Recent results from the PAMELA satellite indicate the presence of a large flux of positrons (relative to electrons) in the cosmic ray spectrum between approximately 10 and 100 GeV. As annihilating dark matter particles in many models are predicted to contribute to the cosmic ray positron spectrum in this energy range, a great deal of interest has resulted from this observation. Here, we consider pulsars (rapidly spinning, magnetized neutron stars) as an alternative source of this signal. After calculating the contribution to the cosmic ray positron and electron spectra from pulsars, we find that the spectrum observed by PAMELA could plausibly originate from such sources. In particular, a significant contribution is expected from the sum of all mature pulsars throughout the Milky Way, as well as from the most nearby mature pulsars (such as Geminga and B0656+14). The signal from nearby pulsars is expected to generate a small but significant dipole anisotropy in the cosmic ray electron spectrum, potentially providing a method by which the Fermi gamma-ray space telescope would be capable of discriminating between the pulsar and dark matter origins of the observed high energy positrons. PACS numbers: 98.70.Sa;97.60.Gb;95.35.+d;98.70.Sa

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. ?? 2013 Elsevier B.V. All rights reserved

It is demonstrated that clusters of galaxies are able to keep cosmic rays for a time exceeding the age of the universe. This phenomenon reveals itself by the production of the di †use Ñux of high-energy gamma and neutrino radiation due to the interaction of the cosmic rays with the intracluster gas. The produced Ñux is determined by the cosmological density of baryons, if a large part of this density is provided ) b , by the intracluster gas. The signal from relic cosmic rays has to be compared with the Ñux produced by the late sources, which can be considered as a background in the search for cosmic-ray production in the past. We calculate this Ñux considering the normal galaxies and active galactic nuclei (AGNs) in the clusters as the sources of cosmic rays. Another potential cosmic-ray source is the shock in the gas accreting to a cluster. We found that this background is relatively high : the di †use Ñuxes produced by relic cosmic rays are of the same order of magnitude that can be expected from AGNs in the clusters. In all cases the predicted di †use gamma-ray Ñux is smaller than the observed one, and the di †use neutrino Ñux can be seen as the small bump at E D 106 GeV over the atmospheric neutrino Ñux. A bright phase in the galaxy evolution can be a source of the relic cosmic rays in clusters, revealing itself by di †use gamma and neutrino radiation. We found that the observation of a signal from the bright phase is better for an individual cluster.

We show that the positron excess measured by the PAMELA experiment in the region between 10 and 100 GeV may well be a natural consequence of the standard scenario for the origin of Galactic cosmic rays. The 'excess' arises because of positrons created as secondary products of hadronic interactions inside the sources, but the crucial physical ingredient which leads to a natural explanation of the positron flux is the fact that the secondary production takes place in the same region where cosmic rays are being accelerated. Therefore secondary positrons (and electrons) participate in the acceleration process and turn out to have a very flat spectrum, which is responsible, after propagation in the Galaxy, for the observed positron 'excess'. This effect cannot be avoided though its strength depends on the values of the environmental parameters during the late stages of evolution of supernova remnants.The PAMELA satellite began its three-year mission in June of 2006, and among its goals there was that of measuring the spectra of cosmic ray positrons up to 270 GeV and electrons up to 2 TeV, each with unprecedented precision [1]. Recent results [2] show that the ratio of positrons to electrons plus positrons (the so-called positron fraction) in the cosmic ray spectrum appears to rise with energy, at least up to ∼ 100 GeV, as already found by previous experiments, including HEAT [3] and AMS-01 [4], although with smaller statistical significance. The clear discrepancy between the observed positron fraction and the predictions of the standard model for the origin and propagation of cosmic rays in the Galaxy, led to many possible explanations, ranging from the annihilation of non-baryonic dark matter [5] to the possibility that new astrophysical sources, especially pulsars [6], could provide the additional positron flux. It is worth recalling that both lines of thought lead to the "correct" spectral slope rather naturally, but they are very different in terms of providing the correct normalization of the positron fluxes. The dark matter interpretation typically requires large, and somewhat artificial, annihilation rates. Such large rates could, in principle, result from dark matter possessing an annihilation cross section in excess of the value predicted for a simple s-wave thermal relic (σv ≈ 3 × 10 −26 cm 3 /sec), for example due to the Sommerfeld effect [7], or from non thermal WIMPs [8]. In the case of pulsars on the other hand, the energetic requirements appear to be all but extreme, although an efficiency factor needs to be introduced by hand [6], and the mechanisms for escape of the pairs from the pulsar environment are basically unknown.

