The escape of charged particles accelerated by diffusive shock acceleration from supernova remnants is shown to be a more complex process than normally appreciated. Using a box model it is shown that ...the high-energy end of the spectrum can exhibit spectral breaks even with no formal escape as a result of geometrical dilution and changing time-scales. It is pointed out that the bulk of the cosmic ray particles at lower energies must be produced and released in the late stages of the remnant's evolution, whereas the high-energy particles are produced early on; this may explain recent observations of slight compositional variations with energy. Escape resulting from ion-neutral friction in dense and partially ionized media is discussed briefly and some comments made on the use of the so-called 'free-escape boundary conditions'. Finally, estimates are made of the total production spectrum integrated over the life of the remnant.
Observations of non-thermal emission from several supernova remnants suggest that magnetic fields close to the blastwave are much stronger than would be naively expected from simple shock compression ...of the field permeating the interstellar medium (ISM). We investigate in some detail a simple model based on turbulence generation by cosmic ray pressure gradients. Previously, this model was investigated using 2D magnetohydrodynamic simulations. Motivated by the well-known qualitative differences between 2D and 3D turbulence, we further our investigations of this model using both 2D and 3D simulations to study the influence of the dimensionality of the simulations on the field amplification achieved. Further, since the model implies the formation of shocks which can, in principle, be efficiently cooled by collisional cooling, we include such cooling in our simulations to ascertain whether it could increase the field amplification achieved. Finally, we examine the influence of different orientations of the magnetic field with respect to the normal of the blastwave. We find that dimensionality has a slight influence on the overall amplification achieved, but a significant impact on the morphology of the amplified field. Collisional cooling has surprisingly little impact, primarily due to the short time which any element of the ISM resides in the precursor region for supernova blastwaves. Even allowing for a wide range of orientations of the magnetic field, we find that the magnetic field can be expected to be amplified by, on average, at least an order of magnitude in the precursors of supernova blastwaves.
A box model is used to study first-order Fermi acceleration driven by magnetic reconnection. It is shown, at least in this simple model, that the spectral index of the accelerated particles is ...related to the total compression in the same way as in diffusive shock acceleration and is not, as has been suggested, a universal E
−5/2 spectrum. The acceleration time-scale is estimated and some comments made about the applicability of the process.
Abstract
Observations of non-thermal emission from several supernova remnants suggest that magnetic fields close to the blast wave are much stronger than would be naively expected from simple shock ...compression of the field permeating the interstellar medium (ISM).
We present a simple model which is capable of achieving sufficient magnetic field amplification to explain the observations. We propose that the cosmic ray pressure gradient acting on the inhomogeneous ISM upstream of the supernova blast wave induces strong turbulence upstream of the supernova blast wave. The turbulence is generated through the differential acceleration of the upstream ISM which occurs as a result of density inhomogeneities in the ISM. This turbulence then amplifies the pre-existing magnetic field.
Numerical simulations are presented which demonstrate that amplification factors of 20 or more are easily achievable by this mechanism when reasonable parameters for the ISM and supernova blast wave are assumed. The length scale over which this amplification occurs is that of the diffusion length of the highest energy non-thermal particles.
Both the acceleration of cosmic rays (CRs) in supernova remnant shocks and their subsequent propagation through the random magnetic field of the Galaxy are deemed to result in an almost isotropic CR ...spectrum. However, the MILAGRO TeV observatory discovered sharp (~10°) arrival anisotropies of CR nuclei. We suggest a mechanism for producing a weak and narrow CR beam which operates en route to the observer. The key assumption is that CRs are scattered by a strongly anisotropic Alfvén wave spectrum formed by the turbulent cascade across the local field direction. The strongest pitch-angle scattering occurs for particles moving almost precisely along the field line. Partly because this direction is also the direction of the minimum of the large-scale CR angular distribution, the enhanced scattering results in a weak but narrow particle excess. The width, the fractional excess, and the maximum momentum of the beam are calculated from a systematic transport theory depending on a single scale l which can be associated with the longest Alfvén wave, which efficiently scatters the beam. The best match to all three characteristics of the beam is achieved at l ~ 1 pc. The distance to a possible source of the beam is estimated to be within a few 100 pc. Possible approaches to the determination of the scale l from the characteristics of the source are discussed. Alternative scenarios of drawing the beam from the galactic CR background are considered. Finally, the beam-related large-scale anisotropic CR component is found to be energy independent, which is also consistent with the observations.
Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy γ rays. The central region of the Milky Way halo is one of the most promising ...targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300 GeV to 70 TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004-2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant γ-ray excess above the background is found. We derive upper limits on the annihilation cross section ⟨σv⟩ for monoenergetic DM lines at the level of 4×10^{-28} cm^{3} s^{-1} at 1 TeV, assuming an Einasto DM profile for the Milky Way halo. For a DM mass of 1 TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300 GeV-70 TeV. Ground-based γ-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two γ-ray photons at the level expected from the thermal relic density for TeV DM particles.
Diffusive shock acceleration is the theory of particle acceleration through multiple shock crossings. In order for this process to proceed at a rate that can be reconciled with observations of ...high-energy electrons in the vicinity of the shock, and for cosmic rays protons to be accelerated to energies up to observed galactic values, significant magnetic field amplification is required. In this review we will discuss various theories on how magnetic field amplification can proceed in the presence of a cosmic ray population. On both short and long length scales, cosmic ray streaming can induce instabilities that act to amplify the magnetic field. Developments in this area that have occurred over the past decade are the main focus of this paper.
Multiwavelength observations of supernova remnants can be explained within the framework of the diffusive shock acceleration theory, which allows effective conversion of the explosion energy into ...cosmic rays. Although models of nonlinear shocks describe the nonthermal component of the emission reasonably well, certain issues, including the heating of the thermal plasma and the related X-ray emission, still remain open. We discuss how the evolution and structure of supernova remnants is affected by strong particle acceleration at the forward shock. We use analytical estimates combined with detailed discussion of the physical processes. The overall dynamics is shown to be relatively insensitive to the amount of particle acceleration, but the post-shock gas temperature can be reduced to a small multiple of the ambient temperature (even as low as six times) with a weak dependence on the shock speed. This is in marked contrast to models with no particle acceleration where the post-shock temperature is insensitive to the ambient temperature and is determined by the square of the shock speed. It thus appears to be possible to effectively suppress thermal X-ray emission from remnants by strong particle acceleration. This might provide a clue to understanding the lack of thermal X-rays from the TeV bright supernova remnant RX J1713.7-3946.
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