Particle acceleration in relativistic magnetic flux-merging events

Abstract

(Received xx; revised xx; accepted xx)Using analytical and numerical methods (fluid and particle-in-cell simulations) we study a number of model problems involving merger of magnetic flux tubes in relativistic magnetically-dominated plasma. Mergers of current-carrying flux tubes (exemplified by the two dimensional "ABC" structures) and zero total current magnetic flux tubes are considered. In all cases regimes of spontaneous and driven evolution are investigated.We identify two stages of particle acceleration during flux mergers: (i) fast explosive prompt X-point collapse and (ii) ensuing island merger. The fastest acceleration occurs during the initial catastrophic X-point collapse, with the reconnection electric field of the order of the magnetic field. During the X-point collapse particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization σ. For plasma magnetization σ 10 2 the spectrum power law index is p > 2; in this case the maximal energy depends linearly on the size of the reconnecting islands. For higher magnetization, σ 10 2 , the spectra are hard, p < 2, yet the maximal energy γ max can still exceed the average magnetic energy per particle, ∼ σ, by orders of magnitude (if p is not too close to unity). The X-point collapse stage is followed by magnetic island merger that dissipates a large fraction of the initial magnetic energy in a regime of forced magnetic reconnection, further accelerating the particles, but proceeds at a slower reconnection rate.

Kinetic Study of Radiation-Reaction-Limited Particle Acceleration During the Relaxation of Unstable Force-Free Equilibria

“…The non-thermal particle fraction increases with the magnetization. Similar studies have also been carried out independently by Lyutikov et al (2016) recently.…”

“…Another possibility involves the polar jet, which has relatively high magnetization and is kink unstablesuggesting that it possesses free magnetic energy. Recently, Lyutikov et al (2016) also proposed that the intermediate latitude post shock flow can produce regions with high magnetization and current carrying filaments, resembling the flux tubes in force-free configurations.…”

“…After the main current layers dissolve, the system enters a turbulent relaxation process, particles diffuse in momentum space, gradually forming a steeper power law connecting the high energy bump with the thermal Maxwellian. There's also an overall heating of the background plasma (see also, Lyutikov et al 2016). At the end of the simulation, the power law has a spectral index of ∼2.9, and the non-thermal particles comprise 12% in number and 36% in energy.…”

Fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited

“…In addition, relativistic reconnection could be important in pair plasmas (see Kagan et al 2015, and references therein), and in particular it should be an ingredient in the modeling of Pulsar Wind Nebulae in order to explain dissipation of magnetic energy in the so-called striped wind region (Coroniti 1990;Kirk & Skjaeraasen 2003;Pétri & Lyubarsky 2007;Amano & Kirk 2013;Takamoto, Pétri & Baty 2015), and the sudden gamma-ray flares observed in the Crab nebula, probably originated in the vicinities of the wind termination shock (Tavani 2013;Baty, Petri & Zenitani 2013;Cerutti et al 2014;Lyutikov et al 2016). Promising results are starting to come from two-fluid relativistic MHD simulations (Barkov et al 2014;Barkov & Komissarov 2016), as a viable alternative to fully kinetic particle-in-cell (PIC) methods, which are more computationally demanding.…”

The Effect of Cooling on Particle Trajectories and Acceleration in Relativistic Magnetic Reconnection

“…These simulations, which focus on the pair-plasma case which is easier to simulate and is directly applicable to the Crab Nebula, have shown that relativistic reconnection can produce hard non-thermal tails of highly accelerated particles that are typically interpreted as power laws (Zenitani & Hoshino 2001, 2005, 2007Zenitani & Hesse 2008;Jaroschek et al 2004Jaroschek et al , 2008Bessho & Bhattacharjee 2005, 2007, 2012Daughton & Karimabadi 2007;Lyubarsky & Liverts 2008;Liu et al 2011;Cerutti et al 2012bCerutti et al , 2013Cerutti et al , 2014Werner et al 2016;Sironi & Spitkovsky 2014;Sironi et al 2016;Guo et al 2014Guo et al , 2015Liu et al 2015;Yuan et al 2016;Lyutikov et al 2016;Kagan et al 2016). In general, the physics of reconnection is similar in two and three dimensions (Sironi & Spitkovsky 2014) unless the initial reconnection structure is inherently three-dimensional, justifying the use of two-dimensional simulations which reduce computational cost and allow larger-scale simulations to be carried out.…”