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We study the magnetospheric evolution of a nonaccreting spinning black hole (BH) with an initially inclined split monopole magnetic field by means of 3D general relativistic magnetohydrodynamic simulations. This serves as a model for a neutron star (NS) collapse or a BH–NS merger remnant after the inherited magnetosphere has settled into a split monopole field creating a striped wind. We show that the initially inclined split monopolar current sheet aligns over time with the BH equatorial plane. The inclination angle evolves exponentially toward alignment, with an alignment timescale that is inversely proportional to the square of the BH angular velocity, where higher spin results in faster alignment. Furthermore, magnetic reconnection in the current sheet leads to exponential decay of event-horizon-penetrating magnetic flux with nearly the same timescale for all considered BH spins. In addition, we present relations for the BH mass and spin in terms of the period and alignment timescale of the striped wind. The explored scenario of a rotating, aligning, and reconnecting current sheet can potentially lead to multimessenger electromagnetic counterparts to a gravitational-wave event due to the acceleration of particles powering high-energy radiation, plasmoid mergers resulting in coherent radio signals, and pulsating emission due to the initial misalignment of the BH magnetosphere.

 

We study a relativistic collisionless electron–positron shock propagating into an unmagnetized ambient medium using 2D particle-in-cell simulations of unprecedented duration and size. The shock generates intermittent magnetic structures of increasingly larger size as the simulation progresses. Toward the end of our simulation, at around 26,000 plasma times, the magnetic coherence scale approachesλ∼ 100 plasma skin depths, both ahead and behind the shock front. We anticipate a continued growth ofλbeyond the time span of our simulation, as long as the shock accelerates particles to increasingly higher energies. The post-shock field is concentrated in localized patches, which maintain a local magnetic energy fractionε_B∼ 0.1. Particles randomly sampling the downstream fields spend most of their time in low field regions (ε_B≪ 0.1) but emit a large fraction of the synchrotron power in the localized patches with strong fields (ε_B∼ 0.1). Our results have important implications for models of gamma-ray burst afterglows.

 

Magnetic reconnection is often invoked as a source of high-energy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model—supported and benchmarked with large-scale three-dimensional kinetic particle-in-cell simulations in electron–positron plasmas—that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factorγ≳ 3σ(here,σis the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is$t_{\mathrm{acc}}\sim \gamma {\eta }_{\mathrm{rec}}^{-1}{\omega }_{\mathrm{c}}^{-1}\simeq 20\gamma {\omega }_{\mathrm{c}}^{-1}$, whereη_rec≃ 0.06 is the inflow speed in units of the speed of light andω_c=eB₀/mcis the gyrofrequency in the upstream magnetic field. They leave the region of active energization aftert_esc, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measuret_escin our simulations and find thatt_esc∼t_accforσ≳ few. This leads to a universal (i.e.,σ-independent) power-law spectrum${\mathit{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$for the particles undergoing active acceleration, and$\mathit{dN}/d\gamma \propto {\gamma }^{-2}$for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources.

 

Abstract Magnetic reconnection can power spectacular high-energy astrophysical phenomena by producing nonthermal energy distributions in highly magnetized regions around compact objects. By means of two-dimensional fully kinetic particle-in-cell (PIC) simulations, we investigate relativistic collisionless plasmoid-mediated reconnection in magnetically dominated pair plasmas with and without a guide field. In X-points, where diverging flows result in a nondiagonal thermal pressure tensor, a finite residence time for particles gives rise to a localized collisionless effective resistivity. Here, for the first time for relativistic reconnection in a fully developed plasmoid chain, we identify the mechanisms driving the nonideal electric field using a full Ohm law by means of a statistical analysis based on our PIC simulations. We show that the nonideal electric field is predominantly driven by gradients of nongyrotropic thermal pressures. We propose a kinetic physics motivated nonuniform effective resistivity model that is negligible on global scales and becomes significant only locally in X-points. It captures the properties of collisionless reconnection with the aim of mimicking its essentials in nonideal magnetohydrodynamic descriptions. This effective resistivity model provides a viable opportunity to design physically grounded global models for reconnection-powered high-energy emission.