Particle Acceleration at Relativistic Shocks

I review the current status of Fermi acceleration theory at relativistic shocks. I first discuss the relativistic shock jump conditions, then describe the non-relativistic Fermi mechanism and the differences introduced by relativistic flows. I present numerical calculations of the accelerated particle spectrum, and examine the maximum energy attainable by this process. I briefly consider the minimum energy for Fermi acceleration, and a possible electron pre-acceleration mechanism.Comment: 17 pages, 4 figures. To appear in "Relativistic Flows in Astrophysics", A.W. Guthmann, M. Georganopoulos, A. Marcowith and K. Manolokou, eds., Lecture Notes in Pysics, Springer Verla

Investigating the machining of tungsten (W) using finite element analysis

Tungsten is extensively used as a plasma facing material in fusion energy reactors. A finite element model was created to simulate the machining of tungsten for the first time by estimating the cutting forces and observing the impact of the variation in tool rake angle. The model was validated through machining experiments involving a specially designed single flute fly cutter which indicated errors of 6% – 34%, depending on the rake angle. This investigation is the first step in understanding the impact of cutting parameters on machining of tungsten. However, the model is affected by the unpredictable impact of tungsten’s deformation behaviour and especially the effects of its brittle nature and low fracture toughness

Probing Pulsar Winds Using Inverse Compton Scattering

We investigate the effects of inverse Compton scattering by electrons and positrons in the unshocked winds of rotationally-powered binary pulsars. This process can scatter low energy target photons to produce gamma rays with energies from MeV to TeV. The binary radio pulsars PSR B1259-63 and PSR J0045-73 are both in close eccentric orbits around bright main sequence stars which provide a huge density of low energy target photons. The inverse Compton scattering process transfers momentum from the pulsar wind to the scattered photons, and therefore provides a drag which tends to decelerate the pulsar wind. We present detailed calculations of the dynamics of a pulsar wind which is undergoing inverse Compton scattering, showing that the deceleration of the wind of PSR B1259-63 due to `inverse Compton drag' is small, but that this process may confine the wind of PSR J0045-73 before it attains pressure balance with the outflow of its companion star. We calculate the spectra and light curves of the resulting inverse Compton emission from PSR B1259-63 and show that if the size of the pulsar wind nebula is comparable to the binary separation, then the gamma-ray emission from the unshocked wind may be detectable by atmospheric Cerenkov detectors or by the new generation of satellite-borne gamma-ray detectors such as INTEGRAL and GLAST. This mechanism may therefore provide a direct probe of the freely-expanding regions of pulsar winds, previously thought to be invisible.Comment: To be published in Astroparticle Physics. 27 pages, 5 figure

Hadronic model for radio-to-TeV gamma-ray emission from PSR B1259-63

We discuss the implications of the recent X-ray and TeV gamma-ray observations of the PSR B1259-63 system (a young rotation powered pulsar orbiting a Be star) for the theoretical models of interaction of pulsar and stellar winds. We show that previously considered models in which the pulsar wind is purely electron loaded have problems to account for the observed behaviour of the system in the TeV and X-ray bands. We develop a model in which the broad band (radio, X-ray and high energy gamma-ray) emission from the binary system is produced in result of collisions of GeV-TeV energy protons accelerated by the pulsar wind and interacting with the stellar disk. In this model the high energy gamma-rays are produced in the decays of secondary neutral pions, while radio and X-ray emission are synchrotron and inverse Compton emission produced by low-energy (< 100 MeV) electrons from the decays of secondary charged pi mesons. This model can explain not only the observed energy spectra, but also the correlations between TeV, X-ray and radio emission components.Comment: Proceeding of "The multi messenger approach to high energy gamma ray sources", Barcelona, June 200

Dissipation in Pulsar Winds

I review the constraints placed on relativistic pulsar winds by comparing optical and X-ray images of the inner Crab Nebula on the one hand with two-dimensional MHD simulations on the other. The various proposals in the literature for achieving the low magnetisation required at the inner edge of the Nebula, are then discussed, emphasising that of dissipation in the striped-wind picture. The possibility of direct observation of the wind is examined. Based on the predicted orientation of the polarisation vector, I outline a new argument suggesting that the off-pulse component of the optical emission of the Crab pulsar originates in the wind.Comment: Invited review presented at the COSPAR Symposium, Paris 200

Inverse Compton Emission of TeV Gamma Rays from PSR B1259-63

We derive light curves for the hard gamma-ray emission, at energies up to several TeV, expected from the unique pulsar/Be-star binary system PSR B1259-63. This is the only known system in our galaxy in which a radio pulsar is orbiting a main sequence star. We show that inverse Compton emission from the electrons and positrons in the shocked pulsar wind, scattering target photons from the Be star, produces a flux of hard gamma-rays that should be above the sensitivity threshold of present day atmospheric Cerenkov detectors. Furthermore, we predict that the flux of hard gamma-rays produced via this mechanism has a characteristic variation with orbital phase that should be observable, and which is not expected from any other mechanism.Comment: To be published in Astroparticle Physics. 24 pages, 8 figure

Fermi acceleration in astrophysical jets

We consider the acceleration of energetic particles by Fermi processes (i.e., diffusive shock acceleration, second order Fermi acceleration, and gradual shear acceleration) in relativistic astrophysical jets, with particular attention given to recent progress in the field of viscous shear acceleration. We analyze the associated acceleration timescales and the resulting particle distributions, and discuss the relevance of these processes for the acceleration of charged particles in the jets of AGNs, GRBs and microquasars, showing that multi-component powerlaw-type particle distributions are likely to occur.Comment: 6 pages, one figure; based on talk at "The multimessenger approach to unidentified gamma-ray sources", Barcelona/Spain, July 2006; accepted for publication in Astrophysics and Space Scienc

Particle Acceleration in Pulsar Wind Nebulae: PIC modelling

We discuss the role of particle-in-cell (PIC) simulations in unveiling the origin of the emitting particles in PWNe. After describing the basics of the PIC technique, we summarize its implications for the quiescent and the flaring emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be emerging that, in addition to the standard scenario of particle acceleration via the Fermi process at the termination shock of the pulsar wind, magnetic reconnection in the wind, at the termination shock and in the Nebula plays a major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae" edited by D. Torres for Springer, based on the invited contributions to the workshop held in Sant Cugat (Barcelona), June 14-17, 201