arXiv subjects

M. Ostrowski

Publications and source records attributed to M. Ostrowski.

88 records · Page 5Linked to original sources

arXiv2000

On energy spectra of UHE cosmic rays accelerated in supergalactic accretion flows

Some ultra-high energy (UHE) cosmic ray (CR) events may be correlated with the Local Supercluster plane. We consider acceleration of such particles at the large-scale accretion flows of matter towards such plane and/or accretion flows onto galaxy clusters. The formed shocks and the general compression flow are expected to allow for CR diffusive acceleration of UHE particles. For a simplified flow geometry we consider a stationary acceleration of such CRs and we discuss influence of model parameters on a particle spectral index. We show that the general convergent flow pattern leads naturally to very flat proton spectra with the phase-space spectral indices $σ\approx 3.0$, as compared to the `canonical' shock value of 4.0.

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arXiv1999

Energetic particle acceleration in shear layers

A plasma velocity shear layer and/or a tangential flow discontinuity provide conditions allowing for energetic particle acceleration. We review such acceleration processes acting both in non-relativistic and in relativistic flows. In heliospheric conditions shear layers can provide particles with energies compatible with the observed values (from several keV up to MeV), while in relativistic extragalactic jets proton energies even in excess of 10^{19} eV can be obtained. Application of the discussed theory to particular astrophysical objects is severely limited by inadequate knowledge of local physical conditions.

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arXiv1999

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.

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arXiv1999

Efficiency of cosmic ray reflections from an ultrarelativistic shock wave

The process of cosmic ray acceleration up to energies in excess of $10^{20}$ eV at relativistic shock waves with large Lorentz factors, $Γ\gg 1$ requires $\sim Γ^2$ particle energy gains at single reflections from the shock (cf. Gallant & Achterberg 1999). In the present comment, applying numerical simulations we address an efficiency problem arising for such models. The actual efficiency of the acceleration process is expected to be substantially lower than the estimates of previous authors.

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arXiv1999

Cosmic-Ray Momentum Diffusion In Magnetosonic Versus Alfvenic Turbulent Field

Energetic particle transport in a finite amplitude magnetosonic and Alfvenic turbulence is considered using Monte Carlo particle simulations, which involve an integration of particle equation of motion. We show that in a low-Betha plasma cosmic ray can be the most important damping process for magnetosonic waves. Assuming such conditions we derive the momentum diffusion coefficient for relativistic particles in the presence of anisotropic finite-amplitude turbulent wave field, for flat and Kolmogorov-type turbulence spectra. We confirm the possibility of larger values of a momentum diffusion coefficient occuring due to transit-time damping resonance interaction in the presence of isotropic fast-mode waves in comparison to the Alfven waves of the same amplitude.

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arXiv1999

Investigation of galactic alignment in LSC galaxy clusters

We investigate the galactic axes orientations within 18 selected clusters, sub-structures of the Local Supercluster. For every cluster we map the parameter Δ_{11} (Flin & Godłowski 1986) describing the galactic axes alignment with respect to a cluster pole changing along the entire celestial sphere. The resulting maps are analyzed for correlations of its maxima with directions from the cluster centre to 1.) the derived `physical' cluster poles, 2.) the Local Supercluster centre, 3.) the Virgo A centre and 4.) the Earth, i.e. along the line of sight (LOS). The strong maxima - with one exception - exist only for non-spiral (NS) sub-samples, with the maximum well correlated with the LOS direction. For clusters with a clearly defined weak maximum the conclusion generally does not change. For the spiral (S) sub-samples the maps are usually at the random noise level. In these cases a weaker, but still existent correlation with the LOS is observed and no other evident correlations are noted. We conclude that the strong systematic effect, generated by the process of galactic axis de-projection from its optical image, is present in the catalogue data. With the use of a simple model for the systematic effect we are able to reproduce the main characteristic features of the maps for NS galaxies. We note, however, a few clusters showing significant differences with respect to this model.

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arXiv1998

Supernova remnants in molecular clouds: on cosmic ray electron spectra

The particle acceleration process at a shock wave, in the presence of the second-order Fermi acceleration in the turbulent medium near the shock, is discussed as an alternative explanation for the observed flat synchrotron spectra of supernova remnants (SNRs) in molecular clouds. We argue that medium Alfvén Mach number shocks considered by Chevalier (1999, ApJ, in press) for such SNRs can naturally lead to the observed spectral indices.

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arXiv1998

Cosmic Ray Acceleration at Relativistic Plasma Flows

Theoretical concepts ofn cosmic ray particle acceleration at relativistic plasma flows -- shocks and shear layers -- are reviewed. We begin with a discussion of mildly relativistic shock waves. The role of oblique field configurations and field perturbations in forming the particle energy spectrum and changing the acceleration time scale is considered. Then, we report on two interesting attempts to consider particle acceleration at ultra-relativistic shocks. Finally, in contrast to the compressive shock discontinuities, we discuss the acceleration processes acting in the boundary layer at the tangential velocity transition. The second-order Fermi acceleration as well as the cosmic ray `viscous' acceleration provide the mechanisms generating energetic particles there.

