Pulsars, transients and relativistic astrophysics

Abstract:

Results from Chandra observations of the two nearest, powerful radio galaxies are summarised.

Abstract:

We discuss the detection of variable circular. polarization (CP) in several scintillating radio sources: PKS 1519-273, PKS 0405-385 and Sgr A*. The CP in PKS 1519-273 is strong, and varies on a timescale of hours to days at frequencies between 1.4 and 8.6 GHz. We argue that the variability is due to scintillation of a compact (15 - 35 muas) component of the source with -3.8 +/- 0.4% circular polarization at 4.8 GHz. We find that no simple model can account for the magnitude and spectrum of the circular polarization in this source. We also interpret the variable CP observed in PKS 0405-385 in terms of scintillation, although the behaviour of the CP is more complicated, and changes character between scintillation epochs. We also discuss CP in Sgr A*, which is found to be variable at 2.5 and 1.4 GHz on a timescale similar to 7 days. The fractional degree of variability in circular polarization greatly exceeds that in total intensity in all three sources.

Abstract:

We have measured Stokes IUQV of several rapidly variable and gamma-bright blazars and detected variable optical circular polarization, occasionally exceeding 1%, in 3C 279. We discuss possible origins for significant amounts of circular polarization (CP) in blazars and suggest direct CP from particles radiating in strong magnetic fields or anisotropies in the particle distribution function as the most plausible explanations.

Abstract:

We review recent relativistic hydrodynamic simulations of jets, and their interpretation in terms of the results from linear stability analysis. These studies show that, interpreted naively, the distribution of synchrotron intensity will in general be a poor guide to the physical state (density and pressure) of the underlying flow, and that even if the physical state can be inferred, it, in turn, may prove to be a poor guide to the source dynamics, in terms of the transport of energy and momentum from the central engine. However, we demonstrate that an interplay of simulation and linear stability analysis provides a powerful tool for elucidating the nature and character of structures that jets may sustain. From such studies we can explain the complex behaviour of observed jets, which manifest both stationary and propagating structures, without recourse to ad hoc macroscopic disturbances. This provides a framework for the interpretation of multi-epoch total intensity data wherein an understanding of the character of individual flow features will allow the effects of physical state and dynamics to be deconvolved.

Abstract:

The standard paradigm for extragalactic radio sources assumes that a description in terms of single-fluid MHD, adiabatic work, and radiation losses should provide an adequate model for the observed radiation and morphology. However this approach omits several physical elements that are now standard in other branches of laboratory, space, and astrophysical plasma physics, and we examine whether current observations of extragalactic radio sources motivate extensions to the standard paradigm. Supernova remnants are analogous to extragalactic radio sources in producing synchrotron radiation from radio to X-ray wavelengths, and electron energies as high as 10(14) eV axe required in some cases. Diffusive shock acceleration can generate these electrons from a mildly relativistic population, but the 'injection' problem of prior acceleration of background electrons to mildly relativistic energies has remained. Here we describe the 'bootstrap' solution of the electron injection problem, by which a shock-reflected proton population drives collective plasma instabilities, which in turn excite waves in the plasma which accelerate thermal electrons to mildly relativistic energies. The physical processes incorporated in this solution lie outside the standard paradigm for extragalactic radio sources, and we discuss their potential relevance.