Current Problems of Modern Stellar Astronomy and Main Research Results

2019

ApJ

Precise measurements of galactic cosmic rays revealed a significant difference between the rigidity spectral indices of protons and helium ions. This finding is a notable contrast to the commonly accepted theoretical prediction that supernova remnant (SNR) shocks accelerate protons and helium ions with the same rigidity alike. Most of the earlier explanations for the "paradox" appealed to SNR environmental factors, such as inhomogeneous p/He mixes in the shock upstream medium, variable ionization states of He, or a multi-SNR origin of the observed spectra. The newest observations, however, are in tension with most of them. In this paper, we show by self-consistent hybrid simulations that such special conditions are not vital for the explanation of the cosmic ray rigidity spectra. In particular, our simulations prove that an SNR shock can modify the chemical composition of accelerated cosmic rays by preferentially extracting them from a homogeneous background plasma without additional, largely untestable assumptions. Our results confirm the earlier theoretical predictions of how the efficiency of injection depends on the shock Mach number M. Its increase with the charge-to-mass ratio saturates at a level that grows with M. We have convolved the time-dependent injection rates of protons and helium ions, obtained from the simulations, with a decreasing shock strength over the active life of SNRs. The integrated SNR rigidity spectrum for p/He ratio compares well with the AMS-02 and PAMELA data.

Section: Introductionmentioning

confidence: 99%

Acceleration of Cosmic Rays in Supernova Shocks: Elemental Selectivity of the Injection Mechanism

Hanusch

Liseykina

2019

ApJ

“…The proton injection is only efficient at quasi-parallel shocks. This follows from simple kinematic considerations (Malkov & Völk 1995) and is supported by Monte-Carlo (Ellison et al 1995) and hybrid simulations (Caprioli & Spitkovsky 2014). The angle beyond which the proton injection drops fast is close to θ cr = π/4.…”

Section: Simulation Resultsmentioning

confidence: 61%

Steepening of Cosmic-Ray Spectra in Shocks with Varying Magnetic Field Direction

Hanusch

Liseykina

et al. 2019

ApJ

Cosmic ray (CR) spectra, both measured upon their arrival at the Earth's atmosphere and inferred from the emission in supernova remnants (SNRs), appear to be significantly steeper than the "standard" diffusive shock acceleration (DSA) theory predicts. Although the reconstruction of the primary spectra introduces an additional steepening due to propagation effects, there is a growing consensus in the CR community that these corrections fall short to explain the newest high-precision data. Using 2D hybrid simulations, we investigate a new mechanism that may steepen the spectrum during the acceleration in SNR shocks.Most of the DSA treatments are limited to homogeneous shock environments. To investigate whether inhomogeneity effects can produce the necessary extra steepening, we assume that the magnetic field changes its angle along the shock front. The rationale behind this approach is the strong dependence of the DSA efficiency upon the field angle, θ Bn . Our results show that the variation of shock obliquity along its face results in a noticeable steepening of the DSA spectrum. Compared to simulations of quasi-parallel shocks, we observe an increase of the spectral index by ∆q = 0.1 − 0.15. Possible extrapolation of the limited simulation results to more realistic SNR conditions are briefly considered.

“…The critical angle ϑ nB = ϑ cr , beyond which the proton injection rate into the DSA drops sharply, is close to ϑ cr π/4. This choice of the source for particle injection is supported by simple theoretical considerations (Malkov & Völk 1995;Völk et al 2003), as well as Monte-Carlo (Ellison et al 1995) and hybrid simulations, e.g., (Thomas & Winske 1990;Caprioli & Spitkovsky 2014). A simple explanation is that for larger ϑ nB the recession of the field line -shock intersection point is too fast for the downstream particles to return upstream and continue acceleration.…”

Section: Particle Acceleration Domain On the Shock Surfacementioning

confidence: 97%

Cosmic-ray Spectrum Steepening in Supernova Remnants. I. Loss-free Self-similar Solution

Aharonian

2019

ApJ

The direct measurements of cosmic rays (CRs), after correction for the propagation effects in the interstellar medium, indicate that their source spectra are likely to be significantly steeper than the canonical E −2 spectrum predicted by the standard Diffusive Shock Acceleration (DSA) mechanism. The DSA has long been held responsible for the production of galactic CRs in supernova remnant (SNR) shocks. The γ-ray "probes" of the acceleration spectra of CRs on-the-spot, inside of the SNRs, lead to the same conclusion. We show that the steep acceleration spectrum can be attributed to the combination of (i) spherical expansion, (ii) tilting of the magnetic field along the shock surface and (iii) shock deceleration. Because of (i) and (ii), the DSA is efficient only on two "polar caps" of a spherical shock where the local magnetic field is within 45 • to its normal. The shock-produced spectrum observed edge-on steepens with the particle energy because the number of freshly accelerated particles with lower energies continually adds up to a growing acceleration region. We demonstrate the steepening effect by obtaining an exact self-similar solution for the particle acceleration at expanding shock surface with an arbitrary energy dependence of particle diffusivity κ. We show that its increase toward higher energy steepens the spectrum, which deeply contrasts with the standard DSA spectrum where κ cancels out.