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1. Spatial Intermittency of Particle Distribution in Relativistic Plasma Turbulence

   https://doi.org/10.3847/1538-4357/accd73  

   Vega, Cristian ; Boldyrev, Stanislav ; Roytershteyn, Vadim ( June 2023 , The Astrophysical Journal)

   Relativistic magnetically dominated turbulence is an efficient engine for particle acceleration in a collisionless plasma. Ultrarelativistic particles accelerated by interactions with turbulent fluctuations form nonthermal power-law distribution functions in the momentum (or energy) space,f(γ)dγ∝γ^−αdγ, whereγis the Lorenz factor. We argue that in addition to exhibiting non-Gaussian distributions over energies, particles energized by relativistic turbulence also become highly intermittent in space. Based on particle-in-cell numerical simulations and phenomenological modeling, we propose that the bulk plasma density has lognormal statistics, while the density of the accelerated particles,n, has a power-law distribution function,$P(n)\mathit{dn}\propto n^{-\beta }\mathit{dn}$. We argue that the scaling exponents are related asβ≈α+ 1, which is broadly consistent with numerical simulations. Non-space-filling, intermittent distributions of plasma density and energy fluctuations may have implications for plasma heating and for radiation produced by relativistic turbulence.

    

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2. The Origin of Power-law Spectra in Relativistic Magnetic Reconnection

   https://doi.org/10.3847/2041-8213/acfe7c  

   Zhang, Hao ; Sironi, Lorenzo ; Giannios, Dimitrios ; Petropoulou, Maria ( October 2023 , The Astrophysical Journal Letters)

   Magnetic reconnection is often invoked as a source of high-energy 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 large-scale three-dimensional kinetic particle-in-cell simulations in electron–positron plasmas—that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factorγ≳ 3σ(here,σis 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$t_{\mathrm{acc}}\sim \gamma {\eta }_{\mathrm{rec}}^{-1}{\omega }_{\mathrm{c}}^{-1}\simeq 20\gamma {\omega }_{\mathrm{c}}^{-1}$, whereη_rec≃ 0.06 is the inflow speed in units of the speed of light andω_c=eB₀/mcis 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_escin our simulations and find thatt_esc∼t_accforσ≳ few. This leads to a universal (i.e.,σ-independent) power-law spectrum${\mathit{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$for the particles undergoing active acceleration, and$\mathit{dN}/d\gamma \propto {\gamma }^{-2}$for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources.

    

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3. Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD

   https://doi.org/10.3847/2041-8213/ad1048  

   Cho 조, Hyerin 혜린 ; Prather, Ben S. ; Narayan, Ramesh ; Natarajan, Priyamvada ; Su, Kung-Yi ; Ricarte, Angelo ; Chatterjee, Koushik ( December 2023 , The Astrophysical Journal Letters)

   Fueling and feedback couple supermassive black holes (SMBHs) to their host galaxies across many orders of magnitude in spatial and temporal scales, making this problem notoriously challenging to simulate. We use a multi-zone computational method based on the general relativistic magnetohydrodynamic (GRMHD) code KHARMA that allows us to span 7 orders of magnitude in spatial scale, to simulate accretion onto a non-spinning SMBH from an external medium with a Bondi radius ofR_B≈ 2 × 10⁵GM_•/c², whereM_•is the SMBH mass. For the classic idealized Bondi problem, spherical gas accretion without magnetic fields, our simulation results agree very well with the general relativistic analytic solution. Meanwhile, when the accreting gas is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic field. The density profile varies as ∼r⁻¹rather thanr^−3/2and the accretion rate$\dot{M}$is consequently suppressed by over 2 orders of magnitude below the Bondi rate${\dot{M}}_{\mathrm{B}}$. We find continuous energy feedback from the accretion flow to the external medium at a level of$\sim {10}^{-2}\dot{M}{c}^2\sim 5\times {10}^{-5}{\dot{M}}_{\mathrm{B}}{c}^2$. Energy transport across these widely disparate scales occurs via turbulent convection triggered by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that accumulate on horizon scales transform the flow dynamics far from the SMBH and naturally explain observed extremely low accretion rates compared to the Bondi rate, as well as at least part of the energy feedback.

    

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4. Kinetic-scale Current Sheets in the Solar Wind at 1 au: Scale-dependent Properties and Critical Current Density

   https://doi.org/10.3847/2041-8213/ac4fc4  

   Vasko, I. Y. ; Alimov, K. ; Phan, T. ; Bale, S. D. ; Mozer, F. S. ; Artemyev, A. V. ( February 2022 , The Astrophysical Journal Letters)

   We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s⁻¹magnetic field resolution. The CSs have thickness,λ,from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λ_pin terms of local proton inertial length,λ_p. We found that the current density is larger for smaller-scale CSs,J₀≈ 6 nAm⁻²· (λ/100 km)^−0.56, but does not statistically exceed a critical value,J_A,corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units:$J_0/J_A\approx 0.17·{(\lambda /{\lambda }_p)}^{-0.51}$. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale:$\mathrm{\Delta }\theta \approx 19°·{(\lambda /{\lambda }_p)}^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.

    

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5. Microphysics of Relativistic Collisionless Electron-ion-positron Shocks

   https://doi.org/10.3847/1538-4357/ac713e  

   Grošelj, Daniel ; Sironi, Lorenzo ; Beloborodov, Andrei M. ( July 2022 , The Astrophysical Journal)

   We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10⁻⁴and pair-loading factorsZ_±≲ 10 are studied, whereZ_±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσ_Lthat decreases withZ_±. Atσ≲σ_Lthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ_±, leading to lowerσ_L. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales as${\overline{E}}_{\mathrm{e}}\propto {\left(Z_{\pm }+1\right)}^{-1}$. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10⁻⁶. The ions then become essentially thermal with mean energy${\overline{E}}_{\mathrm{i}}$, while electrons form a nonthermal tail, extending from$E\sim {\left(Z_{\pm }+1\right)}^{-1}{\overline{E}}_{\mathrm{i}}$to${\overline{E}}_{\mathrm{i}}$. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.

    

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