Generation of Crab Nebulae Wisps by Plasma Drift Instability

Abstract

A new mechanism of wisp formation in the Crab Nebula, without recourse to a shock wave, is presented. The mechanism is based on the plasma processes taking place in the relativistic electron-positron plasma, in particular, on excitation of the drift instability in the slightly inhomogeneous magnetic Ðeld of the nebula. The magnetic Ðeld geometry is assumed to be untwisting spiralÈlike, concentrated around the rotational equatorial plane of the pulsar. We infer that the energy is transported from the pulsar to the nebula not only by the Poynting Ñux but also by the kinetic energy of the particles moving along the Ðeld lines. We consider a speciÐc relativistic electron-positron plasma with Lorentz factor penc B D 10 etrated by an ion beam with It is shown that in such a plasma, taking into account the drift, c b D 106. the generation of low-frequency waves propagating transversely to the magnetic Ðeld is possible. In this way the excited Ðeld causes additional alteration of the magnetic Ðeld geometry. The higher lumi-B r nosity of certain spots is explained by the reorientation of the direction of motion of the synchrotron radiation source (Larmor circle) relative to the observer. The sizes, variability of luminosity, spatial asymmetry, polarization, and transfer velocity, i.e., the main properties of the wisps, are explained. Some predictions are also made.

The Crab Nebula’s Moving Wisps in Radio

“…The precise mechanism of wisp formation, however, is not yet well understood (Bietenholz & Kronberg 1992;Chedia et al 1997;Hester 1998;Arons 1998;Begelman 1999). Also, while the radio spectral index is very uniform over the nebula (Bietenholz et al 1997), suggesting a single and presumably central source for all the radio emitting electrons, their origin in the shock near the pulsar is not yet understood (see e.g., Atoyan 1999;Arons 1998).…”

VLBI imaging of a flare in the Crab nebula: more than just a spot

“…The exact mechanism by which the spin-down energy is released, transported, and dissipated remains poorly understood (cf., Chedia et al 1997;Hester et al 1998;Begelman 1999;Lyutikov 2003;Spitkovsky & Arons 2004;Komissarov & Lyubarsky 2004) -with shocks, plasma instabilities, pair acceleration, Poynting flux, and magnetic field all suggested to affect the evolution of the wind.…”

The Crab Nebula’s Wisps in Radio and Optical

“…This outflow from the pulsar is ultimately responsible for the synchrotron emission we observe from the body of the nebula. The process by which this prodigious transfer of energy happens, however, is not yet well understood (see e.g., Lyubarsky 2003;Lyutikov 2003;Spitkovsky & Arons, 2004;Arons 2002;Begelman 1999;Hester 1998;Chedia et al 1997). The energy spectrum of the particles in the nebula can be deduced from the synchrotron emission spectrum, which has a power-law form extending over many decades of frequency, implying also power-law energy spectra of the electrons.…”

“…This outflow from the pulsar is ultimately responsible for the synchrotron emission we observe from the body of the nebula. The process by which this prodigious transfer of energy happens, however, is not yet well understood (see e.g., Lyubarsky 2003;Lyutikov 2003;Spitkovsky & Arons, 2004;Arons 2002;Begelman 1999;Hester 1998;Chedia et al 1997).…”