modification of the particle acceleration process introducing some features observed in oblique shocks, e.g. particle reflections from the shock. For the parallel mildly relativistic shocks we demonstrate for the first time a (non-monotonic) variation of the accelerated particle spectral index with the turbulence amplitude.

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