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Ultra-precise radio telescopes have spotted a faint glow around the black hole M87* — where before they saw only darkness.
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Radio telescopes scattered across the globe pick up waves from distant black holes. But two telescopes placed almost side by side see only a blurry blob — fine details are beyond them. Usually, such pairs are considered useless and discarded. But that's a mistake: even a blurry blob holds clues.
The authors of the new study noticed that if a source is not just a bright point but is surrounded by a faint halo, the signal from closely spaced telescopes is no longer zero. From a tiny shift, you can tell where the center of the glow lies relative to the bright core. It's like weighing an invisible comet tail by the barely perceptible wobble of its head.
The method is like detective work: from a barely noticeable imbalance in the signal, you reconstruct the shape of something the eye cannot see.
They tested the method on simulated data — it works. Then they took observations of the black hole at the center of galaxy M87 from 2017, 2018, and 2021. It turned out that the faint glow around the famous "ring" is slightly elongated to the northwest — in the same direction as the giant plasma jet. This means we are seeing not just the black hole itself, but also the matter it is pulling in and accelerating.
This method does not require complex models or expensive computations — just a careful look at signals that were previously thrown away. Now astronomers have a simple tool to spot the invisible tails around black holes.
The faint shift they managed to measure is about one thousandth of an arcsecond. That's like spotting a coin from a distance of four thousand kilometers.
In the movie "Interstellar," the black hole Gargantua is surrounded by a glowing accretion disk. The new method helps us see such structures around real black holes.