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1. Global Electron Thermodynamics in Radiatively Inefficient Accretion Flows

   https://doi.org/10.3847/1538-4357/acef12  

   Satapathy, Kaushik; Psaltis, Dimitrios; Özel, Feryal (September 2023, The Astrophysical Journal)

   Abstract In the collisionless plasmas of radiatively inefficient accretion flows, heating and acceleration of ions and electrons are not well understood. Recent studies in the gyrokinetic limit revealed the importance of incorporating both the compressive and Alfvénic cascades when calculating the partition of dissipated energy between the plasma species. In this paper, we use a covariant analytic model of the accretion flow to explore the impact of compressive and Alfvénic heating, Coulomb collisions, compressional heating, and radiative cooling on the radial temperature profiles of ions and electrons. We show that, independent of the partition of heat between the plasma species, even a small fraction of turbulent energy dissipated to the electrons makes their temperature scale with a virial profile and the ion-to-electron temperature ratio smaller than in the case of pure Coulomb heating. In contrast, the presence of compressive cascades makes this ratio larger because compressive turbulent energy is channeled primarily into the ions. We calculate the ion-to-electron temperature in the inner accretion flow for a broad range of plasma properties, mass accretion rates, and black hole spins and show that it ranges between 5 ≲T_i/T_e≲ 40. We provide a physically motivated expression for this ratio that can be used to calculate observables from simulations of black hole accretion flows for a wide range of conditions. 

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2. Using Machine Learning to link black hole accretion flows with spatially resolved polarimetric observables

   https://doi.org/10.1093/mnras/stad466  

   Qiu, Richard; Ricarte, Angelo; Narayan, Ramesh; Wong, George N; Chael, Andrew; Palumbo, Daniel (February 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We introduce a new library of 535 194 model images of the supermassive black holes and Event Horizon Telescope (EHT) targets Sgr A* and M87*, computed by performing general relativistic radiative transfer calculations on general relativistic magnetohydrodynamics simulations. Then to infer underlying black hole and accretion flow parameters (spin, inclination, ion-to-electron temperature ratio, and magnetic field polarity), we train a random forest machine learning model on various hand-picked polarimetric observables computed from each image. Our random forest is capable of making meaningful predictions of spin, inclination, and the ion-to-electron temperature ratio, but has more difficulty inferring magnetic field polarity. To disentangle how physical parameters are encoded in different observables, we apply two different metrics to rank the importance of each observable at inferring each physical parameter. Details of the spatially resolved linear polarization morphology stand out as important discriminators between models. Bearing in mind the theoretical limitations and incompleteness of our image library, for the real M87* data, our machinery favours high-spin retrograde models with large ion-to-electron temperature ratios. Due to the time-variable nature of these targets, repeated polarimetric imaging will further improve model inference as the EHT and next-generation (EHT) continue to develop and monitor their targets. 

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3. Circular Polarization of Simulated Images of Black Holes

   https://doi.org/10.3847/1538-4357/ad5b51  

   Joshi, Abhishek V; Prather, Ben S; Chan, Chi-kwan; Wielgus, Maciek; Gammie, Charles F (September 2024, The Astrophysical Journal)

   Abstract Models of the resolved Event Horizon Telescope (EHT) sources Sgr A* and M87* are constrained by observations at multiple wavelengths, resolutions, polarizations, and time cadences. In this paper, we compare unresolved circular polarization (CP) measurements to a library of models, where each model is characterized by a distribution of CP over time. In the library, we vary the spin of the black hole, the magnetic field strength at the horizon (i.e., both SANE and magnetically arrested disk or MAD models), the observer inclination, a parameter for the maximum ion–electron temperature ratio assuming a thermal plasma, and the direction of the magnetic field dipole moment. We find that Atacama Large Millimeter/submillimeter Array (ALMA) observations of Sgr A* are inconsistent with all edge-on (i= 90°) models. Restricting attention to the MAD models favored by earlier EHT studies of Sgr A*, we find that only models with magnetic dipole moment pointing away from the observer are consistent with ALMA data. We also note that in 26 of the 27 passing MAD models, the accretion flow rotates clockwise on the sky. We provide a table of the means and standard deviations of the CP distributions for all model parameters, along with their trends. 

