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Abstract Binary neutron star mergers can form short-lived magnetar-like remnants whose magnetically dominated polar towers reachB ∼ 10¹⁵–10¹⁶G, but the microphysical composition of these outflows remains poorly understood. Combining tower geometries from general-relativistic magnetohydrodynamics simulations with an analytic treatment of QED and hadronic processes, we argue that magnetic reconnection is the most viable particle acceleration channel in this strongly radiative regime, where the current sheets thin to collisionless scales. Purely leptonic pair loading—including the resonant inverse-Compton scattering of soft photons—is bottlenecked by rapid pitch-angle damping and the tendency of one-photon magnetic conversion to populate low Landau levels. Once protons reach mildly relativistic energies (γ_p ≳ 1.3), however, inelastic proton–proton (pp) collisions inject large-pitch-angle pions that driveπ⁰ → 2γ → e^±cascades with multiplicity ${\mathcal{M}}_{\mathrm{cas}}\simeq 4$ atB = 10¹⁵G, supplying the perpendicular momentum the leptonic channel cannot maintain. This hadronic route dominates pair loading and channels most of the dissipated magnetic energy into thee^±population that could power the nonthermal emission emerging at larger radii. Charged-pion decay, modulated byπ^±synchrotron cooling, further seeds a nonthermal neutrino tail up to ∼300 (σ_p/5) MeV, spectrally distinct from the thermal cooling burst and detectable from sources within ∼100 kpc. 

Abstract Waves launched from the neutron star surface or inner magnetosphere propagate through the magnetosphere as small perturbations, but can grow relative to the background magnetic field and steepen into “monster shocks”—ultrarelativistic magnetized shocks that can power high-energy emission. Such shocks can develop around isolated magnetars, merging binaries, and collapsing neutron stars. They occur in magnetically dominated plasma and are described by relativistic magnetohydrodynamics (MHD). We present global relativistic MHD simulations of monster shocks in unperturbed and perturbed (“wrinkled”) backgrounds with a global dipolar geometry. Our simulations confirm analytical predictions for equatorial shocks and provide new insight into the behavior of oblique shocks off the equator. Simulations where the shock is formed through Alfvén mode to fast mode conversion are also presented, demonstrating the generic nature of the monster shock mechanism. We explore how the presence of additional modes in the magnetosphere modifies the shock behavior. Modes of comparable amplitude can fragment the shock front, substantially reduce the magnetization, produce localized enhancements in the Lorentz factor relative to an unperturbed dipole background, and intermittently generate additional shocks along a line of sight. 

Abstract Recent observations of long period radio transients, such as GLEAM-X J0704–37 and ILT J1101+5521, have revealed a previously unrecognized population of Galactic radio transient sources associated with white dwarf (WD)–M dwarf (MD) binaries. It is an open question how to produce coherent radio emission in these systems, though a model driven by binary interaction seems likely given the nature and correlation of the emission with the binaries’ orbital period. Using kinetic plasma simulations, we demonstrate that the relativistic electron cyclotron maser instability (ECMI) is a viable mechanism for generating radio pulses in WD–MD systems, akin to planetary radio emission, such as that from the Jupiter–Io system. We quantify the relativistic ECMI in the nonlinear regime under conditions relevant for WD radio emission for the first time. Our simulations demonstrate that the ECMI can intrinsically produce partially linearly polarized emission relevant to explaining the observed emission spectra of two Galactic sources, though the precise details will depend on the plasma composition. Our work paves the way for a systematic and fully nonlinear computational modeling of radio emission from interacting WD sources. 

Many astrophysical simulations involve extreme dynamic range of timescales around ‘special points’ in the domain (e.g. black holes, stars, planets, disks, galaxies, shocks, mixing interfaces), where processes on small scales couple strongly to those on large scales. Adaptive resolution, multi-physics, and hybrid numerical methods have enabled tremendous progress on the spatial, physics, and numerical challenges involved. But often the limiter for following the long timescales of global evolution is the extremely short numerical timestep required in some subdomains (which leads to their dominating simulation costs). Recently several approaches have been developed for tackling this in problems where the short timescale solution is sampled and then projected as an effective subgrid model over longer timescales (e.g. ‘zooming in and out’). We generalize these to a family of models where time evolution is modulated by a variable but continuous in space-and-time dilation/stretch factor . This extends previous well-studied approaches (including reduced-speed-of-light and binary orbital dynamics methods), and ensures that the system comes to correct local steady-state solutions, and derive criteria that the dilation factor/timesteps/resolution must obey to ensure good behavior. We present a variety of generalizations to different physics or coupling scales. Compared to previous approaches, this method makes it possible to avoid imprinting arbitrary scales where there is no clear scale-separation, and couples well to Lagrangian or Eulerian methods. It is flexible and easily-implemented and we demonstrate its validity (and limitations) in test problems. We discuss the relationship between these methods and physical time dilation in GRMHD. We demonstrate how this can be used to obtain effective speedup factors exceeding $\gtrsim {10}^4$ in multiphysics simulations. 

