Plasma-Hybridsimulationen von Stoßwellenvorläufern aus Supernovaüberresten
Zusammenfassung der Projektergebnisse
Cosmic rays (CRs) have pronounced effects in the galaxy and provide an appreciable fraction of the human radiation doses at the surface of the Earth. Since Viktor Hess’ historic balloon ascent in 1911 the problem of the CRs origin has pondered physicists. In 1934 two astronomers Walter Baade and Fritz Zwicky knitted the extreme astrophysical events, which they named ”supernova”, with the origin of CRs. The latest direct observations of galactic supernova remnants (SNRs) support a hypothesis that those are the objects where most of the galactic CRs come from. Turning to the leading mechanism of their production, observations favor the diffusive shock acceleration (DSA), a development of the idea proposed suggested by Enrico Fermi in 1949. Modern revolutionary improved observations use the combination of different methods, such as time-of-flight and calorimetric measurements, the deflection of charged particles in a magnetic field, and are able to evaluate the particle spectra with extreme accuracy. For the galactic CRs in the 1 − 500 GeV energy range they have revealed striking deviations from the DSA predictions. Such findings challenge the hypothesis of CR origin in SNRs and raise doubts regarding the DSA as a viable explanation of the observed spectra. PAMELA and AMS-02 measurements of galactic CRs firmly established a significant difference between the proton and helium rigidity spectra, deemed inconsistent with the theoretical predictions, that SNR shocks accelerate protons and helium ions with the same ultrarelativistic rigidity alike. The hybrid simulations focused on the injection phase of the DSA, where elemental similarity does not apply, and analytic modeling, allowed us to demonstrate that the high-precision measurements of elemental spectra with different mass-tocharge (A/Z) ratios are not only consistent with the hypothesis of CR origin in the SNR, but strongly support it. Specifically, our results prove that a SNR shock can modify the chemical composition of accelerated CRs by preferentially extracting them from a homogeneous background plasma without additional untestable assumptions. Our simulations correctly predict the decrease in p/He flux ratio with increasing rigidity at almost exactly the rate ∆q≈0.1 measured for the rigidity range > 10 GV. Another serious challenge for the theory is the significantly steeper, with respect to the DSA predictions, spectra of CRs, both measured upon their arrival at the Earth’s atmosphere and inferred from the emission in SNRs. There is a growing consensus in the CR community that the corrections to the spectra due to propagation effects, fall short to explain the newest high-precision data. Most of the DSA treatments are limited to homogeneous shock environments. To investigate in 2D hybrid simulations whether inhomogeneity can produce the necessary extra steepening, we assume that the magnetic field changes its angle along the shock front and study the influence of a variable orientation of magnetic field on ion acceleration. However limited in resources, our simulations have captured a new physical phenomenon in the DSA – the spectrum steepening associated with the variation of shock obliquity along its face. Compared to the case of quasi-parallel shocks, we observe the increase of the spectral index by ∆q = 0.1 − 0.15. While the spectrum steepening observed in our simulations may be regarded as relatively small, it is likely to be scalable to larger boxes and longer simulation time, so that larger ∆q values is expected for realistic SNR conditions.
Projektbezogene Publikationen (Auswahl)
- Anomalies in cosmic ray composition: Explanation based on mass to charge ratio, 35th International Cosmic Ray Conference; Proceedings of Science 301, PoS(ICRC2017)260 (2017)
A. Hanusch, T. Liseykina, M. Malkov
(Siehe online unter https://doi.org/10.22323/1.301.0260) - Mass to charge dependence of particle injection into DSA, 45th EPS Conference on Plasma Physics, 1392-1395 (2018)
A. Hanusch, T. Liseykina, M. Malkov
- Acceleration of Cosmic Rays in Supernova Shocks: Elemental Selectivity of the Injection Mechanism, Astrophys. J. 872, 108 (2019)
A. Hanusch, T. Liseykina, M. Malkov
(Siehe online unter https://doi.org/10.3847/1538-4357/aafdae) - Steepening of Cosmic-Ray Spectra in Shocks with Varying Magnetic Field Direction, Astrophys. J. 885, 11 (2019)
A. Hanusch, T. Liseykina, M. Malkov, F. Aharonian
(Siehe online unter https://doi.org/10.3847/1538-4357/ab426d)