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4. Electron Heating in the Transrelativistic Perpendicular Shocks of Tilted Accretion Flows

   https://doi.org/10.3847/1538-4357/ad4607  

   Sironi, Lorenzo; Tran, Aaron (June 2024, The Astrophysical Journal)

   Abstract General relativistic magnetohydrodynamic (GRMHD) simulations of black hole tilted disks—where the angular momentum of the accretion flow at large distances is misaligned with respect to the black hole spin—commonly display standing shocks within a few to tens of gravitational radii from the black hole. In GRMHD simulations of geometrically thick, optically thin accretion flows, applicable to low-luminosity sources like Sgr A* and M87*, the shocks have transrelativistic speed, moderate plasma beta (the ratio of ion thermal pressure to magnetic pressure isβ_pi1∼ 1–8), and low sonic Mach number (the ratio of shock speed to sound speed isM_s∼ 1–6). We study such shocks with 2D particle-in-cell simulations, and we quantify the efficiency and mechanisms of electron heating for the special case of preshock magnetic fields perpendicular to the shock direction of propagation. We find that the postshock electron temperatureT_e2exceeds the adiabatic expectationT_e2,adby an amount $T_{\mathrm{e}2}/T_{\mathrm{e}2,\mathrm{ad}}-1\simeq 0.0016M_s^{3.6}$, nearly independent of the plasma beta and of the preshock electron-to-ion temperature ratioT_e1/T_i1, which we vary from 0.1 to unity. We investigate the heating physics forM_s∼ 5–6 and find that electron superadiabatic heating is governed by magnetic pumping atT_e1/T_i1= 1, whereas heating byB-parallel electric fields (i.e., parallel to the local magnetic field) dominates atT_e1/T_i1= 0.1. Our results provide physically motivated subgrid prescriptions for electron heating at the collisionless shocks seen in GRMHD simulations of black hole accretion flows. 

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5. Accretion Flow Morphology in Numerical Simulations of Black Holes from the ngEHT Model Library: The Impact of Radiation Physics

   https://doi.org/10.3390/galaxies11020038  

   Chatterjee, Koushik; Chael, Andrew; Tiede, Paul; Mizuno, Yosuke; Emami, Razieh; Fromm, Christian; Ricarte, Angelo; Blackburn, Lindy; Roelofs, Freek; Johnson, Michael D.; et al (April 2023, Galaxies)

   In the past few years, the Event Horizon Telescope (EHT) has provided the first-ever event horizon-scale images of the supermassive black holes (BHs) M87* and Sagittarius A* (Sgr A*). The next-generation EHT project is an extension of the EHT array that promises larger angular resolution and higher sensitivity to the dim, extended flux around the central ring-like structure, possibly connecting the accretion flow and the jet. The ngEHT Analysis Challenges aim to understand the science extractability from synthetic images and movies to inform the ngEHT array design and analysis algorithm development. In this work, we compare the accretion flow structure and dynamics in numerical fluid simulations that specifically target M87* and Sgr A*, and were used to construct the source models in the challenge set. We consider (1) a steady-state axisymmetric radiatively inefficient accretion flow model with a time-dependent shearing hotspot, (2) two time-dependent single fluid general relativistic magnetohydrodynamic (GRMHD) simulations from the H-AMR code, (3) a two-temperature GRMHD simulation from the BHAC code, and (4) a two-temperature radiative GRMHD simulation from the KORAL code. We find that the different models exhibit remarkably similar temporal and spatial properties, except for the electron temperature, since radiative losses substantially cool down electrons near the BH and the jet sheath, signaling the importance of radiative cooling even for slowly accreting BHs such as M87*. We restrict ourselves to standard torus accretion flows, and leave larger explorations of alternate accretion models to future work. 

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