Nearby supernova remnants and the cosmic-ray spectral hardening at high energies

Recent measurements of cosmic-ray spectra of several individual nuclear species by the CREAM, TRACER, and ATIC experiments indicate a change in the spectral index of the power laws at TeV energies. Possible explanations among others include non linear diffusive shock acceleration of cosmic-rays, different cosmic-ray propagation properties at higher and lower energies in the Galaxy and the presence of nearby sources. In this paper, we show that if supernova remnants are the main sources of cosmic rays in our Galaxy, the effect of the nearby remnants can be responsible for the observed spectral changes. Using a rigidity dependent escape of cosmic-rays from the supernova remnants, we explain the apparent observed property that the hardening of the helium spectrum occurs at relatively lower energies as compared to the protons and also that the spectral hardening does not persist beyond $\sim (20-30)$ TeV energies.Comment: 6 pages, MNRAS accepted, minor text correction

GeV-TeV cosmic-ray spectral anomaly as due to re-acceleration by weak shocks in the Galaxy

Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly understood, suggested explanations include effect of cosmic-ray source spectrum, propagation effects, and the effect of nearby sources. In this paper, we propose that the spectral anomaly might be an effect of re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the younger ones due to their bigger sizes, re-acceleration is expected to be mainly due to weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that such re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, explaining the observed GeV-TeV spectral anomaly.Comment: 12 pages, A&A accepte

GCOS -- The Global Cosmic Ray Observatory

Nature is providing particles with energies exceeding 100 EeV. Their existence imposes immediate questions: Are they ordinary particles, accelerated in extreme astrophysical environments, or are they annihilation or decay products of super-heavy dark matter or other exotic objects? If the particles are accelerated in extreme astrophysical environments, are their sources related to those of high-energy neutrinos, gamma rays, and/or gravitational waves, such as the recently observed mergers of compact objects? The particles can also be used to study physics processes at extreme energies; is Lorentz invariance still valid? Are the particles interacting according to the Standard Model or are there new physics processes? The particles can be used to study hadronic interactions (QCD) in the kinematic forward direction; what is the cross section of protons at center-of-mass energies $\sqrt{s} > 100$ TeV? These questions are addressed at present by installations like the Telescope Array and the Pierre Auger Observatory. After the year 2030, a next-generation observatory will be needed to study the physics and properties of the highest-energy particles in Nature, building on the knowledge harvested from the existing observatories. It should have an aperture at least an order of magnitude bigger than the existing observatories. Recently, more than 200 scientists from around the world came together to discuss the future of the field of multi-messenger astroparticle physics beyond the year 2030. Ideas have been discussed towards the physics case and possible scenarios for detection concepts of the Global Cosmic Ray Observatory - GCOS. A synopsis of the key results discussed during the brainstorming workshop will be presented

Cosmogenic gamma-rays and neutrinos constrain UHECR source models

We use CRPropa 3 to show how the expected cosmogenic neutrino and gamma-ray spectra depend on the maximum energy of ultra-high energy cosmic rays (UHECRs) at their sources, on the spectral index at injection and on the chemical composition of UHECRs. The isotropic diffuse gamma-ray background measured by Fermi/LAT is already close to touching upon a model with co-moving source evolution and with the chemical composition, spectral index and maximum acceleration energy optimized to provide the best fit to the UHECR spectrum and composition measured by the Pierre Auger Collaboration. Additionally, the detectable fraction of protons present at the highest energies in UHECRs, for experiments with sensitivities to the single-flavor neutrino flux at $\sim1$ EeV in the range of $\sim 10^{-8}$ - $10^{-10}$ GeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$, is shown as a function of the evolution of UHECR sources. Experiments that reach this sensitivity will be able to significantly constrain the proton fraction for realistic source evolution models.Comment: Proc. 35th ICRC, Busan, South Korea, PoS(ICRC2017)56

The origin of galactic cosmic rays

The origin of galactic cosmic rays is one of the most interesting unsolved problems in astroparticle physics. Experimentally, the problem is attacked by a multi-disciplinary effort, namely by direct measurements of cosmic rays above the atmosphere, by air shower observations, and by the detection of TeV $\gamma$ rays. Recent experimental results are presented and their implications on the contemporary understanding of the origin of galactic cosmic rays are discussed.Comment: Invited talk given at the Roma International Conference on Astro-Particle physics (RICAP07) June 20th - 22nd, 2007. To be published in Nuclear Instruments and Methods

Models of the Knee in the Energy Spectrum of Cosmic Rays

The origin of the knee in the energy spectrum of cosmic rays is an outstanding problem in astroparticle physics. Numerous mechanisms have been proposed to explain the structure in the all-particle spectrum. In the article basic ideas of several models are summarized, including diffusive acceleration of cosmic rays in shock fronts, acceleration via cannonballs, leakage from the Galaxy, interactions with background particles in the interstellar medium, as well as new high-energy interactions in the atmosphere. The calculated energy spectra and mean logarithmic masses are compiled and compared to results from direct and indirect measurements.Comment: 30 pages, 20 figures accepted by Astroparticle Physics captions of figures 1-3 clarified, references adde

Cosmic-ray composition and its relation to shock acceleration by supernova remnants

An overview is given on the present status of the understanding of the origin of galactic cosmic rays. Recent measurements of charged cosmic rays and photons are reviewed. Their impact on the contemporary knowledge about the sources and acceleration mechanisms of cosmic rays and their propagation through the Galaxy is discussed. Possible reasons for the knee in the energy spectrum and scenarios for the end of the galactic cosmic-ray component are described.Comment: Invited talk given at the 36th COSPAR Scientific Assembly Beijing, China, 16 -- 23 July 2006 - submitted to Advances in Space Research - comments are welcom