Probing Plasma Physics with Spectral Index Maps of Accreting Black Holes on Event Horizon Scales

Abstract

The Event Horizon Telescope (EHT) collaboration has produced the first resolved images of the supermassive black holes at the centre of our galaxy and at the centre of the elliptical galaxy M87. As both technology and analysis pipelines improve, it will soon become possible to produce spectral index maps of black hole accretion flows on event horizon scales. In this work, we predict spectral index maps of both M87* and Sgr A* by applying the general relativistic radiative transfer (GRRT) code IPOLE to a suite of general relativistic magnetohydrodynamic (GRMHD) simulations. We analytically show that the spectral index increases with increasing magnetic field strength, electron temperature, and optical depth. Consequently, spectral index maps grow more negative with increasing radius in almost all models, since all of these quantities tend to be maximised near the event horizon. Additionally, photon ring geodesics exhibit more positive spectral indices, since they sample the innermost regions of the accretion flow with the most extreme plasma conditions. Spectral index maps are sensitive to highly uncertain plasma heating prescriptions (the electron temperature and distribution function). However, if our understanding of these aspects of plasma physics can be tightened, even the spatially unresolved spectral index around 230 GHz can be used to discriminate between models. In particular, Standard and Normal Evolution (SANE) flows tend to exhibit more negative spectral indices than Magnetically Arrested Disk (MAD) flows due to differences in the characteristic magnetic field strength and temperature of emitting plasma.

Accepted for publication in MNRAS

References in corpus (36)

• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
• First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
• First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
• First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
• The Co-Evolution of Galaxies and Supermassive Black Holes: Insights from Surveys of the Contemporary Universe
• First M87 Event Horizon Telescope Results. II. Array and Instrumentation
• First M87 Event Horizon Telescope Results. III. Data Processing and Calibration
• Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre
• First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon
• First Sagittarius A* Event Horizon Telescope Results. VI: Testing the Black Hole Metric
• First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole
• First M87 Event Horizon Telescope Results. VII. Polarization of the Ring
• First Sagittarius A* Event Horizon Telescope Results. III: Imaging of the Galactic Center Supermassive Black Hole
• First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multi-wavelength Observations, Data Processing, and Calibration
• First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass
• The Evolution of Black Hole Mass and Spin in Active Galactic Nuclei
• An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis
• Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon
• Universal Polarimetric Signatures of the Black Hole Photon Ring
• Millimeter light curves of Sagittarius A* observed during the 2017 Event Horizon Telescope campaign
• Polarimetric properties of Event Horizon Telescope targets from ALMA
• Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
• Magnetorotational Turbulence and Dynamo in a Collisionless Plasma
• Evaluation of New Submillimeter VLBI Sites for the Event Horizon Telescope
• State-of-the-art energetic and morphological modelling of the launching site of the M87 jet
• Sgr A* X-ray flares from non-thermal particle acceleration in a magnetically arrested disc
• Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope
• Comparison of the ion-to-electron temperature ratio prescription: GRMHD simulations with electron thermodynamics
• The Flux Distribution of Sgr A*
• Iharm3D: Vectorized General Relativistic Magnetohydrodynamics
• Black Hole Magnetic Fields and Their Imprint on Circular Polarization Images
• Electron and Proton Heating in Transrelativistic Guide Field Reconnection
• Decomposing the Internal Faraday Rotation of Black Hole Accretion Flows
• Unraveling circular polarimetric images of magnetically arrested accretion flows near event horizon of a black hole
• Relative depolarization of the black hole photon ring in GRMHD models of Sgr A* and M87*

Cited by in corpus (3)

• Multi-frequency Black Hole Imaging for the Next-Generation Event Horizon Telescope
• Using Machine Learning to Link Black Hole Accretion Flows with Spatially Resolved Polarimetric Observables
• How Spatially Resolved Polarimetry Informs Black Hole Accretion Flow Models