We present the first self-consistent global simulations of pulsar magnetospheres with operating e ± discharge. We focus on the simple configuration of an aligned or anti-aligned rotator. The star is spun up from zero (vacuum) state to a high angular velocity, and we follow the coupled evolution of its external electromagnetic field and plasma particles using the "particle-in-cell" method. A plasma magnetosphere begins to form through the extraction of particles from the star; these particles are accelerated by the rotation-induced electric field, producing curvature radiation and igniting e ± discharge. We follow the system evolution for several revolution periods, longer than required to reach a quasisteady state. Our numerical experiment puts to test previous ideas for the plasma flow and gaps in the pulsar magnetosphere.We first consider rotators capable of producing pairs out to the light cylinder through photon-photon collisions. We find that their magnetospheres are similar to the previously obtained force-free solutions with a Y-shaped current sheet. The magnetosphere continually ejects e ± pairs and ions. Pair creation is sustained by a strong electric field along the current sheet. We observe powerful curvature and synchrotron emission from the current sheet, consistent with Fermi observations of gamma-ray pulsars.We then study pulsars that can only create pairs in the strong-field region near the neutron star, well inside the light cylinder. We find that both aligned and anti-aligned rotators relax to the "dead" state with suppressed pair creation and electric currents, regardless of the discharge voltage.

It has been suggested that radio telescopes may be sensitive to axion dark matter that resonantly converts to radio photons in the magnetospheres surrounding neutron stars (NSs). In this work, we closely examine this possibility by calculating the radiated power from and projected sensitivity to axion dark matter conversion in ensembles of NSs within astrophysical systems like galaxies and globular clusters. We use population synthesis and evolution models to describe the spatial distributions of NSs within these systems and the distributions of NS properties. Focusing on three specific targets for illustration, the Galactic Center of the Milky Way, the globular cluster M54 in the Sagittarius dwarf galaxy, and the Andromeda galaxy, we show that narrow-band radio observations with telescopes such as the Green Bank Telescope and the future Square Kilometer Array may be able to probe the quantum chromodynamics axion over roughly two orders of magnitude in mass, starting at a fraction of a µeV.

Using numerical simulations we show that low-amplitude Alfvén waves from a magnetar quake propagate to the outer magnetosphere and convert to “plasmoids” (closed magnetic loops) that accelerate from the star, driving blast waves into the magnetar wind. Quickly after its formation, the plasmoid becomes a thin relativistic pancake. It pushes out the magnetospheric field lines, and they gradually reconnect behind the pancake, generating a variable wind far stronger than the normal spindown wind of the magnetar. Repeating ejections drive blast waves in the amplified wind. We suggest that these ejections generate the simultaneous X-ray and radio bursts detected from SGR 1935+2154. A modest energy budget of the magnetospheric perturbation ∼1040 erg is sufficient to produce the observed bursts. Our simulation predicts a narrow (a few milliseconds) X-ray spike from the magnetosphere, arriving almost simultaneously with the radio burst emitted far outside the magnetosphere. This timing is caused by the extreme relativistic motion of the ejecta.

The numerical integration of particle trajectories in curved spacetimes is fundamental for obtaining realistic models of the particle dynamics around massive compact objects such as black holes and neutron stars. Generalized algorithms capable of handling generic metrics are required for studies of both standard spacetimes (Schwarzschild and Kerr metrics) and non-standard spacetimes (e.g. Schwarzschild metric plus non-classical perturbations or multiple black hole metrics). The most commonly employed explicit numerical schemes (e.g. Runge-Kutta) are incapable of producing highly accurate results at critical points, e.g. in the regions close to the event horizon where gravity causes extreme curvature of the spacetime, at an acceptable computational cost. Here, we describe a generalized algorithm for the numerical integration of time-like (massive particles) and null (photons) geodesics in any given 3+1 split spacetime. We introduce a new, exactly energy-conserving implicit integration scheme based on the preservation of the underlying Hamiltonian, and we compare its properties with a standard fourth-order Runge-Kutta explicit scheme and an implicit midpoint scheme. We test the numerical performance of the three schemes against analytic solutions of particle and photon orbits in Schwarzschild and Kerr spacetimes. We also prove the versatility of our framework in handling more exotic metrics such as Morris-Thorne wormholes and quantum-perturbed Schwarzschild black holes. The generalized approach is also discussed in the perspective of future extensions to more complex particle dynamics, e.g. the addition of the Lorentz force acting on charged particles, which allows for test particle diagnostics in GRMHD simulations. *

