Pulsars, transients and relativistic astrophysics

Abstract:

The stratification of entropy and the stratification of angular momentum are closely analogous. The analogy has been developed for a number of different problems in the fluid literature, but its consequences for the behavior of turbulent accretion disks are less appreciated. Of particular interest is the behavior of disks in which angular momentum transport is controlled by convection, and heat transport by dynamical turbulence. In both instances we argue that the transport must proceed "backward" relative to the sense one would expect from a simple enhanced diffusion approach. Reversed angular momentum transport has already been seen in numerical simulations; contragradient thermal diffusion should be amenable to numerical verification as well. These arguments also bear on the observed nonlinear local stability of isolated Keplerian disks: locally generated turbulence in such a disk would require simultaneous inward and outward angular momentum transport, which is, of course, impossible. We also describe a diffusive instability that is the entropy analogue to the magnetorotational instability. It affects thermally stratified layers when Coulomb conduction and a weak magnetic field are present. The plasma must be sufficiently dilute that heat is channeled only along field lines. The criterion for convective instability goes from one of upwardly decreasing entropy to one of upwardly decreasing temperature. The instability remains formally viable if radiative heat transport is also present, but the equilibrium is much more unstable if Coulomb transport is dominant. In that case, the maximum growth rate is of the order of the inverse sound crossing time, independent of the thermal conductivity. The indifference of the growth rate to the conduction coefficient, its simple dynamical scaling, and the replacement in the stability criterion of a conserved quantity (entropy) gradient by a free energy (temperature) gradient are properties similar to those exhibited by the magnetorotational instability.

Abstract:

Low-radio-frequency observations played a remarkable role in the early days of radio astronomy; however, in the subsequent three or four decades their usefulness has largely been in terms of the finding-frequency of surveys. Recent technical innovation at the VLA has meant that spatially well-resolved imaging at low frequencies is now possible. Such imaging is essential to understanding the relationship between the hotspot and lobe emission in classical double radio sources, for example. We here present new images of 3C radio sources at 74 MHz and 330 MHz and discuss their implications.