Very Long Baseline Interferometry (VLBI), the technique used by the EHT to create a virtual Earth-sized dish, has been called the ‘ultimate in delayed gratification’ among astronomers. Radio waves from the edge of a distant supermassive black hole are captured using dishes around the world and the signals stored on banks of hard disk drives. It is only after these disks are brought together, and the stored signals properly combined, that we can achieve a magnifying power equivalent to a telescope that spans the distance between the participating radio dishes. Frustratingly for EHT scientists (and others!), this process takes time.
For one, while the EHT has had data for many months from most of the dishes we used in 2017, disks from the South Pole arrived only in mid December 2017, and have since been properly combined with data from other telescopes. So there has been a long and unavoidable wait to assemble the full data set for one of our primary supermassive black hole targets: Sgr A* at the center of the Milky Way.
Apart from this logistical delay, the EHT team has spent many months first studying the combined data to make sure that all the detrimental effects that could degrade the event horizon image are fully understood. These effects include turbulence in the Earth’s atmosphere as well as random noise and spurious signals added by our own instrumentation. To do this, we use EHT observations of bright quasars (much more distant and brighter cosmic sources observed along with our main targets: Sgr A* and the more massive black hole in galaxy M 87) to calibrate the array. These are sources that have known structure -- or appearance on the sky -- so astronomers can estimate the instrumental effects and compensate for them as they analyze and make images from the raw data.
EHT scientists have been using data from these calibrators to refine techniques for processing the combined data into images. Independent teams within the EHT have developed novel algorithms to convert the raw VLBI data into maps of radio emission on the sky. Using EHT data on the quasars to test these new methods, the teams are all now producing very similar images, giving us confidence that the tools developed over the past year are robust enough to be applied to Sgr A* and M 87 -- black holes large enough that we may be able to see ’silhouettes’ of their event horizons.
Though our EHT collaboration has grown to now include over 200 members, many of us have been occupied recently with planning and carrying out new observations this month. Since we can observe only once per year, during a period of good weather at both Northern and Southern hemisphere sites, many of us have to divert our attention to planning global operations. This April the EHT re-observed Sgr A* and M 87 using an array that included a telescope in Greenland for the first time and captured twice the amount of data recorded in 2017. These new observations, with a greatly improved EHT, will allow us to study changes in our target black hole sources, as well as confirm any results from the 2017 data.
From the outset, the EHT was conceived as a long-term project that would continue to observe black holes and improve the global array over many years. Unlike the amazing black hole mergers detected by LIGO (the Laser Interferometer Gravitational-Wave Observatory), which are events that are over in fractions of a second, the EHT targets can be studied indefinitely. We plan on observing Sgr A* and M 87 each year with an enhanced EHT, because realizing the project’s science goals may require data from multiple annual campaigns. To do this, we have built up over the past year a robust framework that allows the EHT scientists, from over 12 countries and 30 institutes, to work together on instrumentation, observation, theory and simulations.
As the EHT team begins to analyze the 2017 data on Sgr A* and M 87 over the coming months, preliminary images will begin to emerge, and the searches for the signatures of orbiting material around the black holes will be conducted. It is the most exciting time of the project. We will be sure to share what we find after we have put the data and analysis methods through stringent tests to convince ourselves, and independent astronomy colleagues, of what these horizon-resolving observations tell us.
Shep Doeleman,
EHT Director