Radiative Cooling Effects on Plasmoid Formation in Black Hole Accretion Flows with Multiple Magnetic Loops

TL;DR

This study demonstrates that radiative cooling can fundamentally alter black hole accretion flow dynamics with multi-loop magnetic fields by reducing magnetic flux accumulation near the horizon, lowering electron temperatures, and compressing the disk. These macroscopic changes couple to microscopic reconnection physics, producing radiative collapse in current sheets that shortens plasmoid lifetimes while increasing the frequency of plasmoid formation. The cooling-induced layer compression also yields a population of smaller plasmoids and modifies energy transport via negative-energy regions near the ergosphere, potentially influencing reconnection-driven energy extraction. The results highlight radiative cooling as a key factor shaping both the large-scale accretion structure and the microphysics of magnetic reconnection in astrophysical black holes, with implications for interpreting rapid flares and variability in systems like Sgr A$^*$. Caveats include the elevated accretion rates used and the need for higher-resolution, fully 3D modeling with comprehensive radiation feedback to make quantitative predictions for real sources.

Abstract

Context. We investigate the physics of black hole accretion flows, particularly focusing on phenomena like magnetic reconnection and plasmoid formation, which are believed to be responsible for energetic events such as flares observed from astrophysical black holes.Aims. We aim to understand the influence of radiative cooling on plasmoid formation within black hole accretion flows that are threaded by multi-loop magnetic field configurations.Methods. We conducted two- and three-dimensional two-temperature general relativistic magnetohydrodynamic (GRMHD) simulations. By varying the magnetic loop sizes and the mass accretion rate, we explored how radiative cooling alters the accretion dynamics, disk structure, and the properties of reconnection-driven plasmoid chains.Results. Our results demonstrate that radiative cooling suppresses the transition to the magnetically arrested disk (MAD) state by reducing magnetic flux accumulation near the horizon. It significantly modifies the disk morphology by lowering the electron temperature and compressing the disk, which leads to increased density at the equatorial plane and decreased magnetization. Within the current sheets, radiative cooling triggers layer compression and the collapse of plasmoids, shortening their lifetime and reducing their size, while the frequency of plasmoid events increases. Moreover, we observe enhanced negative energy-at-infinity density in plasmoids near the ergosphere, with its peaks corresponding to plasmoid presence.Conclusions. Radiative cooling plays a critical role in shaping both macroscopic accretion flow properties and microscopic reconnection phenomena near black holes. This suggests that radiative cooling may modulate black hole energy extraction through reconnection-driven Penrose processes, highlighting its importance in models of astrophysical black holes.

Figures (18)

• Figure 1: Time evolution of mass accretion rates measured at the event horizon (top) and normalized magnetic flux at the horizon (bottom). The left panels (a,c) correspond to the case with smaller magnetic loops ($\lambda = 40$), while the right panels (b,d) represent the case with larger magnetic loops ($\lambda = 80$). The curves in different colors correspond to the different radiative cooling and time-averaged accretion rates: radiative cooling with $\dot{M}/\dot{M}_{\rm Edd}=1 \times10^{-3}$ (bule, green), radiative cooling with $1 \times10^{-5}$(black, light-blue), and no cooling (red, light-red). In panel (c), the horizontal red and blue lines denote the average magnetic flux of models A and C, respectively. The blue vertical lines in panels (a) and (c) mark the onset of the quasi-steady state. In panels (b) and (d), the light-blue and light-red vertical lines indicate the transition from SANE to MAD for models E and F, respectively. The time is given in units of the light crossing time, $t_g \equiv GM/c^3 = [\text{M}]$.
• Figure 2: Same as Fig. \ref{['fig:Fig1']} but shown in 3D cases (M40n3d and M40a3d).
• Figure 3: Time-averaged electron temperature $\Theta_{\,\rm e}$, plasma $\beta$, and density $\rho$ distributions for three models. From top to bottom, the panels correspond to models A, B, and C. The plasma beta parameter is defined as $\beta = p_{\,\rm g}/p_{\text{mag}}$, where $p_{\,\rm g}$ represents the gas pressure, and $p_{\text{mag}} = b^2/2$ denotes the magnetic pressure. The light black contours in the density profile represent the magnetic field. The average is taken over the time range $t = 8000\,\rm M$ to $13000\,\rm M$.
• Figure 4: The density-weighted scale height as a function of radius for various cooling models A, B, and C.
• Figure 5: Azimuthal and time-averaged polar-angle profiles of the ion-to-electron temperature ratio of the models A, B, and C at $r=15\, \rm M$. Using the same averaging time range as in Fig. \ref{['fig:Fig3']}.