Supermassive black holes in active galactic nuclei are known to launch relativistic jets, which are observed across the entire electromagnetic spectrum and thought to be efficient particle accelerators. Their primary radiation mechanism for radio emission is polarized synchrotron emission produced by a population of nonthermal electrons. In this Letter, we present a global general relativistic magnetohydrodynamical (GRMHD) simulation of a magnetically arrested disk (MAD). After the simulation reaches the MAD state, we show that waves are continuously launched from the vicinity of the black hole and propagate along the interface between the jet and the wind. At this interface, a steep gradient in velocity is present between the mildly relativistic wind and the highly relativistic jet. The interface is, therefore, a shear layer, and due to the shear, the waves generate rollups that alter the magnetic field configuration and the shear layer geometry. We then perform polarized radiation transfer calculations of our GRMHD simulation and find signatures of the waves in both total intensity and linear polarization, effectively lowering the fully resolved polarization fraction. The telltale polarization signatures of the waves could be observable by future very long baseline interferometric observations, e.g., the nextgeneration Event Horizon Telescope.
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Abstract 
Abstract Magnetic reconnection can power spectacular highenergy astrophysical phenomena by producing nonthermal energy distributions in highly magnetized regions around compact objects. By means of twodimensional fully kinetic particleincell (PIC) simulations, we investigate relativistic collisionless plasmoidmediated reconnection in magnetically dominated pair plasmas with and without a guide field. In Xpoints, where diverging flows result in a nondiagonal thermal pressure tensor, a finite residence time for particles gives rise to a localized collisionless effective resistivity. Here, for the first time for relativistic reconnection in a fully developed plasmoid chain, we identify the mechanisms driving the nonideal electric field using a full Ohm law by means of a statistical analysis based on our PIC simulations. We show that the nonideal electric field is predominantly driven by gradients of nongyrotropic thermal pressures. We propose a kinetic physics motivated nonuniform effective resistivity model that is negligible on global scales and becomes significant only locally in Xpoints. It captures the properties of collisionless reconnection with the aim of mimicking its essentials in nonideal magnetohydrodynamic descriptions. This effective resistivity model provides a viable opportunity to design physically grounded global models for reconnectionpowered highenergy emission.more » « less

Abstract Active galactic nuclei in general, and the supermassive black hole in M87 in particular, show bright and rapid gammaray flares up to energies of 100 GeV and above. For M87, the flares show multiwavelength components, and the variability timescale is comparable to the dynamical time of the event horizon, suggesting that the emission may come from a compact region near the nucleus. However, the emission mechanism for these flares is not well understood. Recent highresolution generalrelativistic magnetohydrodynamic simulations show the occurrence of episodic magnetic reconnection events that can power flares near the black hole event horizon. In this work, we analyze the radiative properties of the reconnecting current layer under the extreme plasma conditions applicable to the black hole in M87 from first principles. We show that abundant pair production is expected in the vicinity of the reconnection layer, to the extent that the produced secondary pair plasma dominates the reconnection dynamics. Using analytic estimates backed by twodimensional particleincell simulations, we demonstrate that in the presence of strong synchrotron cooling, reconnection can produce a hard powerlaw distribution of pair plasma imprinted in the outgoing synchrotron (up to a few tens of MeV) and the inverseCompton signal (up to TeV). We produce synthetic radiation spectra from our simulations, which can be directly compared with the results of future multiwavelength observations of M87* flares.

