Dormant black hole discovered outside our galaxy

Glowing brightly about 160 000 light-years away, the Tarantula Nebula is the mos

Glowing brightly about 160 000 light-years away, the Tarantula Nebula is the most spectacular feature of the Large Magellanic Cloud, a satellite galaxy to our Milky Way. This image from VLT Survey Telescope at ESO’s Paranal Observatory in Chile shows the region and its rich surroundings in great detail. It reveals a cosmic landscape of star clusters, glowing gas clouds and the scattered remains of supernova explosions. ESO

The "Police of Black Holes" with participation of the University of Bonn shows a stellar-mass black hole

A team of international experts, renowned for debunking several black hole discoveries, have found a stellar-mass black hole in the Large Magellanic Cloud, a neighbour galaxy to our own. Moreover, they found that the star that gave rise to the black hole vanished without any sign of a powerful explosion. The discovery was made thanks to six years of observations obtained with the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT). The study has been published in the journal "Nature Astronomy".

This artist’s impression shows what the binary system VFTS 243 might look like if we were observing it up close. The system, which is located in the Tarantula Nebula in the Large Magellanic Cloud, is composed of a hot, blue star with 25 times the Sun’s mass and a black hole, which is at least nine times the mass of the Sun. The sizes of the two binary components are not to scale: in reality, the blue star is about 200 000 times larger than the black hole. Note that the ’lensing’ effect around the black hole is shown for illustration purposes only, to make this dark object more noticeable in the image. The inclination of the system means that, when looking at it from Earth, we cannot observe the black hole eclipsing the star.

Stellar-mass black holes are formed when massive stars reach the end of their lives and collapse under their own gravity. In a binary, a system of two stars revolving around each other, this process leaves behind a black hole in orbit with a luminous companion star. The black hole is ’dormant’ if it does not emit high levels of X-ray radiation, which is how such black holes are typically detected. Although astronomers believe they are very common, hardly any dormant black holes are known. They are particularly hard to spot since they do not interact much with their surroundings.

The newly found black hole is at least nine times the mass of our Sun, and orbits a hot, blue star weighing 25 times the Sun’s mass every 10.4 days. To find VFTS 243, the collaboration searched nearly 1000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, looking for the ones that could have black holes as companions.

"For the first time, our team got together to report on a black hole discovery, instead of rejecting one," says study leader Tomer Shenar, who started the study at KU Leuven in Belgium and is now a Marie-Curie Fellow at Amsterdam University, the Netherlands.

Team from the University of Bonn participating

Research groups from all over the world were involved in the study. The team led by Nobert Langer of the Argelander Institute at the University of Bonn has shown through state-of-the-art binary evolution calculations that up to three percent of all stars heavier than 15 solar masses could have a dormant black hole as a companion.

"The results confirm the theory that black holes can exist in close, massive binary stars without emitting significant X-rays. The X-ray-bright black holes found so far thus probably represent only the tip of the iceberg of a very numerous binary star population containing dormant black holes," Langer says.

As members of the " Police of Black Holes," the researchers were extremely skeptical regarding this discovery. But they could not find a plausible explanation for the data that did not involve a black hole. "We identified a ’needle in a haystack’," says Norbert Langer. Though other similar black hole candidates have been proposed, the team claims this is the first ’dormant’ stellar-mass black hole to be unambiguously detected outside our galaxy.

New insights into the formation of black holes

The discovery also allows the team a unique view into the processes that accompany the formation of black holes. Astronomers believe that a stellar-mass black hole forms as the core of a dying massive star collapses, but it remains uncertain whether or not this is accompanied by a powerful supernova explosion.

"The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion," explains Shenar. "Evidence for this ’direct-collapse’ scenario has been emerging recently, but our study arguably provides one of the most direct indications. This has enormous implications for the origin of black-hole mergers in the cosmos."

The team hopes that their work, will enable the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in Milky Way and in the Magellanic Clouds.

Norbert Langer is a member of the Transdisciplinary Research Area "Matter" at the University of Bonn.

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