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Register a new userReconnection in pulsar winds
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The spin-down power of a pulsar is thought to be carried away in an MHD wind in which, at least close to the star, the energy transport is dominated by Poynting flux. The pulsar drives a low-frequency wave in this wind, consisting of stripes of toroidal magnetic field of alternating polarity, propagating in a region around the equatorial plane. The current implied by this configuration falls off more slowly with radius than the number of charged particles available to carry it, so that the MHD picture must, at some point, fail. Recently, magnetic reconnection in such a structure has been shown to accelerate the wind significantly. This reduces the magnetic field in the comoving frame and, consequently, the required current, enabling the solution to extend to much larger radius. This scenario is discussed and, for the Crab Nebula, the range of validity of the MHD solution is compared with the radius at which the flow appears to terminate. For sufficiently high particle densities, it is shown that a low frequency entropy wave can propagate out to the termination point. In this case, the termination shock itself must be responsible for dissipating the wave.
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It is generally thought that most of the spin-down power of a pulsar is carried away in an MHD wind dominated by Poynting flux. In the case of an oblique rotator, a significant part of this energy can be considered to be in a low-frequency wave, consisting of stripes of toroidal magnetic field of alternating polarity, propagating in a region around the equatorial plane. Magnetic reconnection in such a structure has been proposed as a mechanism for transforming the Poynting flux into particle energy in the pulsar wind. We have re-examined this process and conclude that the wind accelerates significantly in the course of reconnection. This dilates the timescale over which the reconnection process operates, so that the wind requires a much larger distance than was previously thought in order to convert the Poynting flux to particle flux. In the case of the Crab, the wind is still Poynting-dominated at the radius at which a standing shock is inferred from observation. An estimate of the radius of the termination shock for other pulsars implies that all except the milli-second pulsars have Poynting-flux dominated winds all the way out to the shock front.
The magnetosphere of a rotating pulsar naturally develops a current sheet beyond the light cylinder (LC). Magnetic reconnection in this current sheet inevitably dissipates a nontrivial fraction of the pulsar spin-down power within a few LC radii. We develop a basic physical picture of reconnection in this environment and discuss its implications for the observed pulsed gamma-ray emission. We argue that reconnection proceeds in the plasmoid-dominated regime, via an hierarchical chain of multiple secondary islands/flux ropes. The inter-plasmoid reconnection layers are subject to strong synchrotron cooling, leading to significant plasma compression. Using the conditions of pressure balance across these current layers, the balance between the heating by magnetic energy dissipation and synchrotron cooling, and Amperes law, we obtain simple estimates for key parameters of the layers --- temperature, density, and layer thickness. In the comoving frame of the relativistic pulsar wind just outside of the equatorial current sheet, these basic parameters are uniquely determined by the strength of the reconnecting upstream magnetic field. For the case of the Crab pulsar, we find them to be of order 10 GeV, $10^{13} cm^{-3}$, and 10 cm, respectively. After accounting for the bulk Doppler boosting due to the pulsar wind, the synchrotron and inverse-Compton emission from the reconnecting current sheet can explain the observed pulsed high-energy (GeV) and VHE (~100 GeV) radiation, respectively. Also, we suggest that the rapid relative motions of the secondary plasmoids in the hierarchical chain may contribute to the production of the pulsar radio emission.
We review current theoretical ideas on pulsar winds and their surrounding nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the striped wind, together with models of magnetic dissipation are discussed. It is shown that the observational signature of this dissipation is likely to be point-like, rather than extended, and that pulsed emission may be produced. The possible pulse shapes and polarisation properties are described. Particle acceleration at the termination shock of the wind is discussed, and it is argued that two distinct mechanisms must be operating, with the first-order Fermi mechanism producing the high-energy electrons (above 1 TeV) and either magnetic annihilation or resonant absorption of ion cyclotron waves responsible for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of the nebula are discussed and compared with observation.
Neutron stars are among the most fascinating astrophysical sources, being characterized by strong gravity, densities about the nuclear one or even above, and huge magnetic fields. Their observational signatures can be extremely diverse across the electromagnetic spectrum, ranging from the periodic and low-frequency signals of radio pulsars, up to the abrupt high-energy gamma-ray flares of magnetars, where energies of ~10^46 erg are released in a few seconds. Fast-rotating and highly magnetized neutron stars are expected to launch powerful relativistic winds, whose interaction with the supernova remnants gives rise to the non-thermal emission of pulsar wind nebulae, which are known cosmic accelerators of electrons and positrons up to PeV energies. In the extreme cases of proto-magnetars (magnetic fields of ~10^15 G and millisecond periods), a similar mechanism is likely to provide a viable engine for the still mysterious gamma-ray bursts. The key ingredient in all these spectacular manifestations of neutron stars is the presence of strong magnetic fields in their constituent plasma. Here we will present recent updates of a couple of state-of-the-art numerical investigations by the high-energy astrophysics group in Arcetri: a comprehensive modeling of the steady-state axisymmetric structure of rotating magnetized neutron stars in general relativity, and dynamical 3-D MHD simulations of relativistic pulsar winds and their associated nebulae.
The cold-dense plasma is occasionally detected in the solar wind with in situ data, but the source of the cold-dense plasma remains illusive. Interchange reconnections (IRs) between closed fields and nearby open fields are well known to contribute to the formation of solar winds. We present a confined filament eruption associated with a puff-like coronal mass ejection (CME) on 2014 December 24. The filament underwent successive activations and finally erupted, due to continuous magnetic flux cancellations and emergences. The confined erupting filament showed a clear untwist motion, and most of the filament material fell back. During the eruption, some tiny blobs escaped from the confined filament body, along newly-formed open field lines rooted around the south end of the filament, and some bright plasma flowed from the north end of the filament to remote sites at nearby open fields. The newly-formed open field lines shifted southward with multiple branches. The puff-like CME also showed multiple bright fronts and a clear southward shift. All the results indicate an intermittent IR existed between closed fields of the confined erupting filament and nearby open fields, which released a portion of filament material (blobs) to form the puff-like CME. We suggest that the IR provides a possible source of cold-dense plasma in the solar wind.
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