Relativistic magnetic reconnection at X-type neutral points

Relativistic effects in the oscillatory damping of magnetic disturbances near two-dimensional X-points are investigated. By taking into account displacement current, we study new features of extremely magnetized systems, in which the Alfv\'en velocity is almost the speed of light. The frequencies of the least-damped mode are calculated using linearized relativistic MHD equations for wide ranges of the Lundquist number S and the magnetization parameter $\sigma$. These timescales approach constant values in the large resistive limit: the oscillation time becomes a few times the light crossing time, irrespective of $\sigma$, and the decay time is proportional to $\sigma$ and therefore is longer for a highly magnetized system.Comment: 6 pages, 4 figure

Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds

By means of two- and three-dimensional particle-in-cell simulations, we investigate the process of driven magnetic reconnection at the termination shock of relativistic striped flows. In pulsar winds and in magnetar-powered relativistic jets, the flow consists of stripes of alternating magnetic field polarity, separated by current sheets of hot plasma. At the wind termination shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated flows), shock-driven reconnection transfers all the magnetic energy of alternating fields to the particles, whose average Lorentz factor increases by a factor of sigma with respect to the pre-shock value. In the limit lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind, the post-shock particle spectrum approaches a flat power-law tail with slope around -1.5, populated by particles accelerated by the reconnection electric field. The presence of a current-aligned "guide" magnetic field suppresses the acceleration of particles only when the guide field is stronger than the alternating component. Our findings place important constraints on the models of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special issue of Computational Science and Discovery on selected research from the 22nd International Conference on Numerical Simulation of Plasma

On the relativistic magnetic reconnection

Reconnection of the magnetic lines of force is considered in case the magnetic energy exceeds the rest energy of the matter. It is shown that the classical Sweet-Parker and Petschek models are generalized straightforwardly to this case and the reconnection rate may be estimated by substituting the Alfven velocity in the classical formulas by the speed of light. The outflow velocity in the Sweet-Parker configuration is mildly relativistic. In the Petschek configuration, the outflow velocity is ultrarelativistic whereas the angle between the slow shocks is very small. Due to the strong compression, the plasma outflow in the Petschek configuration may become strongly magnetized if the reconnecting fields are not exactly antiparallel.Comment: Accepted by MNRA

Hybridization-induced superconductivity from the electron repulsion on a tetramer lattice having a disconnected Fermi surface

Plaquette lattices with each unit cell containing multiple atoms are good candidates for disconnected Fermi surfaces, which are shown by Kuroki and Arita to be favorable for spin-flucutation mediated superconductivity from electron repulsion. Here we find an interesting example in a tetramer lattice where the structure within each unit cell dominates the nodal structure of the gap function. We trace its reason to the way in which a Cooper pair is formed across the hybridized molecular orbitals, where we still end up with a T_c much higher than usual.Comment: 4 pages, 6 figure

Highly magnetized region in pulsar wind nebulae and origin of the Crab gamma-ray flares

The recently discovered gamma-ray flares from the Crab nebula are generally attributed to the magnetic energy release in a highly magnetized region within the nebula. I argue that such a region naturally arises in the polar region of the inner nebula. In pulsar winds, efficient dissipation of the Poynting flux into the plasma energy occur only in the equatorial belt where the energy is predominantly transferred by alternating fields. At high latitudes, the pulsar wind remains highly magnetized therefore the termination shock in the polar region is weak and the postshock flow remains relativistic. I study the structure of this flow and show that the flow at first expands and decelerates and then it converges and accelerates. In the converging part of the flow, the kink instability triggers the magnetic dissipation. The energy release zone occurs at the base of the observed jet. A specific turbulence of relativistically shrinking magnetic loops efficiently accelerates particles so that the synchrotron emission in the hundreds MeV band, both persistent and flaring, comes from this site.Comment: Submitted to MNRA

Magnetic Reconnection in Extreme Astrophysical Environments

Magnetic reconnection is a basic plasma process of dramatic rearrangement of magnetic topology, often leading to a violent release of magnetic energy. It is important in magnetic fusion and in space and solar physics --- areas that have so far provided the context for most of reconnection research. Importantly, these environments consist just of electrons and ions and the dissipated energy always stays with the plasma. In contrast, in this paper I introduce a new direction of research, motivated by several important problems in high-energy astrophysics --- reconnection in high energy density (HED) radiative plasmas, where radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. I identify the key processes distinguishing HED reconnection: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and Compton resistivity); and, at the most extreme end, QED effects, including pair creation. I then discuss the main astrophysical applications --- situations with magnetar-strength fields (exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares and magnetically-powered central engines and jets of GRBs. Here, magnetic energy density is so high that its dissipation heats the plasma to MeV temperatures. Electron-positron pairs are then copiously produced, making the reconnection layer highly collisional and dressing it in a thick pair coat that traps radiation. The pressure is dominated by radiation and pairs. Yet, radiation diffusion across the layer may be faster than the global Alfv\'en transit time; then, radiative cooling governs the thermodynamics and reconnection becomes a radiative transfer problem, greatly affected by the ultra-strong magnetic field. This overall picture is very different from our traditional picture of reconnection and thus represents a new frontier in reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic reconnection). Article is based on an invited review talk at the Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA; February 8-12, 2010). 30 pages, no figure

The theory of pulsar winds and nebulae

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.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and pulsars, 40 years after the discovery", ed W.Becke

Fractal Reconnection in Solar and Stellar Environments

Recent space based observations of the Sun revealed that magnetic reconnection is ubiquitous in the solar atmosphere, ranging from small scale reconnection (observed as nanoflares) to large scale one (observed as long duration flares or giant arcades). Often the magnetic reconnection events are associated with mass ejections or jets, which seem to be closely related to multiple plasmoid ejections from fractal current sheet. The bursty radio and hard X-ray emissions from flares also suggest the fractal reconnection and associated particle acceleration. We shall discuss recent observations and theories related to the plasmoid-induced-reconnection and the fractal reconnection in solar flares, and their implication to reconnection physics and particle acceleration. Recent findings of many superflares on solar type stars that has extended the applicability of the fractal reconnection model of solar flares to much a wider parameter space suitable for stellar flares are also discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016), 33 pages, 18 figure

Pulsed radiation from neutron star winds

The radiation of a pulsar wind is computed assuming that at roughly 10 to 100 light cylinder radii from the star, magnetic energy is dissipated into particle energy. The synchrotron emission of heated particles appears periodic, with, in general, both a pulse and an interpulse. The predicted spacing agrees well with the Crab and Vela pulse profiles.Using parameters appropriate for the Crab pulsar (magnetisation parameter at the light cylinder $\sigma=6\times10^4$, Lorentz factor $\Gamma=250$) agreement is found with the observed total pulsed luminosity. This suggests that the high-energy pulses from young pulsars originate not in the corotating magnetosphere within the light cylinder (as in all other models) but from the radially directed wind well outside it.Comment: 4 pages, 2 figures, accepted for publication in A&A Letter