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Register a new userThe theory of pulsar winds and nebulae
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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.
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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.
This paper deals with the Crab Nebula problem to suggest that particle acceleration takes place not only at the inner shock but also over a larger region in the nebula with disordered magnetic field. Kennel and Cornoniti (1984) constructed a spherically symmetric model of the Crab Nebula and concluded that the pulsar wind which excites the nebular is kinetic-energy dominant (KED) because the nebula flow induced by KED wind is favorable to explain the nebula spectrum and expansion speed. This is true even with new Chandra observation, which provides newly the spatially resolved spectra. We have shown below with 3D modelling and the Chandra image that pure toroidal magnetic field and KED wind are incompatible with the Chandra observation.
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.
Pulsar Wind Nebulae (PWNe) are bubbles or relativistic plasma that form when the pulsar wind is confined by the SNR or the ISM. Recent observations have shown a richness of emission features that has driven a renewed interest in the theoretical modeling of these objects. In recent years a MHD paradigm has been developed, capable of reproducing almost all of the observed properties of PWNe, shedding new light on many old issues. Given that PWNe are perhaps the nearest systems where processes related to relativistic dynamics can be investigated with high accuracy, a reliable model of their behavior is paramount for a correct understanding of high energy astrophysics in general. I will review the present status of MHD models: what are the key ingredients, their successes, and open questions that still need further investigation.
The main goal of our present work is to provide, for the first time, a simple computational tool that can be used to compute the brightness, the spectral index, the polarization, the time variability and the spectrum of the non-thermal light (both synchrotron and inverse Compton, IC) associated with the plasma dynamics resulting from given relativistic magnetohydrodynamics (RMHD) simulations. The proposed method is quite general, and can be applied to any scheme for RMHD and to all non-thermal emitting sources, e.g. pulsar wind nebulae (PWNe), and in particular to the Crab Nebula (CN) as in the present proceeding. Here only the linear optical and X-ray polarization that characterizes the PWNe synchrotron emission is analyzed in order to infer information on the inner bulk flow structure, to provide a direct investigation of the magnetic field configuration, in particular the presence and the strength of a poloidal component, and to understand the origin of some emitting features, such as the knot, whose origins are still uncertain. The inverse Compton radiation is examined to disentangle the different contributions to radiation from the magnetic field and the particle energy distribution function, and to search for a possible hadronic component in the emitting PWN, and thus for the presence of ions in the wind. If hadronic radiation was found in a PWN, young supernova remnants would provide a natural birth-place of the cosmic-rays (CRs) up to the so-called knee in the CR spectrum.
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