Abstract
I outline, from a theoretical and somewhat personal perspective, significant features of Pulsar Wind Nebulae (PWNe) as Cosmic Accelerators. I pay special attention to the recently discovered gamma ray “flares” in the Crab Nebula’s emission, focusing on the possibility, raised by the observations, that the accelerating electric field exceeds the magnetic field, suggesting that reconnection in the persistent current layer (a “current sheet”) plays a significant role in the behavior of this well studied Pevatron. I address the present status of the termination shock model for the particle accelerator that converts the wind flow energy to the observed non-thermal particle spectra, concluding that it has a number of major difficulties related to the transverse magnetic geometry of the shock wave. I discuss recent work on the inferred pair outflow rates, which are in excess of those predicted by existing theories of pair creation, and use those results to point out that the consequent mass loading of the wind reduces the wind’s bulk flow 4-velocity to the point that dissipation of the magnetic field in a pulsar’s wind upstream of the termination shock is restored to life as a viable model for the solution of the “σ” problem. I discuss some suggestions that current starvation in the current flow supporting the structured (“striped”) upstream magnetic field perhaps induces a transition to superluminal wave propagation. I show that current starvation probably does not occur, because those currents are carried in the current sheet separating the stripes rather than in the stripes themselves.
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Notes
I prefer this somewhat old fashioned term for the surface or region where outflow energy converts to non-thermal heat, since as will be seen, the interpretation of the surface as a traditional MHD shock wave does not work, requiring one to endow the word “shock” with unusual properties, if it is to be used in this context.
See Carlson et al. (1994) for a review of earthquake dynamics.
The relativistic MHD wind model for pulsar spin down was introduced by Michel (1969).
For a review of such phenomena, see Paschmann et al. (2002). Of particular significance to pulsars is the fact that the field aligned currents that power narrow auroral arcs consist of precipitating electron beams launched from the reconnection region in the distant magnetotail and counterstreaming ions launched from the planetary atmosphere.
This wrinkled current sheet, frozen into the wind, is the continuation of the sheet separating the closed and open zones interior to the light cylinder, as is apparent in Fig. 9.
I discuss here only the young high voltage nebulae, where pressure due to motion through the interstellar medium and to reverse shocks from the surrounding supernova remnant do not have major effects on the structure—these young systems are the most useful for probing the particle acceleration physics and the plasma properties.
A criticism that also can be leveled at the lighthouse model for high energy pulsed emission—while more popular, it’s not clear that it is unique.
This estimate assumes the fluctuating magnetic field causing scattering and the overall magnetic field causing the accelerated, radiating motion (which includes the fluctuating field) have the same magnitude, as is assumed in models which assume diffusion at the Bohm rate.
That rate assumes only that the radius of curvature of a particle’s orbit be the relativistic Larmor radius, not that particles actually complete full cyclotron orbits.
The picture I favor is close to Coroniti’s model of dissipation in the wind (Coroniti 1990). The striped magnetic structure launched from the pulsar (see Fig. 10) decays interior to the TWS, probably at r∼0.1R TWS (Arons, in preparation), which is possible in the highly mass loaded wind in the Crab and other young pulsars.
The focussing of particles toward the current sheet’s center is a rediscovery of the focussing principle long known to accelerator physicists (e.g. Courant and Snyder 1958). That mechanism can be used as a means to confine particles over distances ∼107 R L =sheet spacing, a length required if voltages of a PetaVolt or more are to be accessed by the linear accelerator, only if the fields are smooth and properly “designed” to a degree extraordinary for an astrophysical configuration, especially when subject to the uncontrolled macroscopic gradients introduced by the instabilities of the current sheet. The Sironi and Spitkovsky simulations of the striped configuration show that when the formal Larmor radius of the particles in the sheets becomes somewhat larger than the spacing, the acceleration saturates, because particles have finite Larmor radius drifts out of the sheet’s core in the “messy” fields of the unstable layer.
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Acknowledgements
The work described here has been supported by NSF grant AST-0507813, NASA grants NNG06GJI08G and NNX09AU05G and DOE grant DE-FC02-06ER41453. I have benefited from discussions with E. Amato, N. Bucciantini, C. Max, R. Romani, J. Scargle, A. Spitkovsky, A.Timokhin and D. Uzdensky.
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With apologies to Geoffrey Chaucer and Margaret Atwood.
Collaborators, none of whom should be held responsible for the content of this paper: D. Alsop, E. Amato, D. Backer, P. Chang, N. Bucciantini, B. Gaensler, Y. Gallant, V. Kaspi, A.B. Langdon, C. Max, E. Quataert, A. Spitkovsky, M. Tavani, A. Timokhin.
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Arons, J. Pulsar Wind Nebulae as Cosmic Pevatrons: A Current Sheet’s Tale. Space Sci Rev 173, 341–367 (2012). https://doi.org/10.1007/s11214-012-9885-1
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DOI: https://doi.org/10.1007/s11214-012-9885-1