Astronomers think they just spotted an ‘invisible’ black hole for the first time

To discover such an invisible black hole, the team of scientists had to combine two different types of observations over several years.

Astronomers took the first-ever direct image of a black hole in 2019, thanks to glowing material in its presence. But many black holes are actually almost impossible to detect. Now another team using the Hubble Space Telescope appears to have finally found something no one has seen before: a completely invisible black hole. The research, which has been posted online and submitted for publication in the Astrophysical Journal, has not yet been peer-reviewed.

Black holes are what remain after large stars die and their cores collapse. They are incredibly dense, with gravity so strong that nothing can move fast enough to escape them, including light. Astronomers are keen to study black holes because they can tell us a lot about how stars die. By measuring the masses of black holes, we can learn more about what was happening in the final moments of stars, when their cores collapsed and their outer layers were pushed out.

It may seem that black holes are by definition invisible – they earned their name after all from their ability to trap light. But we can still detect them through the way they interact with other objects thanks to their strong gravity. Hundreds of small black holes have been detected by the way they interact with other stars.

There are two different approaches for such detection. In “X-ray binary stars” – in which a star and a black hole orbit a common center while producing X-rays – a black hole’s gravitational field can pull material from its companion. The material revolves around the black hole, heating up by friction. The hot material glows brightly in the X-ray light, making the black hole visible, before being sucked into the black hole and disappearing. You can also detect pairs of black holes as they merge, spiraling inward and emitting a brief flash of gravitational waves, which are ripples in spacetime.

There are, however, many rogue black holes that drift through space without interacting with anything, making them difficult to detect. That’s a problem, because if we can’t detect isolated black holes, we can’t learn more about their formation and the death of the stars they came from.

Dark New Horizons

To discover such an invisible black hole, the team of scientists had to combine two different types of observations over several years. This impressive achievement promises a new way to find the previously elusive class of isolated black holes.

Einstein’s general theory of relativity predicted that massive objects would bend light as it passed through them. This means that any light passing very close to an invisible black hole – but not close enough to end up inside – will be deflected in the same way as light passing through a lens. This is called gravitational lensing and can be spotted when a foreground object lines up with a background object, bending its light. The method has already been used to study everything from clusters of galaxies to planets around other stars.

The authors of this new research combined two types of gravitational lensing observations in their search for black holes. It started with them seeing the light from a distant star suddenly grow larger, briefly making it appear brighter before returning to normal. However, they could not see any objects in the foreground that caused the magnification via the process of gravitational lensing. This suggested the object could be a solitary black hole, something that had never been seen before. The problem was that it could also be a faint star.

Determining whether it was a black hole or a faint star took a lot of work, and that’s where the second type of gravitational lensing observations came in. The authors took several resuming images with Hubble for six years, measuring how far the star appeared to move. that its light has been deflected.

Eventually, this allowed them to calculate the mass and distance of the object that caused lensing. They discovered that it was about seven times the mass of our Sun, which is about 5,000 light-years away, which seems far away but is actually relatively close. A star of this size and closeness should be visible to us. As we cannot see, they concluded that it must be an isolated black hole.

Taking so many observations with an observatory like Hubble is not easy. The telescope is very popular and there is a lot of competition for its time. And given how hard it is to confirm an item like this, you might think the chances of finding more aren’t great. Fortunately, we are at the beginning of a revolution in astronomy. It’s thanks to a new generation of facilities, including the ongoing Gaia survey, and upcoming Vera Rubin Observatory and Nancy Grace Roman Space Telescope, all of which will take repeated measurements of large swaths of the sky with unprecedented details.

This is going to be huge for all areas of astronomy. Having regular, high-precision measurements of so much of the sky will allow us to en masse investigate things that change on very short time scales. We’ll study things as varied as asteroids, exploding stars known as supernovae, and planets around other stars in new ways.

As for the search for invisible black holes, this means rather than rejoicing at finding just one, we may soon find so many that it becomes routine. This will allow us to fill in the gaps in our understanding of star death and the creation of black holes.

Ultimately, the galaxy’s invisible black holes are about to have a much harder time hiding. – The Talk | Rappler.com

Adam McMaster is a postgraduate (PhD) student in astronomy at The Open University.

Andrew Norton is a teaching professor of astrophysics at the Open University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.