The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0 σ 10 −1 . The pre-shock magnetic field is orthogonal to the flow, as generically expected for relativistic shocks. We find that relativistic perpendicular shocks propagating in electron-positron plasmas are efficient particle accelerators if the magnetization is σ 10 −3 . For electron-ion plasmas, the transition to efficient acceleration occurs for σ 3 × 10 −5 . Here, the acceleration process proceeds similarly for the two species, since the electrons enter the shock nearly in equipartition with the ions, as a result of strong pre-heating in the self-generated upstream turbulence. In both electron-positron and electron-ion shocks, we find that the maximum energy of the accelerated particles scales in time as ε max ∝ t 1/2 . This scaling is shallower than the so-called (and commonly assumed) Bohm limit ε max ∝ t, and it naturally results from the small-scale nature of the Weibel turbulence generated in the shock layer. In magnetized plasmas, the energy of the accelerated particles increases until it reaches a saturation value ε sat /γ 0 m i c 2 ∼ σ −1/4 , where γ 0 m i c 2 is the mean energy per particle in the upstream bulk flow. Further energization is prevented by the fact that the self-generated turbulence is confined within a finite region of thickness ∝ σ −1/2 around the shock. Our results can provide physically grounded inputs for models of non-thermal emission from a variety of astrophysical sources, with particular relevance to GRB afterglows.

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I show that the relativistic winds of newly born magnetars (neutron stars with petagauss surface magnetic fields) with initial spin rates close to the centrifugal breakup limit, occurring in all normal galaxies with massive star formation, can provide a source of ultrarelativistic light ions with an E À1 injection spectrum, steepening to E À2 at higher energies, with an upper cutoff at 10 21 -10 22 eV. Interactions with the cosmic microwave background yield a spectrum at the Earth that compares favorably with the spectrum of ultrahigh-energy cosmic rays (UHECRs) observed at energies up to a few times 10 20 eV. The fit to the observations suggests that $5%-10% of the magnetars are born with rotation rates and voltages sufficiently high to allow the acceleration of the UHECR. The form the spectrum incident on the Earth takes depends sensitively on the mechanism and the magnitude of gravitational wave losses during the early spin-down of these neutron stars: pure electromagnetic spin-down (the E À1 injection spectrum) yields a GZK feature [a flattening of the E 3 J(E) spectrum] below 10 20 eV, rather than a cutoff, while a moderate GZK cutoff appears if gravitational wave losses are strong enough to steepen the injection spectrum above 10 20 eV. The flux above 10 20 eV comes from magnetars in relatively nearby galaxies (D < 50 Mpc). I outline the probable physics of acceleration of such particles in a magnetar's wind: it is a form of '' surf-riding '' in the approximately force-free fields of the wind. I also show how the high-energy particles can escape with small energy losses from the magnetars' natal supernovae. In particular, I show that the electromagnetic energy emitted by the magnetar '' shreds '' the supernova envelope in times short enough to allow most of the relativistic energy to escape largely unimpeded into the surrounding interstellar medium, where it drives a relativistic blast wave that expands to parsec scale before slowing down to nonrelativistic speeds. I also show that since the ions are accelerated in a region where the magnetic field has the structure of a strong electromagnetic wave but propagate at larger radii through a region of weaker magnetic field near the rotational equator of the outflow, the ultra-high-energy particles escape with negligible adiabatic and radiation losses. The requirement that the magnetars' relativistic winds not overproduce interstellar supershells and unusually large supernova remnants suggests that most of the initial spin-down energy is radiated in kilohertz gravitational waves for several hours after each supernova. For typical distances to events that contribute to E > 100 EeV air showers, the model predicts gravitational wave strains $3 Â 10 À21 . Such bursts of gravitational radiation should correlate with bursts of ultra-highenergy particles. The Auger experiment should see bursts of particles with energy above 100 EeV every few years.

We present a new model for the spectral evolution of pulsar wind nebulae (PWNe) inside supernova remnants (SNRs). The model couples the long‐term dynamics of these systems, as derived in the 1D approximation, with a one‐zone description (all quantities are assumed uniform in the nebula) of the spectral evolution of the emitting plasma. Our goal is to provide a simplified theoretical description that can be used as a tool to put constraints on unknown properties of PWN‐SNR systems: a piece of work that is preliminary to any more accurate and sophisticated modelling. In this paper, we apply the newly developed model to a few objects of different ages and luminosities. We find that an injection spectrum in the form of a broken power law gives a satisfactory description of the emission for all the systems we consider. More surprisingly, we also find that the intrinsic spectral break turns out to be at a similar energy for all sources, in spite of the differences mentioned above. We discuss the implications of our findings on the workings of pulsar magnetospheres, pair multiplicity and on the particle acceleration mechanism(s) that might be at work at the pulsar wind termination shock.