Cosmic Rays X. The cosmic ray knee and beyond: Diffusive acceleration at oblique shocks

Our purpose is to evaluate the rate of the maximum energy and the acceleration rate that cosmic rays acquire in the non-relativistic diffusive shock acceleration as it could apply during their lifetime in various astrophysical sites, where highly oblique shocks exist. We examine numerically (using Monte Carlo simulations) the effect of the diffusion coefficients on the energy gain and the acceleration rate, by testing the role between the obliquity of the magnetic field to the shock normal, and the significance of both perpendicular cross-field diffusion and parallel diffusion coefficients to the acceleration rate. We find (and justify previous analytical work - Jokipii 1987) that in highly oblique shocks the smaller the perpendicular diffusion gets compared to the parallel diffusion coefficient values, the greater the energy gain of the cosmic rays to be obtained. An explanation of the cosmic ray spectrum in high energies, between $10^{15}$eV and about $10^{18}$eV is claimed, as we estimate the upper limit of energy that cosmic rays could gain in plausible astrophysical regimes; interpreted by the scenario of cosmic rays which are injected by three different kind of sources, (a) supernovae which explode into the interstellar medium, (b) Red Supergiants, and (c) Wolf-Rayet stars, where the two latter explode into their pre-supernovae winds.Comment: Accepted in Astronomy and Astrophysics, 9 pages, 8 figures (for the 'Cosmic Rays' series papers

Strong evidences of hadron acceleration in Tycho's Supernova Remnant

Very recent gamma-ray observations of G120.1+1.4 (Tycho's) supernova remnant (SNR) by Fermi-LAT and VERITAS provided new fundamental pieces of information for understanding particle acceleration and non-thermal emission in SNRs. We want to outline a coherent description of Tycho's properties in terms of SNR evolution, shock hydrodynamics and multi-wavelength emission by accounting for particle acceleration at the forward shock via first order Fermi mechanism. We adopt here a quick and reliable semi-analytical approach to non-linear diffusive shock acceleration which includes magnetic field amplification due to resonant streaming instability and the dynamical backreaction on the shock of both cosmic rays (CRs) and self-generated magnetic turbulence. We find that Tycho's forward shock is accelerating protons up to at least 500 TeV, channelling into CRs about the 10 per cent of its kinetic energy. Moreover, the CR-induced streaming instability is consistent with all the observational evidences indicating a very efficient magnetic field amplification (up to ~300 micro Gauss). In such a strong magnetic field the velocity of the Alfv\'en waves scattering CRs in the upstream is expected to be enhanced and to make accelerated particles feel an effective compression factor lower than 4, in turn leading to an energy spectrum steeper than the standard prediction {\propto} E^-2. This latter effect is crucial to explain the GeV-to-TeV gamma-ray spectrum as due to the decay of neutral pions produced in nuclear collisions between accelerated nuclei and the background gas. The self-consistency of such an hadronic scenario, along with the fact that the concurrent leptonic mechanism cannot reproduce both the shape and the normalization of the detected the gamma-ray emission, represents the first clear and direct radiative evidence that hadron acceleration occurs efficiently in young Galactic SNRs.Comment: Minor changes. Accepted for publication in Astronomy & Astrophysic

The Cluster-Merger Shock in 1E 0657-56: Faster than the Speeding Bullet?

Shock waves driven in the intergalactic medium during the merging of galaxy clusters have been observed in X-ray imaging and spectroscopy. Fluid motions inferred from the shock strength and morphology can be compared to the cold dark matter (CDM) distribution inferred from gravitational lensing. A detailed reconstruction of the CDM kinematics, however, must take into account the nontrivial response of the fluid intracluster medium to the collisionless CDM motions. We have carried out two-dimensional simulations of gas dynamics in cluster collisions. We analyze the relative motion of the clusters, the bow shock wave, and the contact discontinuity and relate these to X-ray data. We focus on the "bullet cluster," 1E 0657-56, a near head-on collision of unequal-mass clusters, for which the gas density and temperature jumps across the prominent bow shock imply a high shock velocity 4,700 km/s. The velocity of the fluid shock has been widely interpreted as the relative velocity of the CDM components. This need not be the case, however. An illustrative simulation finds that the present relative velocity of the CDM halos is 16% lower than that of the shock. While this conclusion is sensitive to the detailed initial mass and gas density profile of the colliding clusters, such a decrease of the inferred halo relative velocity would increase the likelihood of finding 1E 0657-56 in a LambdaCDM universe.Comment: 4 pages, 4 figure

Monte Carlo simulations of a diffusive shock with multiple scattering angular distributions

We independently develop a simulation code following the previous dynamical Monte Carlo simulation of the diffusive shock acceleration under the isotropic scattering law during the scattering process, and the same results are obtained. Since the same results test the validity of the dynamical Monte Carlo method for simulating a collisionless shock, we extend the simulation toward including an anisotropic scattering law for further developing this dynamical Monte Carlo simulation. Under this extended anisotropic scattering law, a Gaussian distribution function is used to describe the variation of scattering angles in the particle's local frame. As a result, we obtain a series of different shock structures and evolutions in terms of the standard deviation values of the given Gaussian scattering angular distributions. We find that the total energy spectral index increases as the standard deviation value of the scattering angular distribution increases, but the subshock's energy spectral index decreases as the standard deviation value of the scattering angular distribution increases.Comment: This article include 10 pages, 8 figures, and accepted by Astronomy and Astrophysic

Analysis of Temporal Features of Gamma Ray Bursts in the Internal Shock Model

In a recent paper we have calculated the power density spectrum of Gamma-Ray Bursts arising from multiple shocks in a relativistic wind. The wind optical thickness is one of the factors to which the power spectrum is most sensitive, therefore we have further developed our model by taking into account the photon down-scattering on the cold electrons in the wind. For an almost optically thick wind we identify a combination of ejection features and wind parameters that yield bursts with an average power spectrum in agreement with the observations, and with an efficiency of converting the wind kinetic energy in 50-300 keV emission of order 1%. For the same set of model features the interval time between peaks and pulse fluences have distributions consistent with the log-normal distribution observed in real bursts.Comment: ApJ in press, 2000; with slight revisions; 12 pag, 6 fi