One century ago Viktor Hess carried out several balloon flights that led him to conclude that the penetrating radiation responsible for the discharge of electroscopes was of extraterrestrial origin. One century from the discovery of this phenomenon seems to be a good time to stop and think about what we have understood about Cosmic Rays. The aim of this review is to illustrate the ideas that have been and are being explored in order to account for the observable quantities related to cosmic rays and to summarize the numerous new pieces of observation that are becoming available. In fact, despite the possible impression that development in this field is somewhat slow, the rate of new discoveries in the last decade or so has been impressive, and mainly driven by beautiful pieces of observation. At the same time scientists in this field have been able to propose new, fascinating ways to investigate particle acceleration inside the sources, making use of multifrequency observations that range from the radio, to the optical, to X-rays and gamma rays. These ideas can now be confronted with data.I will mostly focus on supernova remnants as the most plausible sources of Galactic cosmic rays, and I will review the main aspects of the modern theory of diffusive particle acceleration at supernova remnant shocks, with special attention for the dynamical reaction of accelerated particles on the shock and the phenomenon of magnetic field amplification at the shock. Cosmic ray escape from the sources is discussed as a necessary step to determine the spectrum of cosmic rays at the Earth. The discussion of these theoretical ideas will always proceed parallel to an account of the data being collected especially in X-ray and gamma ray astronomy.

The study of the interactions of Cosmic Rays (CR's) with universal diffuse background radiation can provide very stringent tests of the validity of Special Relativity. The interactions we consider are the ones characterized by well defined energy thresholds whose energy position can be predicted on the basis of special relativity. We argue that the experimental confirmation of the existence of these thresholds can in principle put very stringent limits on the scale where special relativity and/or continuity of space-time may possibly break down.

We study the effect of inhomogeneities in the matter distribution of the universe on the Faraday rotation of light from distant QSOs and derive new limits on the cosmological magnetic field. The matter distribution in the Universe is far from being homogeneous and, for the redshifts of interest to rotation measures (RM), it is well described by the observed Ly-α forest. We use a log-normal distribution to model the Ly-α forest and assume that a cosmological magnetic field is frozen into the plasma and is therefore a function of the density inhomogeneities. The Ly-α forest results are much less sensitive to the cosmological magnetic field coherence length than those for a homogeneous universe and show an increase in the magnitude of the expected RM for a given field by over an order of magnitude. The forest also introduces a large scatter in RM for different lines-of-sight with a highly non-gaussian tail that renders the variance and the mean RM impractical for setting limits. The median|RM| is a better statistical indicator which we use to derive the following limits using the observed RM for QSOs between z = 0 and z = 2.5. We set Ω b h 2 = 0.02 and get for cosmological fields coherent accross the present horizon, B H −1 0 ∼ < 10 −9 G in the case of a Ly-α forest which is stronger than the limit for a homogeneous universe, B h H −1 0 ∼ < 2 × 10 −8 G; while for 50 Mpc coherence length, the inhomogeneous case gives B 50Mpc ∼ < 6 × 10 −9 G while the homogeneous limit is B h 50Mpc ∼ < 10 −7 G and for coherence length equal to the Jeans length, B λ J ∼ < 10 −8 G for the Ly-α case while B h λ J ∼ < 10 −6 G.

The long-held notion that the highest energy cosmic rays are of distant extragalactic origin is challenged by observations that events above approximately 1020 eV do not exhibit the expected high-energy cutoff from photopion production off the cosmic microwave background. We suggest that these unexpected ultra-high-energy events are due to iron nuclei accelerated from young strongly magnetized neutron stars through relativistic MHD winds. We find that neutron stars whose initial spin periods are shorter than approximately 10 ms and whose surface magnetic fields are in the 1012-1014 G range can accelerate iron cosmic rays to greater than approximately 1020 eV. These ions can pass through the remnant of the supernova explosion that produced the neutron star without suffering significant spallation reactions or energy loss. For plausible models of the Galactic magnetic field, the trajectories of the iron ions curve sufficiently to be consistent with the observed, largely isotropic arrival directions of the highest energy events.