Session LL03: V: Probing the Structure and Physics of Accretion Flows at Event Horizon Scales with the EHT

From Photons to Black Hole Science: Reconstructing the Variable Structure of Sgr A* with the Event Horizon Telescope

The international Event Horizon Telescope Collaboration (EHTC) has assembled the Event Horizon Telescope (EHT) very long baseline interferometry array and made the first science observations in April 2017. The EHT currently combines eleven millimeter/submillimeter radio observatories into an Earth-sized virtual telescope and thus achieves an angular resolution of about 20 microarcseconds. This unprecedented resolving power, combined with a high sensitivity from wide bandwidths and the large collecting area of the central phased Atacama Large Millimeter/submillimeter Array, allowed us to image the Sagittarius A* and M87* supermassive black holes.

I will discuss the complex EHT data processing pathway -- from the recording of mm radio waves at remote telescope sites to the data processing on supercomputers and the final reconstruction of source models from the sparse interferometric visibilities. I will focus on the analysis of the Sgr A* data, for which we had to deal with the rapid variability of the source and the presence of a strong interstellar scattering screen. For the scientific analysis of the galactic center EHT observation, we made use of exquisite data obtained from a coordinated multi-wavelength campaign.

I will conclude with an outlook of future EHT science enabled by ongoing upgrades of the array and new technical capabilities.   

What the Event Horizon Telescope Reveals on Physics of the Accretion Flow Around Sgr A* and M 87*

We compare EHT images of SgrA* and M87* as well as non-EHT data spanning from the radio to the X-ray to a broad suite of theoretical GRMHD simulations. These simulations span a large parameter space and explore a variety of assumptions about the accretion flow, including aligned and tilted models, thermal and non-thermal electron heating prescriptions, and wind-fed models. For SgrA*, all of the models fail to match one or more of the observational constraints, with the lightcurve and structural variability in particular eliminating nearly all models. For M87*, measurements of the resolved linear polarization constrain the magnetic field structure close to the event horizon. In both cases, observational constraints favor models with strong, dynamically important magnetic fields.   

Probing Gravitational Physics with the Event Horizon Telescope

New horizon-scale images of the Galactic Center black hole Sagittarius A* (Sgr A*) recently published by the Event Horizon Telescope (EHT) allow for new strong-field tests of the Kerr metric in a previously unexplored regime. I will discuss the recent EHT observations of Sgr A* with a particular focus on how these new results can be used to test fundamental physics. I will briefly touch upon the earlier M87 results as well. I will discuss future prospects for the EHT including additional telescopes and a shorter wavelength. Finally, I will introduce a new machine-learning algorithm for EHT data analysis that uses a large library of high-fidelity simulations as a training set.