On Pair Production in the Crab Pulsar

We consider the widespread assumption that coherent pulsar radio emission is based on extended pair production leading to plasma densities highly exceeding the Goldreich-Julian density. We show as an example that the observed low frequency (160 MHz) emission of the Crab pulsar is incompatible to the model of extended pair production. Our results rule out significant pair production if a plasma process is responsible for coherence and the radio emission originates from inside the light cylinder.Comment: accepted for publication in ApJ Letters; 4 pages, no figure

Giant Radio Pulses from the Crab Pulsar

Individual giant radio pulses (GRPs) from the Crab pulsar last only a few microseconds. However, during that time they rank among the brightest objects in the radio sky reaching peak flux densities of up to 1500 Jy even at high radio frequencies. Our observations show that GRPs can be found in all phases of ordinary radio emission including the two high frequency components (HFCs) visible only between 5 and 9 GHz (Moffett & Hankins, 1996). This leads us to believe that there is no difference in the emission mechanism of the main pulse (MP), inter pulse (IP) and HFCs. High resolution dynamic spectra from our recent observations of giant pulses with the Effelsberg telescope at a center frequency of 8.35 GHz show distinct spectral maxima within our observational bandwidth of 500 MHz for individual pulses. Their narrow band components appear to be brighter at higher frequencies (8.6 GHz) than at lower ones (8.1 GHz). Moreover, there is an evidence for spectral evolution within and between those structures. High frequency features occur earlier than low frequency ones. Strong plasma turbulence might be a feasible mechanism for the creation of the high energy densities of ~6.7 x 10^4 erg cm^-3 and brightness temperatures of 10^31 K.Comment: accepted by Advances in Space Research, to appear in the 35th COSPAR assembly proceeding

Shear-Flow Driven Current Filamentation: Two-Dimensional Magnetohydrodynamic Simulations

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional Magnetohydrodynamic (MHD)-simulations. This situation is typical for astrophysical systems like jets, the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.Comment: 32 Pages, 13 Figures. accepted, to appear in Physics of Plasmas (049012

On the rho invariant for manifolds with boundary

This article is a follow up of the previous article of the authors on the analytic surgery of eta- and rho-invariants. We investigate in detail the (Atiyah-Patodi-Singer)-rho-invariant for manifolds with boundary. First we generalize the cut-and-paste formula to arbitrary boundary conditions. A priori the rho-invariant is an invariant of the Riemannian structure and a representation of the fundamental group. We show, however, that the dependence on the metric is only very mild: it is independent of the metric in the interior and the dependence on the metric on the boundary is only up to its pseudo--isotopy class. Furthermore, we show that this cannot be improved: we give explicit examples and a theoretical argument that different metrics on the boundary in general give rise to different rho-invariants. Theoretically, this follows from an interpretation of the exponentiated rho-invariant as a covariantly constant section of a determinant bundle over a certain moduli space of flat connections and Riemannian metrics on the boundary. Finally we extend to manifolds with boundary the results of Farber-Levine-Weinberger concerning the homotopy invariance of the rho-invariant and spectral flow of the odd signature operator.Comment: Published by Algebraic and Geometric Topology at http://www.maths.warwick.ac.uk/agt/AGTVol3/agt-3-22.abs.htm