In cosmic ray physics extensive progress has been made in recent years, both concerning theory and observation. The vast details in direct, indirect and secondary detections on the one hand provide ...the basis for a detailed modeling of the signatures via cosmic-ray transport and interaction, paving the way for the identification of Galactic cosmic-ray sources. On the other hand, the large number of constraints from different channels of cosmic-ray observables challenges these models frequently.
In this review, we will summarize the state-of-the art of the detection of cosmic rays and their secondaries, followed by a discussion what we can learn from coupling our knowledge of the cosmic-ray observables to the theory of cosmic-ray transport in the Galactic magnetic field. Finally, information from neutral secondaries will be added to draw a multimessenger-picture of the non-thermal sky, in which the hypothesis of supernova remnants as the dominant sources survives best. While this has been known since the 1930s, evidence for this scenario is steadily growing, with the first possible detection of hadronic signatures at GeV energies detected for three SNRs with Fermi. The existence of SNRs as PeVatrons, however, is not validated yet. The discussion of this and other open questions concerning the level of anisotropy, composition and spectral shape of the cosmic-ray energy spectrum is reviewed. Future perspectives of how to find the smoking cosmic-ray source gun concludes this review.
The popular JF12 analytic model by Jansson & Farrar provides a quantitative description of the Galaxy's large-scale magnetic field, which is widely used in various astrophysical applications. ...However, both the poloidal X-type component and the spiral disk component of JF12 exhibit regions in which the magnetic divergence constraint is violated. We first propose a cure for this problem, resulting in a truly solenoidal large-scale spiral field. Second, the otherwise straight field lines of the X-type component exhibit kinks in the Galactic plane that, in addition to implying the presence of a singular current sheet, may pose difficulties for e.g., numerical tracing of cosmic-ray particles. We propose and discuss two possible strategies to mitigate this problem. Although all corrections are kept as minimal as possible, the extended set of model parameters will have to be carefully readjusted in order to fully restore the agreement to observational data that the unmodified JF12 field is based on. Furthermore, the performance of our improved version of the field model is quantitatively assessed by test simulations using the CRPropa Galactic cosmic-ray propagation code.
The luminosity ratio of electrons to protons as it is produced in stochastic acceleration processes in cosmic ray sources is an important quantity relevant for several aspects of the modeling of the ...sources themselves. It is usually assumed to be around 1: 100 in the case of Galactic sources, while a value of 1: 10 is typically assumed when describing extragalactic sources. It is supported by observations that the average ratios should be close to these values. At this point, however, there is no possibility to investigate how each individual source behaves. When looking at the physics aspects, a 1: 100 ratio is well supported in theory when making the following assumptions: (1) the total number of electrons and protons that is accelerated are the same; (2) the spectral index of both populations after acceleration is αe=αp≈2.2. In this paper, we reinvestigate these assumptions. In particular, assumption (2) is not supported by observational data of the sources and PIC simulation yield different spectral indices as well. We present the detailed calculation of the electron-to-proton ratio, dropping the assumption of equal spectral indices. We distinguish between the ratio of luminosities and the ratio of the differential spectral behavior, which becomes necessary for cases where the spectral indices of the two particle populations are not the same. We discuss the possible range of values when allowing for different spectral indices concerning the spectral behavior of electrons and protons. Additionally, it is shown that the minimum energy of the accelerated population can have a large influence on the results. We find, in the case of the classical minimum energy of T0,e=T0,p=10 keV, that when allowing for a difference in the spectral indices of up to 0.1 with absolute spectral indices varying between 2.0 < α < 2.3, the luminosity ratio varies between 0.008 < Kep < 0.12. The differential particle number ratio is in the range 0.008<K˜ep<0.25 and depends on the energy.
Abstract
The low-luminosity active galactic nuclei M87, archetype of Fanaroff–Riley I radio galaxies, was observed in a historically quiet state in 2017. While one-zone leptonic jet models alone ...cannot explain the core radio-to-gamma-ray spectrum, we explore a hybrid jet-disk scenario. In this work, we model the overall spectral energy distribution of M87's core with a dominating one-zone lepto-hadronic jet component, coupled with the contribution from the accretion flow. We find close-to-equipartition parameter sets for which the jet component fits the radio-to-optical data as well as the gamma-ray band, while the accretion flow mainly contributes to the X-ray band. The effects of gamma-ray absorption by the extragalactic background light during the propagation toward Earth are probed and are found to be negligible for this model. The neutrino flux produced by such scenarios is also calculated, but remains below the current instruments’ sensitivity.
