Electromagnetic Observables of Weakly Collisional Black Hole Accretion

TL;DR

This work addresses the discrepancy between observed variability in horizon-scale emission from LLAGN and predictions from ideal GRMHD by introducing an Extended GRMHD (EGRMHD) framework that includes Braginskii-like heat conduction and pressure anisotropy. Implemented in KHARMA, the model is applied to MAD and SANE accretion flows around a nonspinning and a rapidly spinning black hole, with synthetic horizon-scale images and spectra generated via ipole and igrmonty. The key finding is that time-averaged images and spectra are largely unchanged relative to ideal GRMHD, but 230 GHz light curves from weakly collisional models show reduced variability, particularly in MAD configurations. This reduction likely stems from weaker turbulent activity and limits on pressure anisotropy imposed by kinetic instabilities, suggesting a path toward reconciling EHT observations with fluid models and motivating future two-fluid or PIC-calibrated closures for electron physics.

Abstract

The black holes in the Event Horizon Telescope sources Messier 87* and Sagittarius A* (SgrA*) are embedded in a hot, collisionless plasma that is fully described in kinetic theory yet is usually modeled as an ideal, magnetized fluid. In this Letter, we present results from a new set of weakly collisional fluid simulations in which leading order kinetic effects are modeled as viscosity and heat conduction. Consistent with earlier, lower-resolution studies, we find that overall flow dynamics remain very similar between ideal and non-ideal models. For the first time, we synthesize images and spectra of SgrA* from weakly collisional models -- assuming an isotropic, thermal population of electrons -- and find that these remain largely indistinguishable from ideal fluid predictions. However, most weakly collisional models exhibit lower light curve variability, with all magnetically dominated models showing a small but systematic decrease in variability.

Figures (9)

• Figure 1: Snapshot from an EGRMHD, MAD, $a_{*} = 15/16$ simulation. (a) Poloidal $(r,\theta)$ slices of fluid rest-mass density (left) and dimensionless temperature (right). (b) Poloidal slices of heat flux normalized by the free-streaming value (left) and pressure anisotropy normalized by magnetic energy density (right). The black contour marks $\sigma = 1$, which separates the accretion disk from the magnetically dominated jet. (c) Mass-weighted distribution of pressure anisotropy as a function of $\beta$ within $r \leq 20r_{\text{g}}$ for the snapshot shown in panels (a) and (b). The black dotted (dashed) lines indicate the mirror (firehose) instability threshold.
• Figure 2: Comparison of time-averaged fluid quantities between Extended and Ideal simulations (MAD, $a_{*} = 15/16$). (a) The top row shows the time- and azimuthally-averaged rest-mass density, $\rho$, and the bottom row displays plasma magnetization, $\sigma$. The left column shows the Extended simulation and the right column shows the Ideal simulation. (b) Angular momentum transport in the disk: components of the density-weighted average $\langle T^{r}_{\phi}\rangle$ normalized by gas pressure $\langle P\rangle$ are plotted as a function of radius. (c) Radial profiles of $\beta$ for Extended and Ideal simulations.
• Figure 3: Electromagnetic observables for MAD $a_{*}=15/16$ simulation with $R_{\text{high}}=160$ at a viewing angle of $30\degree$. (a) and (b) Time-averaged 230 GHz total intensity images for the Extended and Ideal GRMHD simulations, respectively. (c) Comparison of time-averaged spectra for the simulations shown in (a) and (b). (d) Light curves at 230 GHz over a duration of $\Delta t = 15,000~t_{\text{g}}$ ($\sim84$ hours for Sgr A$^*$).
• Figure 4: Three-hour modulation index for all models considered in this work. Marker color represents the magnetization state (MAD vs SANE), while marker shape indicates the black hole spin. Points below the dashed line represent models where the Extended GRMHD simulation produces 230 GHz light curves with lower variability than their Ideal counterparts.
• Figure 5: A schematic flowchart illustrating the sequence of operations during a half-step ($t^{n}\rightarrow t^{n+1/2}$) in the EGRMHD evolution. The gray box marks the Implicit kernel, which iteratively determines the next fluid state.