X-rays reflected and directed forward confirm Einstein’s theory of relativity
Astronomers have captured radiation from the backside of a black hole for the first time. © Dan Wilkins
Theory confirmed: Astronomers have observed light from the back of a black hole for the first time – proving a phenomenon predicted by Einstein’s theory of relativity. The captured X-rays emanated from the far side of a supermassive black hole, were reflected by the plasma ring and deflected in such a way that they appeared on the front side after a delay, as the researchers report in the scientific journal "Nature".
When radiation or matter crosses the event horizon of a black hole, they are irreversibly swallowed up. The black hole therefore appears as a dark shadow, as confirmed by the first photo in 2019. But what does a black hole look like when we look sideways at the bright accretion disk of aspirated material??
Already Albert Einstein postulated in his General Theory of Relativity that in such cases there would have to be such a strong curvature of space-time that light from the back side of the black hole would have to be redirected and become visible at the front side. We see thereby front and back at once. Recent simulations show this effect in impressive detail – even for two orbiting black holes.
Ray bursts at the black hole
Only now have astronomers succeeded in providing the first evidence of this effect: For the first time, they have captured X-rays coming from the backside of a black hole. This observation was made by Dan Wilkins’ team at Stanford University using the XMM-Newton and NuStAR X-ray telescopes. With them, they observed the supermassive black hole in the galaxy I Zwicky 1, 800 million light-years away.
About 60 million kilometers from the event horizon of this black hole, interactions of the infalling plasma with the strong magnetic fields keep producing strong bursts of radiation. These X-ray eruptions are partly reflected by the accretion disk and then light up with a slight delay.
X-ray flashes with echo
But when Wilkins and his team took a closer look at two radiation bursts from I Zwicky 1 that lasted about 2.5 hours, they discovered strange things: the second X-ray burst appeared, in terms of its spectral properties and timing, to be a weaker, delayed echo of the first one. The energies of the individual partial flashes in these outbursts also indicated that they must have been reflected from different parts of the accretion disk.
The radiation of the X-ray bursts on the far side is reflected by the accretion disk and bent forward by the gravity of the black hole around the event horizon. © ESA
Crucially, however, "Analysis of the X-ray flares reveals brief flashes of photons whose characteristics are consistent with emission coming from the back side of the black hole," Wilkins and his team report. "These are particles of light bouncing off the averted side of the disk and bent and amplified by the strong gravitational field around the black hole."
Einstein’s prediction confirmed
This is the first time astronomers have directly observed light from the back of a black hole. They thus prove the effect predicted by Einstein’s theory and the simulations. "The new observations confirm a key prediction of general relativity – that photons directed around the black hole can be detected from the back side," Wilkins and his colleagues state. (Nature, 2021; doi: 10.1038/s41586-021-03667-0)