A. Meli

THE ORIGIN OF COSMIC RAYS: EXPLOSIONS OF MASSIVE STARS WITH MAGNETIC WINDS AND THEIR SUPERNOVA MECHANISM

One prediction of particle acceleration in the supernova (SN) remnants in the magnetic wind of exploding Wolf-Rayet and red supergiant stars is that the final spectrum is a composition of a spectrum E –7/3 and a polar cap component of E –2 at the source. This polar cap component contributes to the total energy content with only a few percent, but dominates the spectrum at higher energy. The sum of both components gives spectra which curve upward. The upturn was predicted to occur always at the same rigidity. An additional component of cosmic rays from acceleration by SNe exploding into the interstellar medium adds another component for hydrogen and for helium. After transport, the predicted spectra J(E) for the wind-SN cosmic rays are E –8/3 and E –7/3; the sum leads to an upturn from the steeper spectrum. An upturn has now been seen by the CREAM mission. Here, we test the observations against the predictions and show that the observed properties are consistent with the predictions. Hydrogen can be shown to also have a noticeable wind-SN component. The observation of the upturn in the heavy element spectra being compatible with the same rigidity for all heavy elements supports the magneto-rotational mechanism for these SNe. This interpretation predicts the observed upturn to continue to curve upward and approach the E –7/3 spectrum. If confirmed, this would strengthen the case that SNe of very massive stars with magnetic winds are important sources of Galactic cosmic rays.

On the origin of ultra high energy cosmic rays : subluminal and superluminal relativistic shocks

Aims. The flux of ultra high energy cosmic rays (UHECRs) at E > 10 18.5 eV is believed to arise in plasma shock environments in extragalactic sources. In this paper, we present a systematic study of cosmic ray (CR) particle acceleration by relativistic shocks, in particular concerning the dependence on bulk Lorentz factor and the angle between the magnetic field and the shock flow. The contribution to the observed diffuse CR spectrum provided by the accelerated particles is discussed. Methods. For the first time, Monte Carlo simulations for super- and subluminal shocks are extended to boost factors up to Γ= 1000 and systematically compared. The source spectra derived are translated into the expected diffuse proton flux from astrophysical sources by folding the spectra with the spatial distribution of active galactic nuclei (AGN) and gamma ray bursts (GRBs). Results of these predictions are compared with UHECR data. Results. While superluminal shocks are shown to be inefficient at providing acceleration to the highest energies (E > 10 18.5 eV), subluminal shocks may provide particles up to 10 21 eV, limited only by the Hillas-criterion. In the subluminal case, we find that mildly-relativistic shocks, thought to occur in jets of AGN (Γ ∼ 10−30), yield energy spectra of dN/dE ∼ E −2 . Highly relativistic shocks expected in GRBs (100 < Γ < 1000), on the other hand, produce spectra as flat as ∼E −1.0 above 10 9.5 GeV. The model results are compared with the measured flux of CRs at the highest energies and it is shown that, while AGN spectra provide an excellent fit, GRB spectra are too flat to explain the observed flux. The first evidence of a correlation between the CR flux above 5.7 × 10 10 GeV and the distribution of AGN provided by Auger are explained by our model. Although GRBs are excluded as the principle origin of UHECRs, neutrino production is expected in these sources either in mildly or highly relativistic shocks. In particular, superluminal shocks in GRBs may be observable via neutrino and photon fluxes, rather than as protons.

Active Galactic Nuclei: Sources for ultra high energy cosmic rays?

The origin of ultra high energy cosmic rays promises to lead us to a deeper understanding of the structure of matter. This is possible through the study of particle collisions at center-of-mass energies in interactions far larger than anything possible with the Large Hadron Collider, albeit at the substantial cost of no control over the sources and interaction sites. For the extreme energies we have to identify and understand the sources first, before trying to use them as physics laboratories. Here we describe the current stage of this exploration. The most promising contenders as sources are radio galaxies and gamma ray bursts. The sky distribution of observed events yields a hint favoring radio galaxies. Key in this quest are the intergalactic and galactic magnetic fields, whose strength and structure are not yet fully understood. Current data and statistics do not yet allow a final judgement. We outline how we may progress in the near future.

ACTIVE GALACTIC NUCLEI: SOURCES FOR ULTRA HIGH ENERGY COSMIC RAYS

Ultra high energy cosmic ray events presently show a spectrum, which we interpret here as galactic cosmic rays due to a starburst, in the radio galaxy Cen A which is pushed up in energy by the shock of a relativistic jet. The knee feature and the particles with energy immediately higher in galactic cosmic rays then turn into the bulk of ultra high energy cosmic rays. This entails that all ultra high energy cosmic rays are heavy nuclei. This picture is viable if the majority of the observed ultra high energy events come from the radio galaxy Cen A, and are scattered by intergalactic magnetic fields across much of the sky.