Black hole whips companion star around at incredible speed

Black holes are among the most extreme objects in the known universe and astronomers using the NASA-ESA XMM-Newton space telescope have now taken detailed observations of one that's a great example of just how extreme they can be. This black hole whips a companion star around it so fast that the star completes one orbit in just 2.4 hours, that's around two million kilometres per hour.

"The companion star revolves around the common centre of mass at a dizzying rate, almost 20 times faster than Earth orbits the Sun. You really wouldn’t like to be on such a merry-go-round in this Galactic fair!" said Erik Kuulkers, who leads the X-ray binary group at the ESA's European Space Astronomy Centre in Spain.

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This black hole, named MAXI J1659-152, is estimated to be around three times the size of our Sun. It is the leftover remnant of a massive star that burned through its stellar fuel very quickly and then collapsed under the crushing weight of its own gravity, setting off a cataclysmic supernova explosion that left only the black hole and its companion behind.

The black hole's companion is a small, cool red dwarf star, about 20 per cent the size of our Sun. It orbits at about 1 million kilometres away (a little over twice the distance from Earth to the Moon), which is close enough for the black hole's immense gravity to tear matter from the star's surface. It spirals in, creating an 'accretion disk' around the singularity. The video above is an animation that shows what the stellar pair looks like as the black hole whips its companion around.

Astronomer first located this pair back in 2010, when the black hole caught their attention by emitting a high-energy burst as it gobbled up the matter it pulled off its companion. The telescopes that first detected it — NASA's Swift telescope and Japan's MAXI satellites — weren't sensitive enough to resolve the binary system, but they could see how the brightness of the x-rays dimmed every 2.4 hours. Investigating further, they used the XMM-Newton space telescope to track it for over 14 hours to verify the orbital period.

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Another interesting thing about this is that the companion star is far too small to have formed this close to its larger companion. If it had, it would have been torn apart when the more massive star self-destructed. It must have formed much further out and then migrated inward at some point. Figuring out how that happened takes some educated guesswork, but I found that 'Bad Astronomer' Phil Plait does a great job of working it out.

This discovery is just incredible. The fact that something big enough to contain over 66 thousand Earths is being whipped around in space at a speed that would reduce our own year down to just 18 days is amazing enough, but the fact that we found these two quite by chance — because the black hole let out a little 'burp' a few years ago — is awesome.

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