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arXiv1998

Energy spectra of cosmic rays accelerated at ultrarelativistic shock waves

Energy spectra of particles accelerated by the first-order Fermi mechanism are investigated at ultrarelativistic shock waves, outside the range of Lorentz factors considered previously. For particle transport near the shock a numerical method involving small amplitude pitch-angle scattering is applied for flows with Lorentz factors $γ$ from 3 to 243. For large $γ$ shocks a convergence of derived energy spectral indices up to the value $σ_\infty \approx 2.2$ is observed for all considered turbulence amplitudes and magnetic field configurations. Recently the same index was derived for $γ$-ray bursts by Waxman [Astrophys. J. Lett. 485, L5 (1997)].

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arXiv1998

Acceleration of UHE Cosmic Ray Particles at Relativistic Jets in Extragalactic Radio Sources

A mechanism of ultra-high energy cosmic ray acceleration in extragalactic radio sources, at the interface between the relativistic jet and the surrounding medium, is discussed as a supplement to the shock acceleration in `hot spots'. Due to crossing the tangential discontinuity of the velocity the particle can gain an amount of energy comparable to the energy gain at the shock crossing. However, the spectrum of particles accelerated at the jet side boundary is expected to be much flatter than the one formed at the shock. Due to this fact, particles accelerated at the boundary can dominate the overall spectrum at highest energies. In conditions characteristic to extragalactic jets' terminal shocks, the mechanism naturally provides the particles with E ~ 10^20 eV and complies with the efficiency requirements. The spectrum formation near the cut-off energy due to action of both the shock acceleration and the tangential discontinuity acceleration is modelled with the Monte Carlo particle simulations. It confirms that the upper energy limit can surpass the shock acceleration estimate.

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arXiv1997

Cosmic Ray Momentum Diffusion in Magnetosonic versus Alfvenic Turbulent Fields

The acceleration of energetic particle transport in high amplitude magnetosonic and Alfvenic turbulence is considered using the method of Monte Carlo particle simulations, involving integration of particle equations of motion. We derive the momentum diffusion coefficient in the presence of anisotropic turbulent wave in the high-magnetized plasma, for a flat and Kolmogorov-type turbulence spectrum. We confirm the quasilinear result (Shlickeiser & Miller (1997) of enhanced values of the momentum diffusion coefficient due to transit-time resonance interaction in the presence of isotropic fast-mode waves as compared to the case of slab Alfvenic waves of the same amplitude. The relation of the momentum diffusion coefficient to the turbulence amplitude and anisotropy is investigated.

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arXiv1996

Diffusion in momentum space as a picture of second-order Fermi acceleration

Energetic particles in a turbulent medium can be subject to second-order Fermi acceleration due to scattering on moving plasma waves. This mechanism leads to growing particle momentum dispersion and, at the same time, increases the mean particle energy. In the most frequently met situations both processes can be represented by a single momentum diffusion term in the particle kinetic equation. In the present paper we discuss the conditions allowing the additional term for regular acceleration to arise. For forward-backward asymmetric scattering centres, besides the diffusive term one should explicitly consider the regular acceleration term in momentum space, which can consist of the first-order ($\propto V$), as well as the second-order ($\propto V^2$) part in the wave velocity V . We derive the condition for the scattering probability in the wave rest frame required for vanishing the regular acceleration term and provide a simple mechanical example illustrating the theoretical concepts. Finally, we address its possible role in cosmic ray acceleration processes.

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arXiv1996

Cosmic Ray Acceleration at Relativistic Shock Waves

Theory of the first-order Fermi acceleration of cosmic ray particles at relativistic shock waves is reviewed. We consider shocks with parallel and oblique, sub- and super-luminal magnetic field configurations and with finite-amplitude magnetic field perturbations. A role of oblique field configurations and field perturbations in forming the cosmic ray energy spectrum and changing the acceleration time scale is discussed.

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arXiv1996

Acceleration of UHE Cosmic Ray Particles at Relativistic Jets in Extragalactic Radio Sources

A mechanism of ultra-high energy (UHE) cosmic ray acceleration in extragalactic radio sources at the interface between the relativistic jet and the ambient medium is discussed as a supplement to the shock acceleration in `hot spots'. Particles accelerated at the jet side boundary are expected to dominate at highest energies. The spectrum formation near the cut-off energy is modeled using the Monte Carlo particle simulations.

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arXiv1996

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, $δB$. We got some interesting results like non-monotonic changes of $T_{acc}^{(c)}$ with $δB$, which arises due to the particle cross-field diffusion.

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arXiv1996

Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries

In the present paper we discuss the modifications introduced into the first-order Fermi shock acceleration process due to a finite extent of diffusive regions near the shock or due to boundary conditions leading to an increased particle escape upstream and/or downstream the shock. In the considered simple example of the planar shock wave we idealize the escape phenomenon by imposing a particle escape boundary at some distance from the shock. Presence of such a boundary (or boundaries) leads to coupled steepening of the accelerated particle spectrum and decreasing of the acceleration time scale. It allows for a semi-quantitative evaluation and, in some specific cases, also for modelling of the observed steep particle spectra as a result of the first-order Fermi shock acceleration. We also note that the particles close to the upper energy cut-off are younger than the estimate based on the respective acceleration time scale. In Appendix A we present a new time-dependent solution for infinite diffusive regions near the shock allowing for different constant diffusion coefficients upstream and downstream the shock.

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