“Luminous Echoes” –First Detection of Light from the Far Side of a Black Hole

Black Hole Horizon


“The galaxy in which this supermassive black hole resides, called “I Zwicky 1”, is what’s known as a Seyfert galaxy. It is a spiral-shaped galaxy with what’s known as an “active galactic nucleus” or “AGN” –where the supermassive black hole in the center of the galaxy is swallowing gas at a high rate, and as the gas falls into the supermassive black hole, enormous amounts of energy are released that power a bright light source in the center of a galaxy that is out-shining all of the stars in the galaxy,” wrote astrophysicist Dan Wilkins, a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory in an email to The Daily Galaxy.

Wilkins’ research focuses on how material spiralling into a supermassive black hole in the centre of a galaxy is able to release huge amounts of energy, powering some of the brightest objects we see in the Universe.

A series of bright flares of X-rays

Wilkins was watching X-rays flung out into the universe by the supermassive black hole at the center of a galaxy 800 million light-years away, when he noticed an intriguing pattern: He observed a series of bright flares of X-rays—exciting, but not unprecedented—and then, the telescopes recorded something unexpected: additional flashes of X-rays that were smaller, later and of different “colors” than the bright flares.

“As far as supermassive black holes go, the one in I Zwicky 1 is relatively nearby and the gas falling into it shines relatively brightly, so we can get a good look at it with our X-ray telescopes,”–explains Wilkins in his email to The Daily Galaxy. “Not only that, but we know from past studies that the I Zwicky 1 is quite erratic. It’s prone to the type of X-ray flares that we saw, so it’s a prime target to look for echoes off of the gas falling into the black hole.”

Warping Space

According to theory, these luminous echoes were consistent with X-rays reflected from behind the black hole—but even a basic understanding of black holes tells us that is a strange place for light to come from.

“Any light that goes into that black hole doesn’t come out, so we shouldn’t be able to see anything that’s behind the black hole,” said Wilkins. “It is another strange characteristic of the black hole, however, that makes this observation possible. “The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself,” Wilkins explained.

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Einstein confirmed

The strange discovery, detailed in a paper published July 28 in Nature, is the first direct observation of light from behind a black hole—a scenario that was predicted by Einstein’s theory of general relativity but never confirmed, until now.

“Fifty years ago, when astrophysicists starting speculating about how the magnetic field might behave close to a black hole, they had no idea that one day we might have the techniques to observe this directly and see Einstein’s general theory of relativity in action,” said Roger Blandford, a co-author of the paper who is the Luke Blossom Professor in the School of Humanities and Sciences and Stanford and SLAC professor of physics and particle physics.

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The mysterious corona

The original motivation behind this research was to learn more about a mysterious feature of certain black holes, called a corona. Material falling into a supermassive black hole powers the brightest continuous sources of light in the universe, and as it does so, forms a corona around the black hole. This light—which is X-ray light—can be analyzed to map and characterize a black hole.

The leading theory for what a corona is starts with gas sliding into the black hole where it superheats to millions of degrees. At that temperature, electrons separate from atoms, creating a magnetized plasma. Caught up in the powerful spin of the black hole, the magnetic field arcs so high above the black hole, and twirls about itself so much that it eventually breaks altogether—a situation so reminiscent of what happens around our own Sun that it borrowed the name “corona.”

“This magnetic field getting tied up and then snapping close to the black hole heats everything around it and produces these high energy electrons that then go on to produce the X-rays,” said Wilkins.

A first glimpse at the far side of a black hole

As Wilkins took a closer look to investigate the origin of the flares, he saw a series of smaller flashes. These, the researchers determined, are the same X-ray flares but reflected from the back of the disk—a first glimpse at the far side of a black hole.

“I’ve been building theoretical predictions of how these echoes appear to us for a few years,” said Wilkins. “I’d already seen them in the theory I’ve been developing, so once I saw them in the telescope observations, I could figure out the connection.”

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Athena on deck…

The mission to characterize and understand coronas continues and will require more observation. Part of that future will be the European Space Agency’s X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics). As a member of the lab of Steve Allen, professor of physics at Stanford and of particle physics and astrophysics at SLAC, Wilkins is helping to develop part of the Wide Field Imager detector for Athena.

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“It’s got a much bigger mirror than we’ve ever had on an X-ray telescope and it’s going to let us get higher resolution looks in much shorter observation times,” said Wilkins. “So, the picture we are starting to get from the data at the moment is going to become much clearer with these new observatories.”

Co-authors of this research are from Saint Mary’s University (Canada), Netherlands Institute for Space Research (SRON), University of Amsterdam and The Pennsylvania State University.

Source: Light bending and X-ray echoes from behind a supermassive black hole, Nature (2021). DOI: 10.1038/s41586-021-03667-0 , www.nature.com/articles/s41586-021-03667-0

The Daily Galaxy, Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Dan Wilkins, Stanford University and Nature. Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL).

Image credit: NASA/ESA/Gaia/DPAC

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