Abstract The origins of the various outbursts of hard Xrays from magnetars (highly magnetized neutron stars) are still unknown. We identify instabilities in relativistic magnetospheres that can explain a range of Xray flare luminosities. Crustal surface motions can twist the magnetar magnetosphere by shifting the frozenin footpoints of magnetic field lines in currentcarrying flux bundles. Axisymmetric (2D) magnetospheres exhibit strong eruptive dynamics, i.e., catastrophic lateral instabilities triggered by a critical footpoint displacement of
ψ _{crit}≳π . In contrast, our new threedimensional (3D) twist models with finite surface extension capture important nonaxisymmetric dynamics of twisted forcefree flux bundles in dipolar magnetospheres. Besides the wellestablished global eruption resulting (as in 2D) from lateral instabilities, such 3D structures can develop helical, kinklike dynamics, and dissipate energy locally (confined eruptions). Up to 25% of the induced twist energy is dissipated and available to power Xray flares in powerful global eruptions, with most of our models showing an energy release in the range of the most common Xray outbursts, ≲10^{43}erg. Such events occur when significant energy builds up while deeply buried in the dipole magnetosphere. Less energetic outbursts likely precede powerful flares, due to intermittent instabilities and confined eruptions of a continuously twisting flux tube. Upon reaching a critical state, global eruptions produce the necessary Poyntingfluxdominated outflows required by models prescribing the fast radio burst production in the magnetar wind—for example, via relativistic magnetic reconnection or shocks. 
Abstract Magnetic reconnection can power bright, rapid flares originating from the inner magnetosphere of accreting black holes. We conduct extremely highresolution (5376 × 2304 × 2304 cells) generalrelativistic magnetohydrodynamics simulations, capturing plasmoidmediated reconnection in a 3D magnetically arrested disk for the first time. We show that an equatorial, plasmoidunstable current sheet forms in a transient, nonaxisymmetric, lowdensity magnetosphere within the inner few Schwarzschild radii. Magnetic flux bundles escape from the event horizon through reconnection at the universal plasmoidmediated rate in this current sheet. The reconnection feeds on the highly magnetized plasma in the jets and heats the plasma that ends up trapped in flux bundles to temperatures proportional to the jet’s magnetization. The escaped flux bundles can complete a full orbit as lowdensity hot spots, consistent with Sgr A* observations by the GRAVITY interferometer. Reconnection near the horizon produces sufficiently energetic plasma to explain flares from accreting black holes, such as the TeV emission observed from M87. The drop in the mass accretion rate during the flare and the resulting lowdensity magnetosphere make it easier for veryhighenergy photons produced by reconnectionaccelerated particles to escape. The extremeresolution results in a converged plasmoidmediated reconnection rate that directly determines the timescales and properties of the flare.more » « less

Alfvén wave collisions are the primary building blocks of the nonrelativistic turbulence that permeates the heliosphere and low to moderateenergy astrophysical systems. However, many astrophysical systems such as gammaray bursts, pulsar and magnetar magnetospheres and active galactic nuclei have relativistic flows or energy densities. To better understand these highenergy systems, we derive reduced relativistic magnetohydrodynamics equations and employ them to examine weak Alfvénic turbulence, dominated by threewave interactions, in reduced relativistic magnetohydrodynamics, including the forcefree, infinitely magnetized limit. We compare both numerical and analytical solutions to demonstrate that many of the findings from nonrelativistic weak turbulence are retained in relativistic systems. But, an important distinction in the relativistic limit is the inapplicability of a formally incompressible limit, i.e. there exists finite coupling to the compressible fast mode regardless of the strength of the magnetic field. Since fast modes can propagate across field lines, this mechanism provides a route for energy to escape strongly magnetized systems, e.g. magnetar magnetospheres. However, we find that the fastAlfvén coupling is diminished in the limit of oblique propagation.more » « less

Alfvén waves as excited in black hole accretion disks and neutron star magnetospheres are the building blocks of turbulence in relativistic, magnetized plasmas. A large reservoir of magnetic energy is available in these systems, such that the plasma can be heated significantly even in the weak turbulence regime. We perform highresolution threedimensional simulations of counterpropagating Alfvén waves, showing that an $E_{B_{\perp }}(k_{\perp }) \propto k_{\perp }^{2}$ energy spectrum develops as a result of the weak turbulence cascade in relativistic magnetohydrodynamics and its infinitely magnetized (forcefree) limit. The plasma turbulence ubiquitously generates current sheets, which act as locations where magnetic energy dissipates. We show that current sheets form as a natural result of nonlinear interactions between counterpropagating Alfvén waves. These current sheets form owing to the compression of elongated eddies, driven by the shear induced by growing higherorder modes, and undergo a thinning process until they breakup into smallscale turbulent structures. We explore the formation of current sheets both in overlapping waves and in localized wave packet collisions. The relativistic interaction of localized Alfvén waves induces both Alfvén waves and fast waves, and efficiently mediates the conversion and dissipation of electromagnetic energy in astrophysical systems. Plasma energization through reconnection in current sheets emerging during the interaction of Alfvén waves can potentially explain Xray emission in black hole accretion coronae and neutron star magnetospheres.more » « less