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The Crab nebula is one of the most efficient accelerators in the Galaxy and the only galactic source showing direct evidence of PeV particles. In spite of this, the physical process behind such effective acceleration is still a deep mystery. While particle acceleration, at least at the highest energies, is commonly thought to occur at the pulsar wind termination shock, the properties of the upstream flow are thought to be non-uniform along the shock surface, and important constraints on the mechanism at work come from exact knowledge of where along this surface particles are being accelerated. Here we use axisymmetric relativistic magnetohydrodynamic simulations to obtain constraints on the acceleration site(s) of particles of different energies in the Crab nebula. Various scenarios are considered for the injection of particles responsible for synchrotron radiation in the different frequency bands, radio, optical and X-rays. The resulting emission properties are compared with available data on the multiwavelength time variability of the inner nebula. Our main result is that the X-ray emitting particles are accelerated in the equatorial region of the pulsar wind. Possible implications on the nature of the acceleration mechanism are discussed. [ABSTRACT FROM AUTHOR]

In recent years, it has become a well-established paradigm that many aspects of the physics of Pulsar Wind Nebulae can be fully accounted for within a relativistic magnetohydrodynamic description. Numerical simulations have proven extremely successful in reproducing the X-ray morphology of the Crab nebula, down to very fine detail. Radio emission, instead, is currently one of the most obscure aspects of the physics of these objects, and one that holds important information about pulsar properties and their role as antimatter factories. Here, we address the question of radio emission morphology and integrated spectrum from the Crab nebula, by using for the first time an axisymmetric dynamical model with parameters chosen to best reproduce its X-ray morphology. Based on our findings we discuss constraints on the origin of the radio emitting particles. [ABSTRACT FROM AUTHOR]

The theory of non-linear diffusive shock acceleration (NLDSA) predicts the formation of a precursor upstream of the shock, where accelerated particles diffuse and induce magnetic field amplification (MFA) through streaming instability. The non-detection of this precursor in X-rays in Chandra observations of the north-eastern region of SN 1006 (G329.6+14.6) led the authors of some previous work to impose an upper limit to the X-ray emission generated by accelerated electrons diffusing in this precursor, at an emissivity level of <1.5 per cent of the emission from the downstream region. This has been used as an argument against Fermi acceleration at this shock. Here, we calculate the spectrum and spatial distribution of accelerated particles in SN 1006 and show that Chandra results (including more recent data) are in perfect agreement with the predictions of NLDSA suggesting efficient particle acceleration and MFA upstream of the shock by a factor of ∼10. [ABSTRACT FROM AUTHOR]