First Detection of Light Echoes From Behind a Supermassive Black Hole

Astronomers have observed X-ray flares emitted from around the supermassive black hole for the first time, proving that Albert Einstein is right, once again.

Researchers were studying X-rays from a supermassive black hole at the center of spiral galaxy “Zwicky 1”, 800 million light-years away, when they discovered this unexpected phenomenon.

In addition to the expected X-ray flashes in front of the black hole, a number of “light echoes” were also detected from a source that the researchers could not initially identify, and they published their findings on July 28 in the journal Nature.

space-time warping

astrophysicist Dan Wilkins 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.

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.

Dan Wilkins, a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory, said “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” .

 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.

The general theory of relativity describes how massive objects can warp the fabric of the universe, called spacetime. Einstein discovered that gravity is not caused by an invisible force, but simply our experience of the bending and warping of spacetime in the presence of matter and energy.

This curved space in turn sets the rules for how energy and matter move, and although light travels in a straight line, light that travels through a highly curved region of spacetime – such as the space around a black hole – will also travel in a curve, in this case, from back to front .

Serendipity and further study

It is not the first time that astronomers have observed a black hole scattering light – which is called gravitational lensing – but it is the first time that they have seen light echoes from an area located behind a black hole.

The astronomers did not originally intend to confirm Einstein’s 1915 theory, but instead hoped to use the European Space Agency’s XMM-Newton space telescopes and NASA’s NuSTAR space telescopes to look at the light emitted by the cloud of superheated particles that form outside a black hole’s point of no return, or “event horizon”.

To understand what happened, we say that the superheated cloud – or the corona – around the black hole turns around and gets hotter as it falls into it, as temperatures in the corona can reach millions of degrees – according to the researchers.

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.

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.”

Now that the researchers have made that observation, their next steps will be to study in detail how light bends around black holes, and to investigate the ways in which coronas of black holes emit bright X-ray flashes.