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Abstract The cores of pulsars are expected to become superconducting soon after birth. The transition to type-II superconductivity is associated with the bunching of magnetic field lines into discrete superconducting flux tubes which possess enormous tension. The coupling of the crust to the flux tubes implies the existence of huge tangential magnetic fields at the crust–core interface. We show that the transition to superconductivity triggers a highly nonlinear response in the Hall drift of the crustal magnetic field, an effect which was neglected in previous numerical modeling. We argue that at the time of the phase transition giant Hall waves are launched from the crust–core interface toward the surface. Our models show that if the crust contains a multipolar magnetic field ∼10¹³G, the amplitude of the Hall waves is ∼10¹⁵G. The elastic deformation of the lattice is included in our models, which allows us to track the time-dependent shear stresses everywhere in the crust. The simulations indicate that the Hall waves may be strong enough to break the crust, and could cause star quakes which trigger rotation glitches and changes in the radio pulse profile. The Hall waves also couple to slow magnetospheric changes, which cause anomalous braking indices. The emission of the giant Hall waves from the crust–core interface facilitates fast flux expulsion from the superconducting core, provided that the flux tubes in the core are themselves sufficiently mobile. For all of the flux tube mobility prescriptions implemented in this work, the core approaches the Meissner state withB= 0 at late times. 

Abstract We perform the first magnetohydrodynamic simulation tracking the magnetosphere of a collapsing magnetar. The collapse is expected for massive rotating magnetars formed in merger events and may occur many hours after the merger. Our simulation suggests a novel mechanism for a gamma-ray burst (GRB), which is uncollimated and forms a delayed high-energy counterpart of the merger gravitational waves. The simulation shows that the collapse launches an outgoing magnetospheric shock, and a hot magnetized outflow forms behind the shock. The outflow is baryon free and uncollimated, and its power peaks on a millisecond timescale. Then, the outflow becomes modulated by the ring-down of the nascent black hole, imprinting its kilohertz quasi-normal modes on the GRB tail. 

Abstract Tayler instability of toroidal magnetic fieldsB_ϕis broadly invoked as a trigger for turbulence and angular momentum transport in stars. This paper presents a systematic revision of the linear stability analysis for a rotating, magnetized, and stably stratified star. For plausible configurations ofB_ϕ, instability requires diffusive processes: viscosity, magnetic diffusivity, or thermal/compositional diffusion. Our results reveal a new physical picture, demonstrating how different diffusive effects independently trigger instability of two types of waves in the rotating star: magnetostrophic waves and inertial waves. It develops via overstability of the waves, whose growth rate sharply peaks at some characteristic wavenumbers. We determine instability conditions for each wave branch and find the characteristic wavenumbers. The results are qualitatively different for stars with magnetic Prandtl numberPm≪ 1 (e.g., the Sun) andPm≫ 1 (e.g., protoneutron stars). The parameter dependence of unstable modes suggests a nonuniversal scaling of the possible Tayler–Spruit dynamo. 

Abstract We investigate how a fast radio burst (FRB) emitted near a magnetar would propagate through its surrounding dipole magnetosphere at radiir= 10⁷–10⁹cm. First, we show that a GHz burst emitted in the O-mode with luminosityL≫ 10⁴⁰erg s⁻¹is immediately damped for all propagation directions except a narrow cone along the magnetic axis. Then, we examine bursts in the X-mode. GHz waves propagating near the magnetic equator behave as magnetohydrodynamic (MHD) waves if they haveL≫ 10⁴⁰erg s⁻¹. The waves develop plasma shocks in each oscillation and dissipate at $r\sim 3\times {10}^8{L}_{42}^{-1/4}$cm. Waves with lowerLor propagation directions closer to the magnetic axis do not obey MHD. Instead, they interact with individual particles and require a kinetic description. The kinetic interaction quickly accelerates particles to Lorentz factors 10⁴–10⁵at the expense of the wave energy, which again results in strong damping of the wave. In either propagation regime, MHD or kinetic, the dipole magnetosphere surrounding the FRB source acts as a pillow absorbing the radio burst and reradiating the absorbed energy in X-rays. These results constrain the origin of observed FRBs. We argue that the observed FRBs avoid damping because they are emitted by relativistic outflows from magnetospheric explosions, so that the GHz waves do not need to propagate through the outer equilibrium magnetosphere surrounding the magnetar. 

Abstract The origin of pulsar radio emission is one of the old puzzles in theoretical astrophysics. In this Letter, we present a global kinetic plasma simulation that shows from first principles how and where radio emission can be produced in pulsar magnetospheres. We observe the self-consistent formation of electric gaps that periodically ignite electron-positron discharge. The gaps form above the polar cap and in the bulk return current. Discharge of the gaps excites electromagnetic modes, which share several features with the radio emission of real pulsars. We also observe the excitation of plasma waves and charge bunches by beam instabilities in the outer magnetosphere. Our numerical experiment demonstrates that global kinetic models can provide deep insight into the emission physics of pulsars and may help interpret their multiwavelength observations. 

