How did astronomers take a picture of our galaxy’s supermassive black hole?
Black Hole

How did astronomers take a picture of our galaxy’s supermassive black hole?

The first image of Sagittarius A* took half a decade to develop. Here’s why.

Last week, the Event Horizon Telescope (EHT) team released its second beautiful and haunting image of a black hole’s accretion disk. This time, the image captured Sagittarius A* (Sgr A*), the supermassive black hole at the center of our own Milky Way galaxy.

Despite being released in 2022, the EHT collaboration began collecting data for this historic image in 2017. But what took them a half-decade to analyse the data and create the final image? The answer is that this form of astronomy is extremely difficult.

Basics of interferometry

The first stage in creating an image of Sgr A* is to construct a telescope that can see it. However, no one instrument has the resolving power required to record the accretion disk, let alone the event horizon, of our supermassive black hole. As a result, astronomers frequently use a technique known as interferometry to improve the resolution of difficult-to-image sights. They can combine data from many telescopes by employing them in tandem. And the wider the separation between the different telescopes, the higher the resolution they can eventually achieve.

Of course, the increased resolution given by interferometry has a cost. To begin with, you lose a lot of potential data – photons that collide with the ground between the telescopes cannot be used in your analysis. You can partially solve this difficulty by adding more telescopes to the interferometer network and watching for several hours, allowing the Earth’s natural rotation to assist your telescopes in covering more ground.

Interferometers require a large amount of data processing since astronomers must integrate all of the individual data streams from each telescope. Most interferometers, such as the Very Large Array in New Mexico and the Atacama Large Millimeter/submillimeter Array in Chile, do this by simply linking the telescopes via physical cables into a central processing correlator. That hardwired approach, however, is not practicable for a global interferometer like EHT.

Event Horizon Telescope: A global scope

ALMASagittariusAcomposite
The Atacama Large Millimeter/submillimeter Array (ALMA), part of the Event Horizon Telescope network, looks up at the Milky Way in this composite. The first image of Sagittarius A* is shown as an inset. ESO/José Francisco Salgado (josefrancisco.org), EHT Collaboration

The Event Horizon Telescope needed to cover the width of our world to attain the resolution required to view the Milky Way’s supermassive black hole, which required telescopes in North America, South America, Europe, and Antarctica. The EHT team had to record every single bit collected by the telescopes during the observing run without any physical cabling, sampling the data up to 64 billion times per second.

Each observing run, which normally last only a few days, produced a genuinely massive amount of data. The data from each telescope was recorded on hard drives, which were then physically transported to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy for processing.

Another delay was caused by the atmosphere, which is every astronomer’s worst enemy. While our atmosphere is excellent for giving the oxygen we require to survive, it is horrible for observing. Significant weather events, or even simple clouds, might completely disrupt an observational run. The EHT team had to monitor weather forecasts at each of their telescopes’ locations throughout the world, waiting for the right moment to use all of the devices at once.

That moment arrived in April of 2017 for Sagittarius A*.

The devil in the details

It was time to process everything when the observations were completed and the physical data storage discs were safely delivered to the two headquarters. That’s when calibration came into play.

When astronomers point their instruments toward a new object, such as Sgr A*, they don’t know whether they’re seeing light emitted by the target or light emitted by some other source of pollution.

SagittariusAraytracing
Using ray tracing and Einstein’s theory of general relativity, the EHT team created numerous depictions of what the Milky Way’s supermassive black hole, Sagittarius A*, might look like. Ben Prather/EHT Theory Working Group/Chi-Kwan Chan

The atmosphere is the most common source of pollution (yes, again). More than 60 miles (100 kilometers) of air separates us from the vacuum of space, and it is continuously fluctuating, with pockets of warm and cool air striving for control. Every time the atmosphere changes, our view of heavenly targets changes slightly.

To adjust for this, the EHT team devoted a portion of each observing run to instrument training on a well-known radio source. They then used observed variations in that source to build a real-time model of atmospheric turbulence and its impact on Sgr A* data, allowing them to remove any atmospheric distortions.

Beyond the atmosphere, there is approximately 26,000 light-years of cosmic material between us and Sagittarius A*. And because interstellar space isn’t a vacuum, dust grains from all throughout the galaxy interfere with radio frequencies detected by the Event Horizon Telescope.

The dust has the effect of gently scattering the radio waves coming from Sgr A*, making it appear wider than it is. The second effect is that big, random clumps of interstellar dust introduce little blotches that aren’t actually part of the black hole system. That meant the team had to work hard to develop models of those effects before they could likewise subtract the from the final image of our home galaxy’s black hole.

Beyond the atmosphere, there is approximately 26,000 light-years of cosmic material between us and Sagittarius A*. And because interstellar space isn’t a vacuum, dust grains from all throughout the galaxy interfere with radio frequencies detected by the Event Horizon Telescope.

The dust has the effect of gently scattering the radio waves coming from Sgr A*, making it appear wider than it is. The second effect is that big, random clumps of interstellar dust introduce little blotches that aren’t actually part of the black hole system. That meant the team had to work hard to develop models of those effects before they could likewise subtract the from the final image of our home galaxy’s black hole.

Finally, scientists have to consider the inherent fluctuation in the disk surrounding Sagittarius A*. Previous, much lower-resolution investigations revealed that the brightness of our supermassive black hole’s disk may double in a matter of years or less. Astronomers have even observed a flare appearing near the black hole and then disappearing within a single day.

The EHT team needed to spend many hours training their telescopes on Sgr A*. All of that data was required to ensure that the signal clearly rose above the noise — otherwise, the observation would be so noisy that it would be practically meaningless. However, because the black hole’s disk changed and varied in brightness over time, it was like photographing a dog chasing its tail. The researchers couldn’t just integrate multiple hours of data into a single blurry mess.

To address this, the researchers separated the data stream into little chunks, each lasting no more than a few minutes. They then processed each chunk separately before combining all of the clean chunks to create a single, average image. The team used distinct software pipelines with different ways for cleaning and processing the data as a self-check for consistency.

After years of planning, days of observation, and years of research, the end result is a stunning depiction of the gravitational goliath hiding in the center of our galaxy: Sagittarius A*.

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