Thursday, August 12, 2021

Newfound black hole may be the closest to Earth

How small can a black hole be? For several decades, astronomers have worked to answer this question by tallying the black holes in our corner of the universe.

They’ve found plenty of big and medium-size ones over the years—including a supermassive monster at the heart of our galaxy. But until recently, they’ve seen no signs of small ones, and that’s presented a long-standing mystery in astrophysics.

Now, astronomers have discovered a black hole with just three times the mass of the sun, making it one of the smallest found to date—and it happens to be the closest known black hole, at just 1,500 light-years from Earth in Monoceros, The Unicorn Constellation.

 By studying this unicorn and other objects like it, researchers hope to get a clearer picture of what happens to stars in the final moments of their lives and why some of them collapse to become black holes while others leave behind dense stellar husks called neutron stars.

Since no light can escape from a black hole, they can only be detected by indirect means. Most known black holes have been found by searching for the x-rays emitted when the invisible object pulls material off an orbiting companion star. As that material heats up in a dense ring around the black hole, known as an accretion disk, it emits radiation that can be detected with x-ray telescopes.

The unicorn, however, was found by a different method. The research team used data from a number of observatories to measure periodic changes in the brightness and spectrum of light coming from a red giant star known as V723 Mon. These types of observations have been used for several decades to search for exoplanets, which can be extremely difficult to spot directly.

The team deduced that an unseen companion object is tugging at the red giant, distorting it into a raindrop shape. The data give the combined mass of both objects, and if the star is heftier than the team’s estimate, it’s possible the unseen object is a neutron star. But the team believes that companion is most likely a small black hole.

Although the unicorn is changing the shape of the red giant, it isn’t pulling material off it. That means there’s no accretion disk and therefore no x-rays, which is why it went unnoticed until now. This lack of x-ray emissions in such “quiet” black holes may account for why so few small ones have been found so far.

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