Stochastic cosmic ray sources and the TeV break in the all-electron spectrum

Despite significant progress over more than 100 years, no accelerator has been unambiguously identified as the source of the locally measured flux of cosmic rays. High-energy electrons and positrons are of particular importance in the search for nearby sources as radiative energy losses constrain their propagation to distances of about 1 kpc around 1 TeV. At the highest energies, the spectrum is therefore dominated and shaped by only a few sources whose properties can be inferred from the fine structure of the spectrum at energies currently accessed by experiments like AMS-02, CALET, DAMPE, Fermi-LAT, H.E.S.S. and ISS-CREAM. We present a stochastic model of the Galactic all-electron flux and evaluate its compatibility with the measurement recently presented by the H.E.S.S. collaboration. To this end, we have MC generated a large sample of the all-electron flux from an ensemble of random distributions of sources. We confirm the non-Gaussian nature of the probability density of fluxes at individual energies previously reported in analytical computations. For the first time, we also consider the correlations between the fluxes at different energies, treating the binned spectrum as a random vector and parametrising its joint distribution with the help of a pair-copula construction. We show that the spectral break observed in the all-electron spectrum by H.E.S.S. and DAMPE is statistically compatible with a distribution of astrophysical sources like supernova remnants or pulsars, but requires a rate smaller than the canonical supernova rate. This important result provides an astrophysical interpretation of the spectrum at TeV energies and allows differentiating astrophysical source models from exotic explanations, like dark matter annihilation. We also critically assess the reliability of using catalogues of known sources to model the electron-positron flux.Comment: 30 pages, 12 figures; extended discussion; accepted for publication in JCA

A new analytic solution for 2nd-order Fermi acceleration

A new analytic solution for 2nd-order Fermi acceleration is presented. In particular, we consider time-dependent rates for stochastic acceleration, diffusive and convective escape as well as adiabatic losses. The power law index q of the turbulence spectrum is unconstrained and can therefore account for Kolmogorov (q = 5/3) and Kraichnan (q = 3/2) turbulence, Bohm diffusion (q = 1) as well as the hard-sphere approximation (q = 2). This considerably improves beyond solutions known to date and will prove a useful tool for more realistic modelling of 2nd-order Fermi acceleration in a variety of astrophysical environments.Comment: 14 pages, 4 figures; comments and references added; to appear in JCA

The 'PAMELA anomaly' indicates a nearby cosmic ray accelerator

We discuss the recently observed `excesses' in cosmic ray electron and positron fluxes which have been widely interpreted as signals of dark matter. By considering the production and acceleration of secondary electrons and positrons in nearby supernova remnants, we predict an additional, harder component that becomes dominant at high energies. The unknown spatial distribution of the supernova remnants introduces a stochastic uncertainty which we estimate analytically. Fitting the prediction for different source distributions to the total electron + positron flux measured by Fermi--LAT fixes all free parameters and allows us to `postdict' the rise in the positron fraction seen by PAMELA. A similar rise in the B/C ratio is predicted at high energies.Comment: 9 pages, 6 figures; accepted for the publication in the proceedings of the ICATPP Conference on Cosmic Rays for Particle and Astroparticle Physics, Villa Olmo (Como), Oct. 201

Origin of Small-Scale Anisotropies in Galactic Cosmic Rays

The arrival directions of Galactic cosmic rays (CRs) are highly isotropic. This is expected from the presence of turbulent magnetic fields in our Galactic environment that repeatedly scatter charged CRs during propagation. However, various CR observatories have identified weak anisotropies of various angular sizes and with relative intensities of up to a level of 1 part in 1,000. Whereas large-scale anisotropies are generally predicted by standard diffusion models, the appearance of small-scale anisotropies down to an angular size of 10 degrees is surprising. In this review, we summarise the current experimental situation for both the large-scale and small-scale anisotropies. We address some of the issues in comparing different experimental results and remaining questions in interpreting the observed large-scale anisotropies. We then review the standard diffusive picture and its difficulty in producing the small-scale anisotropies. Having set the stage, we review the various ideas and models put forward for explaining the small-scale anisotropies.Comment: 60 pages, 16 figures; invited review for Progress in Particle and Nuclear Physics (PPNP

Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei

The excess in the positron fraction reported by the PAMELA collaboration has been interpreted as due to annihilation or decay of dark matter in the Galaxy. More prosaically, it has been ascribed to direct production of positrons by nearby pulsars, or due to pion production during stochastic acceleration of hadronic cosmic rays in nearby sources. We point out that measurements of secondary nuclei produced by cosmic ray spallation can discriminate between these possibilities. New data on the titanium-to-iron ratio from the ATIC-2 experiment support the hadronic source model above and enable a prediction to be made for the boron-to-carbon ratio at energies above 100 GeV. Presently, all cosmic ray data are consistent with the positron excess being astrophysical in origin.Comment: 4 pages, 2 figures (RevTex4); revised to include additional data in figures and references; accepted for publication in PR

Cosmic ray electron and positron spectra at TeV energies

Observations of cosmic ray electrons have made great strides in the last decade and direct observations of the all-electron flux as well as separate electron and positron spectra are now available up to ~ 1 TeV. In this invited contribution to the 2022 edition of the Rencontres de Moriond on Very High Energy Phenomena in the Universe, we review the data on cosmic ray electron and positron spectra at TeV energies and offer general comments on their interpretation. Subsequently, we focus on the study of the stochastic fluctuations and a secondary model for the positron excess.Comment: Contribution to the 2022 Very High Energy Phenomena in the Universe (VHEPU) session of the 56th Rencontres de Moriond. 8 pages, 7 figure

On cosmic ray acceleration in supernova remnants and the FERMI/PAMELA data

We discuss recent observations of high energy cosmic ray positrons and electrons in the context of hadronic interactions in supernova remnants, the suspected accelerators of galactic cosmic rays. Diffusive shock acceleration can harden the energy spectrum of secondary positrons relative to that of the primary protons (and electrons) and thus explain the rise in the positron fraction observed by PAMELA above 10 GeV. We normalize the hadronic interaction rate by holding pion decay to be responsible for the gamma-rays detected by HESS from some SNRs. By simulating the spatial and temporal distribution of SNRs in the Galaxy according to their known statistics, we are able to then fit the electron (plus positron) energy spectrum measured by Fermi. It appears that IceCube has good prospects for detecting the hadronic neutrino fluxes expected from nearby SNRs.Comment: 13 pages, 8 figures (ReVTeX 4); Clarifying comments added; Revised prediction for B/C added (new Fig.8); References added; To appear in Phys. Rev.

Fingerprints of Galactic Loop I on the Cosmic Microwave Background

We investigate possible imprints of galactic foreground structures such as the "radio loops" in the derived maps of the cosmic microwave background. Surprisingly there is evidence for these not only at radio frequencies through their synchrotron radiation, but also at microwave frequencies where emission by dust dominates. This suggests the mechanism is magnetic dipole radiation from dust grains enriched by metallic iron or ferrimagnetic materials. This new foreground we have identified is present at high galactic latitudes, and potentially dominates over the expected $B$-mode polarization signal due to primordial gravitational waves from inflation.Comment: 5 pages, 4 figures; matches published versio