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Abstract The black holes in the Event Horizon Telescope sources Messier 87* and Sagittarius A* (Sgr A*) 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 nonideal models. For the first time, we synthesize images and spectra of Sgr A* 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. 

Abstract Using very-long-baseline interferometry (VLBI) observations at (sub)millimeter wavelengths, the Event Horizon Telescope (EHT) currently achieves the finest angular resolution of any astronomical facility, necessary for imaging the horizon-scale structure around supermassive black holes. A significant calibration challenge for high-frequency VLBI stems from rapid variations in the atmospheric water vapor content above each telescope in the array, which induce corresponding fluctuations in the phase of the correlated signal that limit the coherent integration time and thus the achievable sensitivity. In this paper, we introduce a model that describes station-based phase corruptions jointly with a parameterization for the source structure. We adopt a Gaussian process (GP) prescription for the time evolution of these phase corruptions, which provides sufficient flexibility to capture even highly erratic phase behavior. The use of GPs permits the application of a Kalman filtering algorithm for numerical marginalization of these phase corruptions, which permits efficient exploration of the remaining parameter space. Our model also removes the need to specify an arbitrary “reference station” during calibration, instead establishing a global phase zero-point by enforcing the GPs at all stations to have fixed mean and finite variance. We validate our method using a real EHT observation of the blazar 3C 279, demonstrating that our approach yields calibration solutions that are consistent with those determined by the EHT Collaboration. The model presented here can be straightforwardly extended to incorporate frequency-dependent phase behavior, such as is relevant for the frequency phase transfer calibration technique. 

Abstract The Event Horizon Telescope (EHT) enables the exploration of black hole accretion flows at event-horizon scales. Fitting ray-traced physical models to EHT observations requires the generation of synthetic images, a task that is computationally demanding. This study leveragesALINet, a generative machine learning model, to efficiently produce radiatively inefficient accretion flow (RIAF) images as a function of the specified physical parameters.ALINethas previously been shown to be able to interpolate black hole images and their associated physical parameters after training on a computationally tractable set of library images. We utilize this model to estimate the uncertainty introduced by a number of anticipated unmodeled physical effects, including interstellar scattering and intrinsic source variability. We then use this to calibrate physical parameter estimates and their associated uncertainties from RIAF model fits to mock EHT data via a library of general relativistic magnetohydrodynamics models. 

Abstract The Event Horizon Telescope (EHT) has produced horizon-resolving images of Sagittarius A* (Sgr A*). Scattering in the turbulent plasma of the interstellar medium distorts the appearance of Sgr A* on scales only marginally smaller than the fiducial resolution of EHT. The scattering process both diffractively blurs and adds stochastic refractive substructures that limits the practical angular resolution of EHT images of Sgr A*. We explore the ability of a novel recurrent neural network machine learning framework to mitigate these scattering effects, after training on sample data that are agnostic to general relativistic magnetohydrodynamics (GRMHD). We demonstrate that if instrumental limitations are negligible, it is possible to nearly completely mitigate interstellar scattering at a wavelength of 1.3 mm. We validate and quantify the fidelity of this scattering mitigation scheme with physically relevant GRMHD simulations. We find that we can accurately reconstruct resolved structures at the scale of 3μas, well below the nominal instrumental resolution of EHT, 24μas. 

Abstract We introduceMahakala, aPython-based, modular, radiative ray-tracing code for curved spacetimes. We employ Google’sJAXframework for accelerated automatic differentiation, which can efficiently compute Christoffel symbols directly from the metric, allowing the user to easily and quickly simulate photon trajectories through non-Kerr spacetimes.JAXalso enablesMahakalato run in parallel on both CPUs and GPUs.Mahakalanatively uses the Cartesian Kerr–Schild coordinate system, which avoids numerical issues caused by the pole in spherical coordinate systems. We demonstrateMahakala’s capabilities by simulating 1.3 mm wavelength images (the wavelength of Event Horizon Telescope observations) of general relativistic magnetohydrodynamic simulations of low-accretion rate supermassive black holes. The modular nature ofMahakalaallows us to quantitatively explore how different regions of the flow influence different image features. We show that most of the emission seen in 1.3 mm images originates close to the black hole and peaks near the photon orbit. We also quantify the relative contribution of the disk, forward jet, and counterjet to 1.3 mm images. 

Abstract Models of highly sub-Eddington accretion onto black holes commonly use a single-fluid model for the collisionless, near-horizon plasma. These models must specify an equation of state. It is common to use an ideal gas withp = (γ − 1)uandγ = 4/3, 13/9, or 5/3, but these produce significantly different outcomes. We discuss the origins of this discrepancy and the assumptions underlying the single-fluid model. The main result of this investigation is that under conditions relevant to low-luminosity black hole accretion the best choice of single-fluid adiabatic index is close to but slightly less than 5/3. Along the way we provide a simple equilibrium model for the relation between the ion-to-electron dissipation ratio and the ion-to-electron temperature ratio, and explore the implications for electron temperature fluctuations in Event Horizon Telescope sources. 

