For the first time, astronomers have managed to capture a radio image showing two black holes orbiting each other. The observation confirmed the existence of black hole pairs. In the past, astronomers have only managed to image individual black holes.

An international team of researchers has succeeded in imaging two black holes orbiting each other at the centre of a quasar called OJ287. Quasars are extremely bright galactic cores, whose light is produced when a supermassive black hole at the centre of the galaxy devours the cosmic gas and dust around it.

In the past, astronomers have managed to image the black hole in the centre of the Milky Way and in a nearby galaxy called Messier 87.

“Quasar OJ287 is so bright that it can be detected even by amateur astronomers with private telescopes. What is special about OJ287 is that it has been thought to harbour not one but two black holes circling each other in a twelve-year orbit, which produces an easily recognisable pattern of light variations in the same period,” says first author of the research article Mauri Valtonen from the University of Turku, Finland. The findings of the research group were published in the Astrophysical Journal.

Quasar discovered "by accident" in the 19th century

The earliest observations of OJ287 can be traced through old photographs all the way to the 19th century, when the region of the sky in question was first photographed by astronomers. Back then, however, it was inconceivable that black holes existed, let alone quasars. OJ287 was "accidentally" included in pictures while astronomers focused on other objects. Aimo Sillanpää, who was at the time a master's student at the University of Turku, noticed as early as 1982 that the brightness of the object changed regularly over a 12-year period. He went on to study OJ287 as a university researcher, assuming that the brightness variation was caused by two black holes orbiting each other. Hundreds of astronomers have been intensively monitoring the quasar to see if the theory is correct and to get a complete picture of the black holes' orbital motion.

The mystery of the orbit was finally solved four years ago by Doctoral Researcher Lankeswar Dey from Mumbai, India, working part-time at the University of Turku. The only question that remained was whether both black holes could be detected at the same time.

It was answered by NASA's TESS satellite that detected light from both black holes. However, they were still only visible as a single dot, because images using normal light do not have high enough resolution to show the black holes separately.

What was needed was an image with 100,000 times higher resolution, which is possible with radio telescopes.

Scientists identified new “wagging tail” jet from the smaller black hole

In this latest study, the astronomers compared the earlier theoretical calculations with a radio image. The two black holes were there in the image, just where they were expected to be. This gave the researchers an answer to a question that has been open for 40 years: whether black-hole pairs exist in the first place.

“For the first time, we managed to get an image of two black holes circling each other. In the image, the black holes are identified by the intense particle jets they emit. The black holes themselves are perfectly black, but they can be detected by these particle jets or by the glowing gas surrounding the hole," Valtonen says.

The researchers also identified a completely new kind of a jet emanating from a black hole. The jet coming out of the smaller black hole is twisted like a jet of a rotating garden hose. This is because the smaller black hole moves fast around the primary black hole of OJ287, and its jet is diverted depending on its current motion. The researchers liken it to “a wagging tail” which should be seen twisting in different directions in the coming years when the smaller black hole changes its speed and direction of motion.

"The image of the two black holes was captured with a radio telescope system that included the RadioAstron satellite. It was in operation a decade ago, when OJ287 was imaged. The satellite's radio antenna went half-way to the Moon, which greatly improved the resolution of the image. In recent years, we have only been able to use Earth-based telescopes, where the image resolution is not as good,” Valtonen says.