Abstract In this work, we investigate gravitational-wave signatures of a proposed subsolar-mass merger scenario resulting from fragmentation inside a collapsar accretion disk. This scenario has gained recent interest with the electromagnetic transient AT2025ulz, a possible superkilonova counterpart candidate to the subthreshold gravitational wave event S250818k. One prediction of fragmentation is the formation of multiple smaller neutron-star fragments, some of which might merge hierarchically. Such mergers are expected not only to produce individual electromagnetic counterparts but also, because of their repeated capture and merger dynamics, to impart kicks to the system and thereby drive orbital eccentricity. By performing numerical relativity simulations of hierarchical subsolar-mass compact-object mergers modeled as black holes in a disklike geometry consistent with this scenario, we demonstrate the buildup of potentially large eccentricity for the final merger, of ordere ≃ 0.6 initially, and show that, because of the short lifetime of the system, a substantial part of this eccentricity, up toe ≃ 0.1, can survive until the final neutron star–black hole merger in the general case. As a result, future detections of eccentricities in potential subsolar-mass gravitational-wave candidate events would be a strong indicator for a hierarchical formation scenario. In the extreme case, where we observe repeated mergers to lead to the formation of a solar-mass neutron star, the expected binary parameters can be in a regime similar to those of the eccentric neutron star–black hole merger event GW200105. 

Abstract Many transients believed to originate from magnetars are thought to be triggered by crustal activity, which feeds back on the surrounding magnetosphere. These perturbations, through a variety of proposed mechanisms, can convert a fraction of the magnetic energy stored in the magnetosphere, as well as the energy injected by crustal activity itself into electromagnetic emission, including X-ray bursts and fast radio bursts. We here provide a first glimpse of this process by coupling magnetoelastic dynamics simulations of the crust to fully three-dimensional relativistic resistive force-free electrodynamic simulations of the magnetosphere. Our simulations demonstrate that the elastodynamical motions of the surface launch a series of fast magnetosonic and Alfvén waves into the magnetosphere. These waves rapidly enter a nonlinear regime, ultimately giving rise to a wide range of phenomena, including monster shock formation, relativistic blast waves, trapped Alfvén waves, nonlinear Alfvén wave ejecta, and transient equatorial current sheets interacting with these waves. After the initial nonlinear phase, the magnetosphere is partially combed out, resembling a strongly perturbed split monopole configuration. Our results can offer hints and potential constraints on fast radio burst emission mechanisms, in particular for hyperactive repeating sources, by placing tight bounds on energy conversion efficiency and possible quasiperiodic imprints on magnetospheric waves by elastic oscillations of the crust. 

Abstract Binary neutron star mergers can produce extreme magnetic fields, some of which can lead to strong magnetar-like remnants. While strong magnetic fields have been shown to affect the dynamics of outflows and angular momentum transport in the remnant, they can also crucially alter the properties of nuclear matter probed in the merger. In this work, we provide a first assessment of the latter, determining the strength of the pressure anisotropy caused by Landau-level quantization and the anomalous magnetic moment. To this end, we perform the first numerical relativity simulation with a magnetic polarization tensor and a magnetic-field-dependent equation of state using a new algorithm we present here, which also incorporates a mean-field dynamo model to control the magnetic field strength present in the merger remnant. Our results show that—in the most optimistic case—corrections to the anisotropy can be in excess of 10% and are potentially largest in the outer layers of the remnant. This work paves the way for a systematic investigation of these effects. 

Abstract A significant interest has emerged recently in assessing whether collimated and ultrarelativistic outflows can be produced by a long-lived remnant from a binary neutron star (BNS) merger, with different approaches leading to different outcomes. To clarify some of the aspect of this process, we report the results of long-term (i.e., ∼110 ms) state-of-the-art general relativistic magnetohydrodynamics simulations of the inspiral and merger of a BNS system of magnetized stars. We find that after ∼50 ms from the merger anα–Ω dynamo driven by the magnetorotational instability sets in in the densest regions of the disk and leads to the breakout of the magnetic field lines from the accretion disk around the remnant. The breakout is responsible for generating a collimated, magnetically driven outflow with only mildly relativistic velocities and for a violent eruption of electromagnetic energy. We provide evidence that this outflow is partly collimated via a Blandford–Payne mechanism. Finally, by including or not the radiative transport via neutrinos, we determine the role they play in the launching of the collimated wind. In this way, we conclude that the mechanism of magnetic field breakout we observe is robust and takes place even without neutrinos. Contrary to previous expectations, the inclusion of neutrino absorption and emission leads to a smaller baryon pollution in polar regions and hence accelerates the occurrence of the breakout, yielding a larger electromagnetic luminosity. Given the mildly relativistic nature of these disk-driven breakout outflows, it is difficult to consider them responsible for the jet phenomenology observed in short gamma-ray bursts.