It is an exciting time in the Event Horizon Telescope (EHT) project. After many years of preparation, our team mounted a week-long observing campaign in April of this year that linked together 8 telescopes in Hawaii, the South Pole, Arizona, Spain, Mexico and Chile. This global array targeted two supermassive black holes, one at the center of the Milky Way galaxy, and the other at the heart of M87, a giant elliptical galaxy about 50 million light years away. For each of these black holes, the EHT has the magnifying power and sensitivity to form images of the mm wavelength light emitted by the hot gas near the event horizon. Data recorded at all the sites has been shipped back to two central processing facilities at MIT and the Max Planck Institute for Radioastronomy where signals from all participating telescopes are being combined. The power of this technique is that the EHT delivers an angular resolution comparable to a telescope as large as the distance between the EHT sites.
There are no guarantees of what the EHT will see. Eintstein’s General Theory of Relativity predicts that the EHT should see a silhouette formed by the intense gravity of the black hole warping the light from infalling hot gas. The dynamics of matter may also be detected as hot blobs of material orbit the black hole and shear into turbulent flows. But the proof will be in the team’s analysis of the data, and that is still just getting started. Because the collected data are combined long after the observations are made, the technique used by the EHT (Very Long Baseline Interferometry, or VLBI) is well known for its quality of delayed gratification.
On the technical side, the EHT has broken new ground by making VLBI observations at the shortest wavelengths to date. And the array has been extended to bandwidths, or data capture rates, that are more than 10 times what was possible just a few years ago. Extension to include the South Pole Telescope means that the EHT is truly an Earth-sized instrument.
Parallel advances in theory are providing direction for analysis techniques through detailed modeling and simulations of black hole accretion. Information on current EHT work in both of these areas can be found on this website.
During the observations, EHT members at all sites ticked through detailed checklists each day to ensure things were ready: Hydrogen maser atomic clocks stable, high speed data recorders on line, signal processing instruments tuned up, synchronization to GPS complete. The EHT can tell if the position of an entire telescope is off by a millimeter, and if the timing of electronic systems are shifted by a trillionth of a second, so all of this matters. And after waiting over a decade to make these observations, you go through the checklist twice. The one thing beyond anyone’s control is the weather. At a central command room at the Smithsonian Astrophysical Observatory in Cambridge, MA, weather data was collected each day from around the array and an often agonizing decision made on whether to fire off an evening of observations based on predictions and the experience of staff at all the sites. Will the heavy clouds surrounding a mountain top telescope dissipate, or will they settle in for the night? Is the weather risky at many sites, or maybe just one? And even if the sky above clears up, might ground conditions early in the evening leave a dish iced up and unusable? In the end, the weather was overwhelmingly excellent and we triggered 5 days of observing out of a possible 10-day window.
So far, the data processing centers have confirmed that all the sites in the EHT worked well, except of course for the South Pole, where the hard disks used to record the data are being stored until the station re-opens in September and flights are allowed in and out. This is very welcome news, but at this stage no results on the two main targets, SgrA* and M87, are available. Over the coming months, the EHT team will continue data processing and refining analysis tools with focus then shifting to investigations of predicted strong gravity black hole signatures.
The EHT website will be updated with developments, and also has background material, news, science results and educational resources.
Shep Doeleman
EHT Director