In some low-luminosity accreting supermassive black hole systems, the supply of plasma in the funnel region can be a problem. It is believed that a local region with an unscreened electric field can exist in the black hole magnetosphere, accelerating particles and producing high-energy gamma-rays that can create e ± pairs. We carry out time-dependent self-consistent 1D PIC simulations of this process, including inverse-Compton scattering and photon tracking. We find a highly time-dependent solution where a macroscopic gap opens quasi-periodically to create e ± pairs and high-energy radiation. If this gap is operating at the base of the jet in M87, we expect an intermittency on the order of a few r g /c, which coincides with the timescale of the observed TeV flares from the same object. For Sagittarius A * the gap electric field can potentially grow to change the global magnetospheric structure, which may explain the lack of a radio jet at the center of our galaxy.

The magnetospheres of magnetars are believed to be filled with electron-positron plasma generated by electric discharge. We present a first numerical experiment demonstrating this process in an axisymmetric magnetosphere with a simple threshold prescription for pair creation, which is applicable to the inner magnetosphere with an ultrastrong field. The e  discharge occurs in response to the twisting of the closed magnetic field lines by a shear deformation of the magnetar surface, which launches electric currents into the magnetosphere. The simulation shows the formation of an electric "gap" with an unscreened electric field (E B 0 ¹ · ) that continually accelerates particles along the magnetic field lines and sustains pair creation. The accelerating voltage is self-regulated to the threshold of the e  discharge. It controls the rate of energy release and the lifetime of the magnetic twist. The simulation follows the global evolution of the twisted magnetosphere over a long time and demonstrates its gradual resistive untwisting. A vacuum cavity forms near the star and expands, gradually erasing magnetospheric electric currents j. The active j-bundle shrinks with time and its footprints form shrinking hot spots on the magnetar surface bombarded by the created particles.

This is the second paper in a series where we examine the physics of pair producing gaps in low-luminosity accreting supermassive black hole systems. In this paper, we carry out time-dependent self-consistent fully general relativistic 1D particle-in-cell simulations of the gap, including full inverse Compton scattering and photon tracking. Similar to the previous paper, we find a highly time-dependent solution where a macroscopic vacuum gap can open quasiperiodically, producing bursts of pairs and high energy radiation. We present the light curve, particle spectrum, and photon spectrum from this process. Using an empirical scaling relation, we rescale the parameters to the inferred values at the base of the jet in M87, and find that the observed TeV flares could potentially be explained by this model under certain parameter assumptions.

Recent modeling of Neutron Star Interior Composition Explorer (NICER) observations of the millisecond pulsar PSR J0030+0451 suggests that the magnetic field of the pulsar is non-dipolar. We construct a magnetic field configuration where foot points of the open field lines closely resemble the hotspot configuration from NICER observations. Using this magnetic field as input, we perform forcefree simulations of the magnetosphere of PSR J0030+0451, showing the three-dimensional structure of its plasma-filled magnetosphere. Making simple and physically motivated assumptions about the emitting regions, we are able to construct the multi-wavelength lightcurves that qualitatively agree with the corresponding observations. The agreement suggests that multipole magnetic structures are the key to modeling this type of pulsars, and can be used to constrain the magnetic inclination angle and the location of radio emission.