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---------------------- 2331 - Picture of a Blackhole
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- Blackholes are extremely camera shy. Supermassive blackholes, located at the centers of galaxies, make themselves visible by spewing bright jets of charged particles or by flinging away or ripping up nearby stars. Up close, these blackholes are surrounded by glowing accretion disks of infalling material. But because a blackhole’s extreme gravity prevents light from escaping, the blackhole itself remains entirely invisible.
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- Telescopes look instead for the silhouette of a blackhole’s event horizon, the perimeter inside which nothing can be seen or escape. That is what the Event Horizon Telescope, or EHT, did in April 2017, collecting data that has now yielded the first image of a supermassive blackhole, the one inside the galaxy M87.
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- Creating this first-ever portrait of a blackhole was tricky. Blackholes take up a minuscule sliver of sky and, from Earth, appear very faint. The project of imaging M87’s blackhole required observatories across the globe working in tandem as one virtual Earth-sized radio dish with sharper vision than any single observatory could achieve on its own.
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- This blackhole is around 6.5 billion times the mass of our Sun. M87 is viewed from 55 million light-years away. The black hole is only about 42 microarcseconds across on the sky. That’s smaller than an orange on the moon would appear to someone on Earth.
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- Only a telescope with unprecedented resolution could pick out something so tiny. (For comparison, the Hubble Space Telescope can distinguish objects only about as small as 50,000 microarcseconds.)
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- A telescope’s resolution depends on its diameter: The bigger the dish, the clearer the view. Getting a crisp image of a supermassive blackhole would require a planet-sized radio dish.
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- A technique called very long baseline interferometry combines radio waves seen by many telescopes all around the Earth all at once. The telescopes effectively work together like one giant dish. The diameter of that virtual dish is equal to the length of the longest distance, or baseline, between two telescopes in the network. For the EHT in 2017, that was the distance from the South Pole to Spain.
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- In 2009 there was a network of just four observatories in Arizona, California and Hawaii. They got the first good look at the base of one of the plasma jets spewing from the center of M87’s blackhole But the four telescope combination did not yet have the magnifying power to reveal the blackhole silouette itself.
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- By 2017, there were eight observing stations in North America, Hawaii, Europe, South America and the South Pole. These eight radio observatories teamed up in 2017 to work together as a global telescope, called the Event Horizon Telescope network.
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- Their mission was to image a supermassive black hole for the first time. Data from seven were used to create a picture of the blackhole inside the galaxy M87.
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- The EHT observing campaigns are best run within about 10 days in late March or early April, when the weather at every observatory promises to be the most cooperative. Researchers’ biggest enemy is water in the atmosphere, like rain or snow, which can muddle with the millimeter-wavelength radio waves that the EHT’s telescopes are tuned to. But, planning for weather on several continents can be a logistical headache.
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- When the skies are clear enough to observe, researchers steer the telescopes at each EHT observatory toward the vicinity of a supermassive black hole and begin collecting radio waves.
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- On their own, the data from each observing station look like nonsense. But taken together using the very long baseline interferometry technique, these data can reveal a blackhole’s appearance.
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- Picture a pair of radio dishes aimed at a single target, in this case the ring-shaped silhouette of a blackhole. The radio waves emanating from each bit of that ring must travel slightly different paths to reach each telescope. These radio waves can interfere with each other, sometimes reinforcing one another and sometimes canceling each other out. The interference pattern seen by each telescope depends on how the radio waves from different parts of the ring are interacting when they reach that telescope’s location.
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- M87’s supermassive black hole spits out bright jets of charged subatomic particles that extend out thousands of light-years. For simple targets, such as individual stars, the radio wave patterns picked up by a single pair of telescopes provide enough information for researchers to work backward and figure out what distribution of light must have produced those data.
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- But, for a source with complex structure, like a blackhole, there are too many possible solutions for what the image could be. Researchers need more data to work out how a blackhole’s radio waves are interacting with each other, offering more clues about what the blackhole looks like.
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- The ideal array has as many baselines of different lengths and orientations as possible. Telescope pairs that are farther apart can see finer details, because there’s a bigger difference between the pathways that radio waves take from the black hole to each telescope. The EHT includes telescope pairs with both north-south and east-west orientations, which change relative to the blackhole as Earth rotates.
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- In order to put all this together the observations from each observatory, researchers need to record times for their data with exquisite precision. For that, they use hydrogen maser atomic clocks, which lose about one second every 100 million years.
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- The data was recorded at a rate of 64 gigabits per second, which is about 1,000 times faster than your home internet connection. These data are then transferred to MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy in Bonn, Germany, for processing in a special kind of supercomputer called a correlator.
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- But each telescope station amasses hundreds of terabytes of information during a single observing campaign. This is far too much to send over the internet. So the researchers use the next best option, They mail it.
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- Combining the EHT data still isn’t enough to render a vivid picture of a supermassive blackhole. There are mathematical rules about how much randomness any given picture can contain, how bright it should be and how likely it is that neighboring pixels will look similar. Those basic guidelines can inform how software decides which potential images, or data interpretations, make the most sense.
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- The EHT’s blackhole observations are expected to help answer questions like how some supermassive black holes, including M87’s, launch such bright plasma jets. Understanding how gas falls into and feeds blackholes could also help solve the mystery of how some blackholes grew so quickly in the early universe.
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- The EHT could also be used to find pairs of supermassive black holes orbiting one another. The EHT doesn’t have many viable targets other than supermassive blackholes. There are few other things in the universe that appear as tiny but luminous as the space surrounding a supermassive blackhole.
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- April 10, 2019
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-------------------------- Thursday, April 11, 2019 --------------------------
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