Abstract Magnetospheres of neutron stars can be perturbed by star quakes, interaction in a binary system, or sudden collapse of the star. The perturbations are typically in the kilohertz band and excite magnetohydrodynamic waves. We show that compressive magnetospheric waves steepen into monster shocks, possibly the strongest shocks in the Universe. The shocks are radiative, i.e., the plasma energy is radiated before it crosses the shock. As the kilohertz wave with the radiative shock expands through the magnetosphere, it produces a bright X-ray burst. Then, it launches an approximately adiabatic blast wave, which will expand far from the neutron star. These results suggest a new mechanism for X-ray bursts from magnetars and support the connection of magnetar X-ray activity with fast radio bursts. Similar shocks may occur in magnetized neutron-star binaries before they merge, generating an X-ray precursor of the merger. Powerful radiative shocks are also predicted in the magnetosphere of a neutron star when it collapses into a black hole, producing a bright X-ray transient. 

Abstract Nonlinear effects are crucial for the propagation of fast radio bursts (FRBs) near the source. We study the filamentation of FRBs in the relativistic winds of magnetars, which are commonly invoked as the most natural FRB progenitors. As a result of filamentation, the particle number density and radiation intensity develop strong gradients along the direction of the wind magnetic field. A steady state is reached when the plasma pressure balances the ponderomotive force. In such a steady state, particles are confined in periodically spaced thin sheets, and electromagnetic waves propagate between them as in a waveguide. We show the following. (i) The dispersion relation resembles that in the initial homogeneous plasma, but the effective plasma frequency is determined by the separation of the sheets, not directly by the mean particle density. (ii) The contribution of relativistic magnetar winds to the dispersion measure of FRBs could be several orders of magnitude larger than previously thought. The dispersion measure of the wind depends on the properties of individual bursts (e.g., the luminosity) and therefore can change significantly among different bursts from repeating FRBs. (iii) Induced Compton scattering is suppressed because most of the radiation propagates in near-vacuum regions. 

ABSTRACT We perform 2D particle-in-cell simulations of magnetic reconnection in electron-ion plasmas subject to strong Compton cooling and calculate the X-ray spectra produced by this process. The simulations are performed for trans-relativistic reconnection with magnetization 1 ≤ σ ≤ 3 (defined as the ratio of magnetic tension to plasma rest-mass energy density), which is expected in the coronae of accretion discs around black holes. We find that magnetic dissipation proceeds with inefficient energy exchange between the heated ions and the Compton-cooled electrons. As a result, most electrons are kept at a low temperature in Compton equilibrium with radiation, and so thermal Comptonization cannot reach photon energies $$\sim 100\,$$ keV observed from accreting black holes. Nevertheless, magnetic reconnection efficiently generates $$\sim 100\,$$ keV photons because of mildly relativistic bulk motions of the plasmoid chain formed in the reconnection layer. Comptonization by the plasmoid motions dominates the radiative output and controls the peak of the radiation spectrum Epk. We find Epk ∼ 40 keV for σ = 1 and Epk ∼ 100 keV for σ = 3. In addition to the X-ray peak around 100 keV, the simulations show a non-thermal MeV tail emitted by a non-thermal electron population generated near X-points of the reconnection layer. The results are consistent with the typical hard state of accreting black holes. In particular, we find that the spectrum of Cygnus X-1 is well explained by electron-ion reconnection with σ ∼ 3. 

Abstract The most common form of magnetar activity is short X-ray bursts, with durations from milliseconds to seconds, and luminosities ranging from 10³⁶–10⁴³erg s⁻¹. Recently, an X-ray burst from the galactic magnetar SGR 1935+2154 was detected to be coincident with two fast radio burst (FRB) like events from the same source, providing evidence that FRBs may be linked to magnetar bursts. Using fully 3D force-free electrodynamics simulations, we show that such magnetar bursts may be produced by Alfvén waves launched from localized magnetar quakes: a wave packet propagates to the outer magnetosphere, becomes nonlinear, and escapes the magnetosphere, forming an ultra-relativistic ejecta. The ejecta pushes open the magnetospheric field lines, creating current sheets behind it. Magnetic reconnection can happen at these current sheets, leading to plasma energization and X-ray emission. The angular size of the ejecta can be compact, ≲1 sr if the quake launching region is small, ≲0.01 sr at the stellar surface. We discuss implications for the FRBs and the coincident X-ray burst from SGR 1935+2154. 

Abstract We examine the possibility that fast radio bursts (FRBs) are emitted inside the magnetosphere of a magnetar. On its way out, the radio wave must interact with a low-densitye^±plasma in the outer magnetosphere at radiiR= 10⁹–10¹⁰cm. In this region, the magnetospheric particles have a huge cross section for scattering the wave. As a result, the wave strongly interacts with the magnetosphere and compresses it, depositing the FRB energy into the compressed field and the scattered radiation. The scattered spectrum extends to theγ-ray band and triggerse^±avalanche, further boosting the opacity. These processes choke FRBs, disfavoring scenarios with a radio source confined atR≪ 10¹⁰cm. Observed FRBs can be emitted by magnetospheric flare ejecta transporting energy to large radii.