General relativistic magnetohydrodynamic (GRMHD) simulations of black hole tilted disks—where the angular momentum of the accretion flow at large distances is misaligned with respect to the black hole spin—commonly display standing shocks within a few to tens of gravitational radii from the black hole. In GRMHD simulations of geometrically thick, optically thin accretion flows, applicable to lowluminosity sources like Sgr A* and M87*, the shocks have transrelativistic speed, moderate plasma beta (the ratio of ion thermal pressure to magnetic pressure is
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Abstract β _{pi1}∼ 1–8), and low sonic Mach number (the ratio of shock speed to sound speed isM _{s}∼ 1–6). We study such shocks with 2D particleincell simulations, and we quantify the efficiency and mechanisms of electron heating for the special case of preshock magnetic fields perpendicular to the shock direction of propagation. We find that the postshock electron temperatureT _{e2}exceeds the adiabatic expectationT _{e2,ad}by an amount , nearly independent of the plasma beta and of the preshock electrontoion temperature ratio ${T}_{\mathrm{e}2}/{T}_{\mathrm{e}2,\mathrm{ad}}\phantom{\rule{0.25em}{0ex}}1\simeq 0.0016{M}_{s}^{3.6}$T _{e1}/T _{i1}, which we vary from 0.1 to unity. We investigate the heating physics forM _{s}∼ 5–6 and find that electron superadiabatic heating is governed by magnetic pumping atT _{e1}/T _{i1}= 1, whereas heating byB parallel electric fields (i.e., parallel to the local magnetic field) dominates atT _{e1}/T _{i1}= 0.1. Our results provide physically motivated subgrid prescriptions for electron heating at the collisionless shocks seen in GRMHD simulations of black hole accretion flows. 
Abstract We measure the thermal electron energization in 1D and 2D particleincell simulations of quasiperpendicular, lowbeta (
β _{p}= 0.25) collisionless ion–electron shocks with mass ratiom _{i}/m _{e}= 200, fast Mach number –4, and upstream magnetic field angle ${\mathcal{M}}_{\mathrm{ms}}=1$θ _{Bn}= 55°–85° from the shock normal . It is known that shock electron heating is described by an ambipolar, $\stackrel{\u02c6}{\mathit{n}}$ parallel electric potential jump, ΔB ϕ _{∥}, that scales roughly linearly with the electron temperature jump. Our simulations have –0.2 in units of the preshock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure $\mathrm{\Delta}{\varphi}_{\parallel}/(0.5{m}_{\mathrm{i}}{{u}_{\mathrm{sh}}}^{2})\sim 0.1$ϕ _{∥}, including the use of de Hoffmann–Teller frame fields, agree to tensofpercent accuracy. Neglecting offdiagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ _{∥}in our lowβ _{p}shocks. We further focus on twoθ _{Bn}= 65° shocks: a ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$ ) case with a long, 30 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}1.8$d _{i}precursor of whistler waves along , and a $\stackrel{\u02c6}{\mathit{n}}$ ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}7$ ) case with a shorter, 5 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}3.2$d _{i}precursor of whistlers oblique to both and $\stackrel{\u02c6}{\mathit{n}}$ ;B d _{i}is the ion skin depth. Within the precursors,ϕ _{∥}has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the , ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$θ _{Bn}= 65° case,ϕ _{∥}shows a weak dependence on the electron plasmatocyclotron frequency ratioω _{pe}/Ω_{ce}, andϕ _{∥}decreases by a factor of 2 asm _{i}/m _{e}is raised to the true proton–electron value of 1836. 
Abstract We study a relativistic collisionless electron–positron shock propagating into an unmagnetized ambient medium using 2D particleincell simulations of unprecedented duration and size. The shock generates intermittent magnetic structures of increasingly larger size as the simulation progresses. Toward the end of our simulation, at around 26,000 plasma times, the magnetic coherence scale approaches
λ ∼ 100 plasma skin depths, both ahead and behind the shock front. We anticipate a continued growth ofλ beyond the time span of our simulation, as long as the shock accelerates particles to increasingly higher energies. The postshock field is concentrated in localized patches, which maintain a local magnetic energy fractionε _{B}∼ 0.1. Particles randomly sampling the downstream fields spend most of their time in low field regions (ε _{B}≪ 0.1) but emit a large fraction of the synchrotron power in the localized patches with strong fields (ε _{B}∼ 0.1). Our results have important implications for models of gammaray burst afterglows. 
ABSTRACT We perform nonradiative twodimensional particleincell simulations of magnetic reconnection for various strengths of the guide field (perpendicular to the reversing field), in magnetically dominated electron–positron plasmas. Magnetic reconnection under such conditions could operate in accretion disc coronae around black holes. There, it has been suggested that the transrelativistic bulk motions of reconnection plasmoids containing inverseComptoncooled electrons could Comptonupscatter soft photons to produce the observed nonthermal hard Xrays. Our simulations are performed for magnetizations 3 ≤ σ ≤ 40 (defined as the ratio of enthalpy density of the reversing field to plasma enthalpy density) and guide field strengths 0 ≤ Bg/B0 ≤ 1 (normalized to the reversing field strength B0). We find that the mean bulk energy of the reconnected plasma depends only weakly on the flow magnetization but strongly on the guide field strength – with Bg/B0 = 1 yielding a mean bulk energy twice smaller than Bg/B0 = 0. Similarly, the dispersion of bulk motions around the mean – a signature of stochasticity in the plasmoid chain’s motions – is weakly dependent on magnetization (for σ ≳ 10) but strongly dependent on the guide field strength – dropping by more than a factor of two from Bg/B0 = 0 to Bg/B0 = 1. In short, reconnection in strong guide fields (Bg/B0 ∼ 1) leads to slower and more ordered plasmoid bulk motions than its weak guide field (Bg/B0 ∼ 0) counterpart.

