Observational Constraints on Pulsar Wind Theories

Two-dimensional, relativistic, MHD simulations of pulsar-wind powered nebulae provide strong constraints on the properties of the winds themselves. In particular, they confirm that Poynting flux must be converted into particle energy close to or inside the termination shock front, emphasising the puzzle known as the $\sigma$ paradox. To distinguish between the different possible resolutions of this paradox, additional observational constraints are required. In this paper, I briefly discuss two recents developments in this respect: the modelling of high time-resolution optical polarimetry of the Crab pulsar, and the detection of the pulsar/Be star binary PSR 1259-63 in TeV energy gamma-rays.Comment: Paper presented at the workshop on "Stellar End Products", Granada 13-15 April 200

Reality and the Meaning of Evil: On the Moral Causality of Signs

ABSTRACT: “Evil is really only a privation.” This philosophical commonplace reflects an ancient solution to the problem of theodicy in one of its dimensions: is evil of such a nature that it must have God as its author? Stated in this particular way, it also reflects the commonplace identification of the real with natural being—the realm of what exists independently of human thought and perspectives—as opposed to all that is termed, by comparison, “merely subjective” and “unreal”. If we stick with this way of construing the meaning of “reality”, then by the excellent arguments of the tradition we are also stuck with defending the sufficiency of privation as a response to what evil “really is”. In this article, we argue against both ways of being stuck. We argue, first, that a one-sided focus upon the being of nature blocks an adequate understanding of the world we actually live in: the semiotically constituted lifeworld that is the proper locus of human realities, including moral evil. We argue, second, that the positivity of moral evil consists not only, nor even primarily, in the positivity of “action” as such, but in structures of objectivity engendered by creative reason that oppose the due end, and that involve a specific genus of pure object which we call a mystical daydream. Like any objects, these objects are communicable and formative in relation to the lifeworld, within which they in turn engender further interpretants for both those who do and those who suffer evil, thanks to the causality of signs

Superluminal Waves and the Structure of Pulsar Wind Termination Shocks

The termination shock of a pulsar wind is located roughly where the ram pressure matches that of the surrounding medium. Downstream of the shock, MHD models of the diffuse nebular emission suggest the plasma is weakly magnetized. However, the transition from a Poynting-dominated MHD wind to a particle-dominated flow is not well understood. We discuss a solution of this "sigma problem" in which a striped wind converts into a strong, superluminal electromagnetic wave. This mode slows down as it propagates radially, and its ram pressure tends to a constant value at large radius, a property we use to match the solution to the surrounding nebula. The wave thus forms a pre-cursor to the termination shock, which occurs at the point where the wave dissipates. Possible damping and dissipation mechanisms are discussed qualitatively.Comment: 4 pages, 2 figures. Proceedings of the "Electromagnetic Radiation from Pulsars and Magnetars" conference, April 24-27, 2012, Zielona Gora, Polan

Rapid cosmic-ray acceleration at perpendicular shocks in supernova remnants

Perpendicular shocks are shown to be rapid particle accelerators that perform optimally when the ratio $u_{\rm s}$ of the shock speed to the particle speed roughly equals the ratio $1/\eta$ of the scattering rate to the gyro frequency. We use analytical methods and Monte-Carlo simulations to solve the kinetic equation that governs the anisotropy generated at these shocks, and find, for $\eta u_{\rm s}\approx1$, that the spectral index softens by unity and the acceleration time increases by a factor of two compared to the standard result of diffusive shock acceleration theory. These results provide a theoretical basis for the thirty-year-old conjecture that a supernova exploding into the wind of a Wolf-Rayet star may accelerate protons to an energy exceeding $10^{15}\,$eV.Comment: 12 pages, 2 figures, accepted for publication in Ap

The "sigma" problem of the Crab pulsar wind

The conversion of the Crab pulsar wind from one dominated by Poynting flux close to the star to one dominated by particle-born energy at the termination shock is considered. The idea put forward by Coroniti (1990) and criticised by Lyubarsky & Kirk (2001) that reconnection in a striped wind is responsible, is generalised to include faster prescriptions for the a priori unknown dissipation rate. Strong acceleration of the wind is confirmed, and the higher dissipation rates imply complete conversion of Poynting flux into particle-born flux within the unshocked wind.Comment: 4 pages, to appear in "Young Neutron Stars and Their Environments" (IAU Symposium 218, ASP Conference Proceedings), eds F. Camilo and B. M. Gaensle

Large-Scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence

We calculate the large-scale cosmic-ray (CR) anisotropies predicted for a range of Goldreich-Sridhar (GS) and isotropic models of interstellar turbulence, and compare them with IceTop data. In general, the predicted CR anisotropy is not a pure dipole; the cold spots reported at 400 TeV and 2 PeV are consistent with a GS model that contains a smooth deficit of parallel-propagating waves and a broad resonance function, though some other possibilities cannot, as yet, be ruled out. In particular, isotropic fast magnetosonic wave turbulence can match the observations at high energy, but cannot accommodate an energy dependence in the shape of the CR anisotropy. Our findings suggest that improved data on the large-scale CR anisotropy could provide a valuable probe of the properties - notably the power-spectrum - of the interstellar turbulence within a few tens of parsecs from Earth.Comment: 20 pages, 12 figures. Published in The Astrophysical Journa

Inductive spikes and gamma-ray flares from the Crab Nebula

The ~400 MeV flaring emission from the Crab Nebula is naturally explained as the result of an abrupt reduction in the mass-loading of the pulsar wind. Very few particles are then available to carry the current required to maintain wave activity, causing them to achieve high Lorentz factors. When they penetrate the Nebula, a tightly beamed, high luminosity burst of hard gamma-rays results, with characteristics similar to the observed flares. This mechanism may operate in other powerful pulsars, such as J0537-6910 (PWN N 157B), B0540-69, B1957+20 and J0205+6449 (3C 58).Comment: Talk presented at the 7th Fermi Symposium, Garmisch-Partenkirchen, October 201