Context. The observed power laws in space and time profiles of energetic particles in the heliosphere can be the result of an underlying superdiffusive transport behavior. Such anomalous, ...non-Gaussian transport regimes can arise, for example, as a consequence of intermittent structures in the solar wind. Non-diffusive transport regimes may also play a critical role in other astrophysical environments such as supernova remnant shocks. Aims. To clarify the role of superdiffusion in the transport of particles near shocks, we study the solutions of a fractional diffusion-advection equation to investigate this issue. A fractional generalization of the Laplace operator, the Riesz derivative, provides a model of superdiffusive propagation. Methods. We obtained numerical solutions to the fractional transport equation by means of pseudo-particle trajectories solving the associated stochastic differential equation driven by a symmetric, stable Lévy motion. Results. The expected power law profiles of particles upstream of the plasma shock, where particles are injected, can be reproduced with this approach. The method provides a full, time-dependent solution of the fractional diffusion-advection equation. Conclusions. The developed models enable a quantitative comparison to energetic particle properties based on a comprehensive, superdiffusive transport equation and allow for an application in a number of scenarios in astrophysics and space science.
Fanaroff-Riley (FR) 0 radio galaxies compose a new class of radio galaxies, which are usually weaker but much more numerous than the well-established class of FR 1 and FR 2 galaxies. The latter ...classes have been proposed as sources of the ultra-high-energy cosmic rays (UHECRs) with energies reaching up to ∼1020 eV. Based on this conjecture, the possibility of UHECR acceleration and survival in an FR 0 source environment is examined in this work.
In doing so, an average spectral energy distribution (SED) based on data from the FR 0 catalog (FR0CAT) is compiled. The resulting photon fields are used as targets for UHECRs, which suffer from electromagnetic pair production, photo-disintegration, photo-meson production losses, and synchrotron radiation. Multiple mechanisms are discussed to assess the UHECR acceleration probability, including Fermi-I order and gradual shear accelerations, and particle escape from the source region.
This work shows that in a hybrid scenario, combining Fermi and shear accelerations, FR 0 galaxies can contribute to the observed UHECR flux, as long as Γj≳1.6, where shear acceleration starts to dominate over escape. Even in less optimistic scenarios, FR 0s can be expected to contribute to the cosmic-ray flux between the knee and the ankle. Our results are relatively robust with respect to the realized magnetic turbulence model and the speed of the accelerating shocks.
Although several theories exist for the origin of cosmic rays (CRs) in the region between the spectral "knee" and "ankle," this problem is still unsolved. A variety of observations suggest that the ...transition from Galactic to extragalactic sources occurs in this energy range. In this work, we examine whether a Galactic wind that eventually forms a termination shock far outside the Galactic plane can contribute as a possible source to the observed flux in the region of interest. Previous work by Bustard et al. estimated that particles can be accelerated to energies above the "knee" up to Rmax = 1016 eV for parameters drawn from a model of a Milky Way wind. A remaining question is whether the accelerated CRs can propagate back into the Galaxy. To answer this crucial question, we simulate the propagation of the CRs using the low-energy extension of the CRPropa framework, based on the solution of the transport equation via stochastic differential equations. The setup includes all relevant processes, including three-dimensional anisotropic spatial diffusion, advection, and corresponding adiabatic cooling. We find that, assuming realistic parameters for the shock evolution, a possible Galactic termination shock can contribute significantly to the energy budget in the "knee" region and above. We estimate the resulting produced neutrino fluxes and find them to be below measurements from IceCube and limits by KM3NeT.
The low-luminosity Active Galactic Nuclei M87, archetype of Fanaroff-Riley I radio-galaxies, was observed in a historically quiet state in 2017. While one-zone leptonic jet models alone cannot ...explain the core radio-to-gamma-ray spectrum, we explore a hybrid jet-disc scenario. In this work, we model the overall spectral energy distribution of M87's core with a dominating one-zone lepto-hadronic jet component, coupled with the contribution from the accretion flow. We find close-to-equipartition parameter sets for which the jet component fits the radio-to-optical data as well as the gamma-ray band, while the accretion flow mainly contributes to the X-ray band. The effects of gamma-ray absorption by the Extragalactic Background Light during the propagation towards Earth are probed and are found to be negligible for this model. The neutrino flux produced by such scenarios is also calculated, but remains below the current instruments' sensitivity.
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