Active Galactic Nuclei and Gamma Ray Bursts: Relativistic shocks and the origin of ultra high energy cosmic rays

The importance of explaining and achieving the highest observed cosmic ray energies by the first order Fermi acceleration mechanism in a given astrophysical environment is a recurring theme in astroparticle physics. In this contribution, we discuss acceleration of cosmic rays in relativistic shock formations, focusing on numerical studies concerning proton acceleration efficiency by subluminal and superluminal shocks, emphasising the dependence of the scattering model, shock Lorentz factor and the angle between the magnetic field and the shock front. We present a diffuse cosmic ray prediction model, where results are compared with the measured flux of cosmic rays at the highest energies. We show that steeper Active Galactic Nuclei spectra provide an excellent fit. Our model explains well the first evidence of a correlation between the cosmic ray flux above 5.7 ×1010 GeV and the distribution of Active Galactic Nuclei provided by AUGER. On the other hand, Gamma Ray Burst spectra being flatter can hardly explain the highest energy observed flux. We note though that recent Fermi observations of GRB090816c indicate very flat spectra which are expected within our model predictions and support evidence that GRB particle spectra can be flat, when the shock Lorentz factor is of order ∼1000.