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.