Context: Features in the spectra of primary cosmic rays (CRs) provide invaluable information on the propagation of these particles in the Galaxy. In the rigidity region around a few hundred GV, such features have been measured in the proton and heliu
m spectra by the PAMELA experiment and later confirmed with a higher significance by AMS-02. We investigate the implications of these datasets for the scenario in which CRs propagate under the action of self-generated waves. Aims: We show that the recent data on the spectrum of protons and helium nuclei as collected with AMS-02 and Voyager are in very good agreement with the predictions of a model in which the transport of Galactic CRs is regulated by self-generated waves. We also study the implications of the scenario for the boron-to-carbon ratio: although a good overall agreement is found, at high energy we find marginal support for a (quasi) energy independent contribution to the grammage, that we argue may come from the sources themselves Results: A break in the spectra of all nuclei is found at rigidity of a few hundred GV, as a result of a transition from self-generated waves to pre-existing waves with a Kolmogorov power spectrum. Neither the slope of the diffusion coefficient, nor its normalisation are free parameters. Moreover, at rigidities below a few GV, CRs are predicted to be advected with the self-generated waves at the local Alfven speed. This effect, predicted in our previous work, provides an excellent fit to the Voyager data on the proton and helium spectra at low energies, providing additional support to the model.
While there is some level of consensus on a Galactic origin of cosmic rays up to the knee ($E_{k}sim 3times 10^{15}$ eV) and on an extragalactic origin of cosmic rays with energy above $sim 10^{19}$ eV, the debate on the genesis of cosmic rays in the
intermediate energy region has received much less attention, mainly because of the ambiguity intrinsic in defining such a region. The energy range between $10^{17}$ eV and $sim 10^{19}$ eV is likely to be the place where the transition from Galactic to extragalactic cosmic rays takes place. Hence the origin of these particles, though being of the highest importance from the physics point of view, it is also one of the most difficult aspects to investigate. Here I will illustrate some ideas concerning the sites of acceleration of these particles and the questions that their investigation may help answer, including the origin of underline{ultra} high energy cosmic rays.
The last decade has been dense with new developments in the search for the sources of Galactic cosmic rays. Some of these developments have confirmed the tight connection between cosmic rays and supernovae in our Galaxy, through the detection of gamm
a rays and the observation of thin non-thermal X-ray rims in supernova remnants. Some other, such as the detection of features in the spectra of some chemicals opened new questions on the propagation of cosmic rays in the Galaxy and on details of the acceleration process. Here I will summarize some of these developments and their implications for our understanding of the origin of cosmic rays. I will also discuss some new avenues that are being pursued in testing the supernova origin of Galactic cosmic rays.
The origin of the bulk of cosmic rays (CRs) observed at Earth is the topic of a century long investigation, paved with successes and failures. From the energetic point of view, supernova remnants (SNRs) remain the most plausible sources of CRs up to
rigidity ? 10^6-10^7 GV. This confidence somehow resulted in the construction of a paradigm, the so-called SNR paradigm: CRs are accelerated through diffusive shock acceleration in SNRs and propagate diffusively in the Galaxy in an energy dependent way. Qualitative confirmation of the SNR acceleration scenario has recently been provided by gamma ray and X-ray observations. Diffusive propagation in the Galaxy is probed observationally through measurement of the secondary to primary nuclei flux ratios (such as B/C). There are however some weak points in the paradigm, which suggest that we are probably missing some physical ingredients in our models. The theory of diffusive shock acceleration at SNR shocks predicts spectra of accelerated particles which are systematically too hard compared with the ones inferred from gamma ray observations. Moreover, hard injection spectra indirectly imply a steep energy dependence of the diffusion coefficient in the Galaxy, which in turn leads to anisotropy larger than the observed one. Moreover recent measurements of the flux of nuclei suggest that the spectra have a break at rigidity ? 200 GV, which does not sit well with the common wisdom in acceleration and propagation. In this paper I will review these new developments and suggest some possible implications.