Abstract The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact “coronal” region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a largescale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, Xray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particleincell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is
below the break and ${\mathit{dn}}_{p}/{\mathit{dE}}_{p}\propto {E}_{p}^{1}$ above the break, with IceCube neutrino observations suggesting ${\mathit{dn}}_{p}/{\mathit{dE}}_{p}\propto {E}_{p}^{s}$s ≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal Xrays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron–positron pairs that makes the corona moderately Compton thick. 
Abstract Magnetic reconnection is often invoked as a source of highenergy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model—supported and benchmarked with largescale threedimensional kinetic particleincell simulations in electron–positron plasmas—that elucidates the physics governing the generation of powerlaw energy spectra in relativistic reconnection. Particles with Lorentz factor
γ ≳ 3σ (here,σ is the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is , where ${t}_{\mathrm{acc}}\sim \gamma \phantom{\rule{0.25em}{0ex}}{\eta}_{\mathrm{rec}}^{1}{\omega}_{\mathrm{c}}^{1}\simeq 20\phantom{\rule{0.25em}{0ex}}\gamma \phantom{\rule{0.25em}{0ex}}{\omega}_{\mathrm{c}}^{1}$η _{rec}≃ 0.06 is the inflow speed in units of the speed of light andω _{c}=eB _{0}/mc is the gyrofrequency in the upstream magnetic field. They leave the region of active energization aftert _{esc}, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measuret _{esc}in our simulations and find thatt _{esc}∼t _{acc}forσ ≳ few. This leads to a universal (i.e.,σ independent) powerlaw spectrum for the particles undergoing active acceleration, and ${\mathit{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma}^{1}$ for the overall particle population. Our results help to shed light on the ubiquitous presence of powerlaw particle and photon spectra in astrophysical nonthermal sources. $\mathit{dN}/d\gamma \propto {\gamma}^{2}$ 
ABSTRACT The nonlinear interaction between electromagnetic waves and plasmas attracts significant attention in astrophysics because it can affect the propagation of Fast Radio Bursts (FRBs) – luminous millisecondduration pulses detected at radio frequency. The filamentation instability (FI) – a type of nonlinear wave–plasma interaction – is considered to be dominant near FRB sources, and its nonlinear development may also affect the inferred dispersion measure of FRBs. In this paper, we carry out fully kinetic particleincell simulations of the FI in unmagnetized pair plasmas. Our simulations show that the FI generates transverse density filaments, and that the electromagnetic wave propagates in near vacuum between them, as in a waveguide. The density filaments keep merging until force balance between the wave ponderomotive force and the plasma pressure gradient is established. We estimate the merging timescale and discuss the implications of filament merging for FRB observations.

Abstract In galaxy clusters, the intracluster medium (ICM) is expected to host a diffuse, longlived, and invisible population of “fossil” cosmicray electrons (CRe) with 1–100 MeV energies. These CRe, if reaccelerated by 100× in energy, can contribute synchrotron luminosity to cluster radio halos, relics, and phoenices. Reacceleration may be aided by CRe scattering upon the ionLarmorscale waves that spawn when ICM is compressed, dilated, or sheared. We study CRe scattering and energy gain due to ion cyclotron (IC) waves generated by continuously driven compression in 1D fully kinetic particleincell simulations. We find that pitchangle scattering of CRe by IC waves induces energy gain via magnetic pumping. In an optimal range of ICresonant momenta, CRe may gain up to ∼10%–30% of their initial energy in one compression/dilation cycle with magnetic field amplification ∼3–6×, assuming adiabatic decompression without further scattering and averaging over initial pitch angle.more » « less

ABSTRACT We study the linear stability of a planar interface separating two fluids in relative motion, focusing on the symmetric configuration where the two fluids have the same properties (density, temperature, magnetic field strength, and direction). We consider the most general case with arbitrary sound speed cs, Alfvén speed vA, and magnetic field orientation. For the instability associated with the fast mode, we find that the lower bound of unstable shear velocities is set by the requirement that the projection of the velocity on to the fluidframe wavevector is larger than the projection of the Alfvén speed on to the same direction, i.e. shear should overcome the effect of magnetic tension. In the frame where the two fluids move in opposite directions with equal speed v, the upper bound of unstable velocities corresponds to an effective relativistic Mach number $M_{\rm re}\equiv v/v_{\rm {f}\perp }\sqrt{(1v_{\rm {f}\perp }^2)/(1v^2)} \cos \theta =\sqrt{2}$, where $v_{\rm {f}\perp }=[v_{\rm {A}}^2+c_{\rm s}^2(1v_{\rm {A}}^2)]^{1/2}$ is the fast speed assuming a magnetic field perpendicular to the wavevector (here, all velocities are in units of the speed of light), and θ is the laboratoryframe angle between the flow velocity and the wavevector projection on to the shear interface. Our results have implications for shear flows in the magnetospheres of neutron stars and black holes – both for single objects and for merging binaries – where the Alfvén speed may approach the speed of light.