High-energy Emission from Turbulent Electron-ion Coronae of Accreting Black Holes

TL;DR

This study analyzes energy dissipation and high-energy emission in radiatively dense black-hole coronae via 2D radiative PIC simulations that include self-consistent Compton scattering. The corona self-organizes into a trans-sonic, trans-Alfvénic, two-temperature state with $T_i \gg T_e$, where ions receive $q_i \sim 0.6$–$0.7$ of the dissipated energy and electrons radiate efficiently, producing a Comptonized X-ray spectrum with a peak near $\sim$100 keV. Nonthermal particle populations arise: ions develop extended tails ( $p \gtrsim 3$ ) and electrons form hard tails due to reconnection at intense current sheets, yielding a MeV tail increasingly pronounced with higher $\ell$ and $\sigma_e$. The simulated spectra closely match X-ray observations of the AGN NGC 4151, supporting the model as a viable description of coronal dissipation and its radiative output, while highlighting the MeV band as a diagnostic for microphysical acceleration processes and the potential for cosmic-ray production in AGN coronae.

Abstract

We develop a model of particle energization and emission from strongly turbulent black-hole coronae. Our local model is based on a set of 2D radiative particle-in-cell simulations with an electron-ion plasma composition, injection and diffusive escape of photons and charged particles, and self-consistent Compton scattering. We show that a radiatively compact turbulent corona generates extended nonthermal ion distributions, while producing X-ray spectra consistent with observations. As an example, we demonstrate excellent agreement with observed X-ray spectra of NGC 4151. The predicted emission spectra feature an MeV tail, which can be studied with future MeV-band instruments. The MeV tail is shaped by nonthermal electrons accelerated at turbulent current sheets. We also find that the corona regulates itself into a two-temperature state, with ions much hotter than electrons. The ions carry away roughly 60% to 70% of the dissipated power, and their energization is driven by a combination of shocks and reconnecting current sheets, embedded into the turbulent flow.

Figures (7)

• Figure 1: Time evolution and approach to steady state in our radiative PIC simulations of turbulence with ion magnetizations $\sigma_{\rm i} = 0.035,\, 0.1,\, 0.19$. Shown from top to bottom are the box-averaged compactness $\ell$, proper electron kinetic temperature $T_{\rm e}$, proper ion kinetic temperature $T_{\rm i}$, the turbulent sonic Mach number $M_{\rm s}$, and the Alfvénic Mach number $M_{\rm A}$.
• Figure 2: Visualization of turbulent fields in our simulation with $\sigma_{\rm i} = 0.19$ at time $t = 6.86\, S/v_{\rm A}$. The left panels show the proper ion and electron kinetic temperatures ($T_{\rm i}$ and $T_{\rm e}$). In the middle panels we show the magnetic field magnitude $B$ and the out-of-plane electric current $J_z$. Finally, the right panels depict the ion density $n$ and the density of fast-mode fluctuations $n_{\rm fast}$. For easier comparison with the total density $n$, we include the mean value in the visualization of the fast mode (i.e., $n_{\rm fast} = \delta n_{\rm fast} + n_0$). An animated version of this figure is available online at https://youtu.be/0W-b242WMhw. The animation lasts 50 s and shows the spatiotemporal evolution of the turbulent fields from the start ($t v_{\rm A}/S=0$) to the end ($t v_{\rm A}/S = 10.3$) of the simulation, in the same format as the static figure.
• Figure 3: Turbulent energy spectra in our simulation with $\sigma_{\rm i}=0.19$, averaged over the steady state from $t = 4\,S/v_{\rm A}$ until the end of the run at $t = 10.3\,S/v_{\rm A}$. In the bottom panel we show the 1D energy spectrum ${\mathcal{E}}(k_\perp)$ as a function of the perpendicular wavenumber $k_\perp$. Power-law slopes are shown for reference. In the top panel we show the relative energy content of fluctuations identified as the MHD Alfvén, slow, and fast mode (see Appendix \ref{['sec:modes']} for details).
• Figure 4: Steady-state particle spectra in our simulations with different strengths of the ion magnetization $\sigma_{\rm i}$. The middle panel shows the ion energy spectra and the bottom panel the electron spectra. Energy is measured in units of the particles' own rest mass ($m_{\rm i}c^2$ for ions and $m_{\rm e}c^2$ for electrons). Power-law slopes are indicated with black dotted lines for reference. In the bottom panel we fit a Maxwellian distribution to the low-energy part of the spectrum (dashed black curve). The top panel shows the ion escape time $t_{\rm esc,i}$. Finally, the inset shows the ion heating fraction.
• Figure 5: Comparison of predicted emission spectra with X-ray observations of NGC 4151. Dashed curves show the intrinsic steady-state spectra obtained directly from our PIC simulations with different strengths of the radiative compactness $\ell \approx 0.6,\, 2.5,\, 8.5$. Solid curves show the spectra corrected for absorption and reflection. Shaded bands indicate the ranges over which the simulated spectra vary during the averaging interval from $t \approx 4 S/v_{\rm A}$ to $t \approx 10 S/v_{\rm A}$.