Black hole shadows reveal a new way to study them


In 2019, the world was stunned by the first-ever image of a black hole, a monster with a mass of 6.5 billion suns at the center of the galaxy Messier 87. The image was made possible by the telescope Event Horizon – a global network of radio-synchronized antennas that act like a giant telescope.

Now astronomers have discovered a potential new way to measure the “shadows” of two supermassive black holes as they collide. This could give researchers an easier way to measure black holes that are smaller than M87 and reside in more distant galaxies.

They developed an imaging technique – described in complementary studies published in Physical examination letters and Physical examination D – which relies on two requirements: first, you need two merging supermassive black holes, and second, you need to look at the pair almost sideways.

“It took years and considerable effort by dozens of scientists to create this high-resolution image of M87’s black holes,” says first author Jordy Davelaar, a researcher at Columbia University in the US, and the Center for Computational Astrophysics at the Flatiron Institute. “This approach only works for the largest and closest black holes – the pair at the core of M87 and potentially our own Milky Way.”

This new method could allow researchers to study black holes that are currently beyond the reach of conventional imaging techniques.

“With our technique, you measure the brightness of black holes over time: you don’t need to spatially resolve every object,” says Davelaar. “It should be possible to find this signal in many galaxies.”

So what is the technique?

It relies on a previously hidden signal from a well-known phenomenon called gravitational lensing.

The massive gravitational pull of a black hole causes surrounding gas, dust, and particles to fall toward it and form a disc-like structure that heats up and emits light. This is called an accretion disk.

In binary black hole systems, as one black hole passes in front of the other, you can observe (from a sideways perspective) a bright flash of light as the ring of light around the farthest black hole is amplified or “lensed” by the gravitational force. nearest black hole field.

A simulation of gravitational lensing in a pair of merged supermassive black holes. Credit: Jordy Devalaar/Columbia University

But the researchers discovered an unexpected additional hidden signal: a distinctive dip in the brightness of this flare that matches the “shadow” cast by the black hole’s event horizon behind it.

The event horizon is the point at which nothing can escape a black hole’s gravitational pull – not even light – and when the lens is directly over the farthest black hole, the shadow of the event horizon is enlarged.

Depending on the mass of the black holes and the proximity of their orbits, this subtle dimming can last from a few hours to a few days. So if scientists measure how long it lasts, they can estimate the size and shape of that shadow.

“This dark spot tells us about the size of the black hole, the shape of the spacetime around it, and how matter falls into the black hole near its horizon,” says co-author Zoltán Haiman, professor of astronomy at Columbia. University.

Astronomers became interested in this area of ​​research when NASA’s Kepler planet-hunting space telescope, while looking for tiny dips in brightness corresponding to a planet passing in front of its star, instead detected flares of a suspected pair of black holes merged at the center of a distant galaxy in the early universe.

They named the distant galaxy “Spikey” after the lens flare-triggered brightness spikes, and they built a computer model to learn more about it.

Unexpectedly, however, their simulated pair of black holes also produced a periodic dip in brightness each time one rotated past the other.

At first the team thought this was the result of a coding error, but eventually realized that each dip in brightness closely matched the time it took for the black hole closest to the viewer to pass the shadow of the black hole on the back. .

Observing a merger of supermassive black holes from the side, the black hole closest to the viewer magnifies the black hole further via gravitational lensing. The researchers discovered a brief dip in brightness corresponding to the “shadow” of the more distant black hole, allowing the viewer to gauge its size. 1 credit

Now they’re looking for more telescopic data to confirm the dip they observed in the Kepler data and verify that there is indeed a pair of merged black holes at Spikey’s center. If the technique holds true, it could be applied to other suspect pairs of merging supermassive black holes.

“Even if only a tiny fraction of these black hole binaries have the right conditions to measure our proposed effect, we could find many of these black hole troughs,” says Davelaar.

In fact, the researchers estimate that this drop should be detectable in about 1% of the 150 supermassive black hole binary candidates found so far.

black hole hunters

The documentary black hole hunters – an entry to the SCINEMA International Science Film Festival from 2020 – follows Professor Tamara Davis as she meets scientists who are on an ambitious quest to hunt and photograph a black hole for the first time.

You can watch the full movie here.


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