The wealth of data collected in the last few years thanks to the Pierre Auger Observatory and recently to the Telescope Array made the problem of the origin of ultra high energy cosmic rays a genuinely experimental/observational one. The apparently c
ontradictory results provided by these experiments in terms of spectrum, chemical composition and anisotropies do not allow to reach any final conclusions as yet. Here I will discuss some of the theoretical challenges imposed by these data: in particular I will discuss some issues related to the transition from Galactic to extragalactic cosmic rays and how the different models confront our understanding of Galactic cosmic rays in terms of supernova remnants paradigm. I will also discuss the status of theories aiming at describing acceleration of cosmic rays to the highest energies in relativistic shocks and unipolar inductors.
We reconsider the model of neutrino production during the bright phase, first suggested in 1977, in the light of modern understanding of the role of Pop III stars and acceleration of particles in supernova shocks. We concentrate on the production of
cosmogenic UHE neutrinos in supernova explosions that accompany the death of Pop III stars. Accelerated protons produce neutrinos in collisions with CMB photons. We deliberately use simplified assumptions which make our results transparent. Pop III stars are assumed to be responsible for the reionization of the universe as observed by WMAP. Since the evolution of Pop III stars is much faster than the Hubble rate, we consider the burst of UHE proton production to occur at fixed redshift (z_b=10-20). We discuss the formation of collisionless shocks and particle acceleration in the early universe. The composition of accelerated particles is expected to be proton dominated. A simple calculation is presented to illustrate the fact that the diffuse neutrinos flux from the bright phase burst is concentrated in a relatively narrow range around $7.5 times 10^{15}(20/z_b)^2$ eV. The $ u_mu$ flux may be detectable by IceCube without violating the cascade upper limit and the expected energetics of SNe associated with Pop III stars. A possible signature of the neutrino production from Pop III stars may be the detection of resonant neutrino events. For the burst at $z_b=20$ and $bar{ u}_e$-flux at the cascade upper limit, the number of resonant events in IceCube may be as high as 10 events in 5 years of observations. These events have equal energies, $E=6.3times 10^{15}$ eV, in the form of e-m cascades. Given the large uncertainties in the existing predictions of UHE cosmogenic neutrino fluxes, we argue that neutrinos from the first stars might become one of the most reliable hopes for UHE neutrino astronomy.
We show that a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-alfvenic fastly growing mode which has the same dispersion relation as that previously found by
cite{bell04} by treating the plasma in the MHD approximation. The kinetic approach allows us to investigate the dependence of the dispersion relation of these waves on the microphysics of the current which compensates the cosmic ray flow. We also show that a resonant and a non-resonant mode may appear at the same time and one of the two may become dominant on the other depending on the conditions in the acceleration region. We discuss the role of the unstable modes for magnetic field amplification and particle acceleration in supernova remnants at different stages of the remnant evolution.
We discuss the signatures of the transition from galactic to extragalactic cosmic rays in different scenarios, giving most attention to the dip scenario. The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons in the energy ra
nge $1times 10^{18} - 4times 10^{19}$ eV, which is caused by electron-positron pair production on the cosmic microwave background (CMB) radiation. The dip scenario provides a simple physical description of the transition from galactic to extragalactic cosmic rays. Here we summarize the signatures of the pair production dip model for the transition, most notably the spectrum, the anisotropy and the chemical composition. The main focus of our work is however on the description of the features that arise in the elongation rate and in the distribution of the depths of shower maximum $X_{rm max}$ in the dip scenario. We find that the curve for $X_{max}(E)$ shows a sharp increase with energy, which reflects a sharp transition from an iron dominated flux at low energies to a proton dominated flux at $Esim 10^{18}$ eV. We also discuss in detail the shape of the $X_{max}$ distributions for cosmic rays of given energy and demonstrate that this represents a powerful tool to discriminate between the dip scenario and other possible models of the transition.
