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Abstract Rotational evolution of stellar radiative zones is an old puzzle. We argue that angular momentum transport by turbulent processes induced by differential rotation is insufficient, and propose that a key role is played by “magnetic webs.” We define magnetic webs as stable magnetic configurations that enforce corotation of their coupled mass shells, and discuss their resistance to differential torques that occur in stars. Magnetic webs are naturally expected in parts of radiative zones that were formerly convective, retaining memory of extinguished dynamos. For instance, red giants with moderate massesM ≳ 1.3M_⊙likely contain a magnetic web deposited on the main sequence during the retreat of the central convective zone. The web couples the helium core to the hydrogen envelope of the evolving red giant and thus reduces spin-up of the contracting core. The magnetic field and the resulting slower rotation of the core are both consistent with asteroseismic observations, as we illustrate with a stellar evolution model with mass 1.6M_⊙. Evolved massive stars host more complicated patterns of convective zones that may leave behind many webs, transporting angular momentum toward the surface. Efficient web formation likely results in most massive stars dying with magnetized and slowly rotating cores. 

Abstract The Tayler instability (TI) of toroidal magnetic fields is a candidate mechanism for driving turbulence, angular momentum (AM) transport, and dynamo action in stellar radiative zones. Recently V. A. Skoutnev & A. M. Beloborodov (2024) revisited the linear stability analysis of a toroidal magnetic field in a rotating and stably stratified fluid. In this paper, we extend the analysis to include both thermal and compositional stratification, allowing for general application to stars. We formulate an analytical instability criterion for use as a “toggle switch” in stellar evolution codes. It determines when and where in a star the TI develops with a canonical growth rate as assumed in existing prescriptions for AM transport based on Tayler–Spruit dynamo. We implement such a toggle switch in the MESA stellar evolution code and map out the stability of each mode of the TI on a grid of stellar evolution models. In evolved lower-mass stars, the TI becomes suppressed in the compositionally stratified layer around the hydrogen-burning shell. In higher-mass stars, the TI can be active throughout their radiative zones but at different wavenumbers than previously expected. 

Abstract The merger of a black hole (BH) and a neutron star (NS) in most cases is expected to leave no material around the remnant BH; therefore, such events are often considered as sources of gravitational waves without electromagnetic counterparts. However, a bright counterpart can emerge if the NS is strongly magnetized, as its external magnetosphere can experience radiative shocks and magnetic reconnection during/after the merger. We use magnetohydrodynamic simulations in the dynamical spacetime of a merging BH–NS binary to investigate its magnetospheric dynamics. We find that compressive waves excited in the magnetosphere develop into monster shocks as they propagate outward. After swallowing the NS, the BH acquires a magnetosphere that quickly evolves into a split-monopole configuration and then undergoes an exponential decay (balding), enabled by magnetic reconnection and also assisted by the ringdown of the remnant BH. This spinning BH drags the split monopole into rotation, forming a transient pulsar-like state. It emits a striped wind if the swallowed magnetic-dipole moment is inclined to the spin axis. We predict two types of transients from this scenario: (1) a fast radio burst emitted by the shocks as they expand to large radii; and (2) an X-ray/γ-ray burst emitted by thee^±outflow heated by magnetic dissipation. 

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 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 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 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 particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10⁻⁴and pair-loading factorsZ_±≲ 10 are studied, whereZ_±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσ_Lthat decreases withZ_±. Atσ≲σ_Lthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ_±, leading to lowerσ_L. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales as ${\overline{E}}_{\mathrm{e}}\propto {\left(Z_{\pm }+1\right)}^{-1}$. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10⁻⁶. The ions then become essentially thermal with mean energy ${\overline{E}}_{\mathrm{i}}$, while electrons form a nonthermal tail, extending from $E\sim {\left(Z_{\pm }+1\right)}^{-1}{\overline{E}}_{\mathrm{i}}$to ${\overline{E}}_{\mathrm{i}}$. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.