Black-Hole Binary

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Black-Hole Binary

Millions of black holes inhabit our home galaxy, the Milky Way. Most of them probably roam the galaxy alone, so we never see them. Luckily for science, though, many of them have companion stars. That makes it possible for astronomers to “see” and learn about the black holes.

An example is a system called Swift J1357. It was discovered in 2011 by Swift, a space telescope that studies the X-ray sky. The system is thousands of light-years away, and appears to be outside the galaxy’s disk of stars.

J1357 consists of two objects: a black hole and a small, faint companion star. The black hole’s gravity pulls in hot gas from the companion. The gas spirals around the black hole, heating up and forming a faint disk.

The black hole appears to be at least nine times the mass of the Sun. It and the companion orbit each other once every two and a half hours — one of the tighter orbits of any known black-hole binary system.

J1357 produces outbursts of X-rays every few years. They may occur when too much gas piles up in the disk. It gets so hot that it causes an eruption of particles and energy — an outburst that reveals more about this intriguing system.

Swift J1357 is much too faint to see with the eye alone, but we can spot its location. It’s close to the left of Spica, the brightest star of Virgo. It’s about a third of the way up the southeastern sky at nightfall.

Script by Damond Benningfield

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