Shadows of a supermassive black hole binary system uncovers new method for studying 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 centre of the galaxy Messier 87. The image was made possible thanks to the Event Horizon Telescope – a global network of synchronised radio dishes which act as one giant telescope.

Now, astronomers have discovered a potential new method for measuring the ”shadows” of two supermassive black holes that are in the process of colliding. This could give researchers an easier way to measure black holes which are smaller than M87 and reside in more distant galaxies.

They have devised an imaging technique – outlined in complementary studies published in Physical Review Letters and Physical Review D – that relies on two requirements: first, you need two supermassive black holes in the throes of merging, and second, you need you need to be looking at the pair almost side-on.

“It took years and a massive effort by dozens of scientists to make that high-resolution image of the M87 black holes,” says first author Jordy Davelaar, a researcher at Columbia University, US, and the Flatiron Institute’s Center for Computational Astrophysics. “That approach only works for the biggest and closest black holes – the pair at the heart 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 the black holes over time: you don’t need to resolve each object spatially,” explains 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 in towards it and form a disc-like structure that heats up and radiates light. This is called an accretion disc.

In black hole binary systems, as one black hole passes in front of the other you can observe (from a sideways vantage point) a bright flash of light as the glowing ring around the black hole farther away is magnified or “lensed” by the gravitational field of the closer black hole.

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

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

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

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

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

The astronomers became interested in this area of research when NASA’s planet-hunting Kepler space telescope, while scanning for tiny dips in brightness which correspond to a planet passing in front of its star, instead detected the flares of a suspected pair of merging black holes at the centre of a far-off galaxy in the early universe.

They named the distant galaxy “Spikey” after the spikes in brightness triggered by the lensing effect 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 orbited in front of the other.

At first, the team thought this was the result of a coding mistake, but eventually realised that each dip in brightness closely matched the time taken for the black hole closest to the viewer to pass in front of the shadow of the black hole at the back. 

Observing a supermassive black hole merger side-on.
Observing a supermassive black hole merger side-on, the black hole closest to the viewer magnifies the black hole farther away via the the gravitational lensing effect. Researchers discovered a brief dip in brightness corresponding to the ‘shadow’ of the black hole farther away, allowing the viewer to measure its size. Credit: Nicoletta Baroloini

Now, they’re looking for other telescopic data to confirm the dip they observed in the Kepler data and verify that there is indeed a pair of merging black holes at the centre of Spikey. If the technique checks out, it could be applied to other suspected pairs of merging supermassive black holes.

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

In fact, the researchers estimate that this dip should be detectable in about 1% of the 150 total candidate supermassive black hole binaries found so far.

Black hole hunters

The documentary Black Hole Hunters – a SCINEMA International Science Film Festival entry 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 film in full here.

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