Abstract The Event Horizon Telescope (EHT) has produced resolved images of the supermassive black holes (SMBHs) Sgr A* and M87*, which present the largest shadows on the sky. In the next decade, technological improvements and extensions to the array will enable access to a greater number of sources, unlocking studies of a larger population of SMBHs through direct imaging. In this paper, we identify 12 of the most promising sources beyond Sgr A* and M87* based on their angular size and millimeter flux density. For each of these sources, we make theoretical predictions for their observable properties by ray tracing general relativistic magnetohydrodynamic models appropriately scaled to each target’s mass, distance, and flux density. We predict that these sources would have somewhat higher Eddington ratios than M87*, which may result in larger optical and Faraday depths than previous EHT targets. Despite this, we find that visibility amplitude size constraints can plausibly recover masses within a factor of 2, although the unknown jet contribution remains a significant uncertainty. We find that the linearly polarized structure evolves substantially with the Eddington ratio, with greater evolution at larger inclinations, complicating potential spin inferences for inclined sources. We discuss the importance of 345 GHz observations, milli-Jansky baseline sensitivity, and independent inclination constraints for future observations with upgrades to the EHT through ground updates with the next-generation EHT program and extensions to space through the black hole Explorer. 

Abstract We used the NSF Jansky Very Large Array at a frequencyν = 22 GHz to study the nearest billion-solar-mass black hole (BH), in the early-type galaxy NGC 3115 at a distance of 9.7 Mpc. We localize a faint continuum nucleus, with flux densityS_{22 GHz} = 48.2 ± 6.4μJy, to a FWHM diameterd_{22 GHz} < 59 mas (2.8 pc). We find no evidence for adjacent emission within a stagnation region of radiusR_sta ∼ 360 mas (17 pc) identified in a recent hydrodynamic simulation tailored to NGC 3115. Within that region, the simulated gas flow developed into an advection-dominated accretion flow (ADAF). The nucleus’ luminosity densityL_{22 GHz} = 5.4 × 10¹⁷W Hz⁻¹is about 60 times that of Sagittarius A^⋆. The nucleus’ spectral index ${\alpha }_{10\mathrm{GHz}}^{22\mathrm{GHz}}=-1.85\pm 0.18$(S_ν ∝ ν^α) indicates optically thin synchrotron emission. The spectral energy distribution of the nucleus peaks nearν_peak = 9 GHz. Modeling this radio peak as an ADAF implies a BH massM_ADAF = (1.2 ± 0.2) × 10⁹M_⊙, consistent with previous estimates of (1–2) × 10⁹M_⊙from stellar or hot-gas dynamics. Also, the Eddington-scaled accretion rate for NGC 3115, ${\dot{M}}_{\mathrm{ADAF}}/{\dot{M}}_{\mathrm{Edd}}=1.2_{-0.6}^{+1.0}\times 10^{-8}$, is about 4–8 times lower than recent estimates for Sagittarius A^⋆. 

Abstract Very long baseline interferometry observations reveal that relativistic jets like the one in M87 have a limb-brightened, double-edged structure. Analytic and numerical models struggle to reproduce this limb-brightening. We propose a model in which we invoke anisotropy in the distribution function of synchrotron-emitting nonthermal electrons such that electron velocities are preferentially directed parallel to magnetic field lines, as suggested by recent particle-in-cell simulations of electron acceleration and the effects of synchrotron cooling. We assume that the energy injected into nonthermal electrons is proportional to the jet Poynting flux, and we account for synchrotron cooling via a broken power-law energy distribution. We implement our emission model in both general relativistic magnetohydrodynamic (GRMHD) simulations and axisymmetric force-free electrodynamic (GRFFE) jet models and produce simulated jet images at multiple scales and frequencies using polarized general relativistic radiative transfer. We find that the synchrotron emission is concentrated parallel to the local helical magnetic field and that this feature produces limb-brightened jet images on scales ranging from tens of microarcseconds to hundreds of milliarcseconds in M87. We present theoretical predictions for horizon-scale M87 jet images at 230 and 345 GHz that can be tested with next-generation instruments. Due to the scale-invariance of the GRMHD and GRFFE models, our emission prescription can be applied to other targets and serve as a foundation for a unified description of limb-brightened synchrotron images of extragalactic jets. 

Abstract We continue our previous work, H.-S. Chan et al., to investigate how variations in the electron temperature prescription parameter,R_Low, influence the 3 hr variability at 230 GHz,M_ΔT, in magnetically arrested disk (MAD) models of Sagittarius A* (Sgr A*), through analyzing a series of general-relativistic magnetohydrodynamics and ray-tracing simulations. For models with a black hole spina > 0, we discovered that increasingR_Lowrenders the photon ring more optically thick, obscuring the varying accretion flows that contribute to the variability. However, asR_Lowincreases further, MAD flux eruptions become more pronounced, compensating for the decrease inM_ΔT. For models with spina < 0, although a higherR_Lowalso increases the optical thickness of the fluid, voids within the optically thick gas fail to cover the entire photon ring. Similarly, flux eruptions become more prominent asR_Lowincreases further, contributing to the observed rise inM_ΔTrelative toR_Low. For black holes with spina= 0, although the effect of increasing optical depth is still present, their 230 GHz light curves, and henceM_ΔT, are insensitive to changes inR_Low. Furthermore, we found that the variability of the 230 GHz light curves atR_Low = 1 might correlate with fluctuations in the internal energy of the gas near the black hole, and we listed potential causes and solutions to the over-variability problem. Our findings highlight potential approaches for refiningM_ΔTto better align with observations when modeling Sgr A*.