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1. Synthetic gamma-ray light curves of Kerr black hole magnetospheric activity from particle-in-cell simulations

   https://doi.org/10.1051/0004-6361/202040158  

   Crinquand, B.; Cerutti, B.; Dubus, G.; Parfrey, K.; Philippov, A. (June 2021, Astronomy & Astrophysics)

   Context. The origin of ultra-rapid flares of very high-energy radiation from active galactic nuclei remains elusive. Magnetospheric processes, occurring in the close vicinity of the central black hole, could account for these flares. Aims. Our aim is to bridge the gap between simulations and observations by synthesizing gamma-ray light curves in order to characterize the activity of a black hole magnetosphere, using kinetic simulations. Methods. We performed global axisymmetric 2D general-relativistic particle-in-cell simulations of a Kerr black hole magnetosphere. We included a self-consistent treatment of radiative processes and plasma supply, as well as a realistic magnetic configuration, with a large-scale equatorial current sheet. We coupled our particle-in-cell code with a ray-tracing algorithm in order to produce synthetic light curves. Results. These simulations show a highly dynamic magnetosphere, as well as very efficient dissipation of the magnetic energy. An external supply of magnetic flux is found to maintain the magnetosphere in a dynamic state, otherwise the magnetosphere settles in a quasi-steady Wald-like configuration. The dissipated energy is mostly converted to gamma-ray photons. The light curves at low viewing angle (face-on) mainly trace the spark gap activity and exhibit high variability. On the other hand, no significant variability is found at high viewing angle (edge-on), where the main contribution comes from the reconnecting current sheet. Conclusions. We observe that black hole magnetospheres with a current sheet are characterized by a very high radiative efficiency. The typical amplitude of the flares in our simulations is lower than is detected in active galactic nuclei. These flares could result from the variation in parameters external to the black hole. 

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2. Monster Radiative Shocks in the Perturbed Magnetospheres of Neutron Stars

   https://doi.org/10.3847/1538-4357/acf659  

   Beloborodov, Andrei_M (December 2023, The Astrophysical Journal)

   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. 

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3. Relativistic coronal mass ejections from magnetars

   https://doi.org/10.1093/mnras/stad2192  

   Sharma, Praveen; Barkov, Maxim_V; Lyutikov, Maxim (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We study dynamics of relativistic coronal mass ejections (CMEs), from launching by shearing of foot-points (either slowly – the ‘Solar flare’ paradigm, or suddenly – the ‘star quake’ paradigm), to propagation in the preceding magnetar wind. For slow shear, most of the energy injected into the CME is first spent on the work done on breaking through the overlaying magnetic field. At later stages, sufficiently powerful CMEs may lead to the ‘detonation’ of a CME and opening of the magnetosphere beyond some equipartition radius req, where the decreasing energy of the CME becomes larger than the decreasing external magnetospheric energy. Post-CME magnetosphere relaxes via the formation of a plasmoid-mediated current sheet, initially at ∼req, and slowly reaching the light cylinder. Both the location of the foot-point shear and the global magnetospheric configuration affect the frequent/weak versus rare/powerful CME dichotomy – to produce powerful flares, the slow shear should be limited to field lines that close in near the star. After the creation of a topologically disconnected flux tube, the tube quickly (at ∼ the light cylinder) comes into force-balance with the preceding wind and is passively advected/frozen in the wind afterward. For fast shear (a local rotational glitch), the resulting large amplitude Alfvén waves lead to the opening of the magnetosphere (which later recovers similarly to the slow shear case). At distances much larger than the light cylinder, the resulting shear Alfvén waves propagate through the wind non-dissipatively. 

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4. Safety First: Stability and Dissipation of Line-tied Force-free Flux Tubes in Magnetized Coronae

   https://doi.org/10.3847/1538-4357/ad3206  

   Rugg, N.; Mahlmann, J. F.; Spitkovsky, A. (May 2024, The Astrophysical Journal)

   Abstract Magnetized plasma columns and extended magnetic structures with both footpoints anchored to a surface layer are an important building block of astrophysical dissipation models. Current loops shining in X-rays during the growth of plasma instabilities are observed in the corona of the Sun and are expected to exist in highly magnetized neutron star magnetospheres and accretion disk coronae. For varying twist and system sizes, we investigate the stability of line-tied force-free flux tubes and the dissipation of twist energy during instabilities using linear analysis and time-dependent force-free electrodynamics simulations. Kink modes (m= 1) and efficient magnetic energy dissipation develop for plasma safety factorsq≲ 1, whereqis the inverse of the number of magnetic field line windings per column length. Higher-order fluting modes (m> 1) can distort equilibrium flux tubes forq> 1 but induce significantly less dissipation. In our analysis, the characteristic pitch ${\tilde{\mu }}_0$of flux-tube field lines determines the growth rate ( $\propto {\tilde{\mu }}_0^3$) and minimum wavelength of the kink instability ( $\propto {\tilde{\mu }}_0^{-1}$). We use these scalings to determine a minimum flux tube length for the growth of the kink instability for any given ${\tilde{\mu }}_0$. By drawing analogies to idealized magnetar magnetospheres with varying regimes of boundary shearing rates, we discuss the expected impact of the pitch-dependent growth rates for magnetospheric dissipation in magnetar conditions. 

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5. Gamma-Ray Bursts and Kilonovae from the Accretion-induced Collapse of White Dwarfs

   https://doi.org/10.3847/2041-8213/ada1cc  

   Cheong_張, Patrick_Chi-Kit_志杰; Pitik, Tetyana; Longo_Micchi, Luís_Felipe; Radice, David (January 2025, The Astrophysical Journal Letters)

   Abstract We present the first seconds-long 2D general relativistic neutrino magnetohydrodynamic simulations of accretion-induced collapse (AIC) in rapidly rotating, strongly magnetized white dwarfs (WDs), which might originate as remnants of double-WD mergers. This study examines extreme combinations of magnetic fields and rotation rates, motivated both by the need to address the limitations of 2D axisymmetric simulations and to explore the physics of AIC under rare conditions that, while yet to be observationally confirmed, may be consistent with current theoretical models and account for unusual events. Under these assumptions, our results demonstrate that, if realizable, such systems can generate relativistic jets and neutron-rich outflows with properties consistent with long gamma-ray bursts (LGRBs) accompanied by kilonovae, such as GRB 211211A and GRB 230307A. These findings highlight the potential role of AIC in heavyr-process element production and offer a framework for understanding rare LGRBs associated with kilonova emission. Longer-duration 3D simulations are needed to fully capture magnetic field amplification, resolve instabilities, and determine the fate of the energy retained by the magnetar at the end of the simulations. 

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