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Register a new userSecondary cosmic-ray nucleus spectra disfavor particle transport in the Galaxy without reacceleration
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The precise observations of Galactic cosmic ray fluxes of the secondary family, such as Li, Be, B, are expected to have significant implications on our understanding of the cosmic ray origin and propagation. Here we employ the recent very precise measurements of those species by the Alpha Magnetic Spectrometer on the International Space Station, together with their parent species (C and O), as well as the data collected by the Voyager-1 spacecraft outside the heliosphere and the Advanced Composition Explorer, to investigate the propagation of cosmic rays in the Milky Way. We consider the diffusion of cosmic rays plus reacceleration or convection effect during the propagation, and find that the reacceleration model can fit the data significantly better than the convection model. We further find that for the reacceleration model, the spectral hardenings of both the primary and secondary particles can be well described by the injection hardening without including additional propagation hardening. This is due to that the reacceleration effect results in a steeper secondary-to-primary ratio at low energies, and can thus naturally reproduce the fact that the secondary spectra harden more than the primary spectra found by AMS-02.
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Charged particles scattering on moving inhomogenities of the magnetised interstellar medium can gain energy through the process of second-order Fermi acceleration. This energy gain depletes in turn the magnetic wave spectrum around the resonance wave-vector $ksim 1/R_L$, where $R_L$ is the Larmor radius of the charged particle. This energy transfer can prohibit the cascading of magnetic turbulence to smaller scales, leading to a drop in the diffusion coefficient and allowing the efficient exchange of charged dark matter particles in the disk and the halo. As a result, terrestial limits from direct detection experiments apply to charged dark matter. Together with the no-observation of a drop in the diffusion coefficient, this excludes charged dark matter for $10^3 GeVlesssim m/q lesssim 10^{11} GeV$, even if the charged dark matter abundance is only a small part of the total relic abundance.
The emission mechanism for hard $gamma$-ray spectra from supernova remnants (SNRs) is still a matter of debate. Recent multi-wavelength observations of TeV source HESS J1912+101 show that it is associated with an SNR with an age of $sim 100$ kyrs, making it unlikely produce the TeV $gamma$-ray emission via leptonic processes. We analyzed Fermi observations of it and found an extended source with a hard spectrum. HESS J1912+101 may represent a peculiar stage of SNR evolution that dominates the acceleration of TeV cosmic rays. By fitting the multi-wavelength spectra of 13 SNRs with hard GeV $gamma$-ray spectra with simple emission models with a density ratio of GeV electrons to protons of $sim 10^{-2}$, we obtain reasonable mean densities and magnetic fields with a total energy of $sim 10^{50}$ ergs for relativistic ions in each SNR. Among these sources, only two of them, namely SN 1006 and RCW 86, favor a leptonic origin for the $gamma$-ray emission. The magnetic field energy is found to be comparable to that of the accelerated relativistic ions and their ratio has a tendency of increase with the age of SNRs. These results suggest that TeV cosmic rays mainly originate from SNRs with hard $gamma$-ray spectra.
In ten years of observations, the IceCube neutrino observatory has revealed a neutrino sky in tension with previous expectations for neutrino point source emissions. Astrophysical objects associated with hadronic processes might act as production sites for neutrinos, observed as point sources at Earth. Instead, a nearly isotropic flux of astrophysical neutrinos is observed up to PeV energies, prompting a reassessment of the assumed transport and production physics. This work applies a new physical explanation for neutrino production from populations of active galactic nuclei (AGN) and starburst galaxies to three years of public IceCube point source data. Specifically, cosmic rays (CRs) produced at such sources might interact with extragalactic background light and gas along the line of sight, generating a secondary neutrino flux. This model is tested alongside a number of typical flux weighting schemes, in all cases the all-sky flux contribution being constrained to percent levels of the reported IceCube diffuse astrophysical flux.
We show that the complex shape of the cosmic ray (CR) spectrum, as recently measured by PAMELA and inferred from Fermi-LAT gamma-ray observations of molecular clouds in the Gould belt, can be naturally understood in terms of basic plasma astrophysics phenomena. A break from a harder to a softer spectrum at blue rigidity Rsimeq 10 GV follows from a transition from transport dominated by advection of particles with Alfven waves to a regime where diffusion in the turbulence generated by the same CRs is dominant. A second break at Rsimeq 200 GV happens when the diffusive propagation is no longer determined by the self-generated turbulence, but rather by the cascading of externally generated turbulence (for instance due to supernova (SN) bubbles) from large spatial scales to smaller scales where CRs can resonate. Implications of this scenario for the cosmic ray spectrum, grammage and anisotropy are discussed.
The paper investigates the overall and detailed features of cosmic ray (CR) spectra in the knee region using the scenario of nuclei-photon interactions around the acceleration sources. Young supernova remnants can be the physical realities of such kind of CR acceleration sites. The results show that the model can well explain the following problems simultaneously with one set of source parameters: the knee of CR spectra and the sharpness of the knee, the detailed irregular structures of CR spectra, the so-called component B of Galactic CRs, and the electron/positron excesses reported by recent observations. The coherent explanation serves as evidence that at least a portion of CRs might be accelerated at the sources similar to young supernova remnants, and one set of source parameters indicates that this portion mainly comes from standard sources or from a single source.
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