Black Holes

Black Holes Radiate Jets of Light, and Scientists Now Know Why

Despite the name, black holes aren't strictly black. Those infinitely massive sinkholes in space actually shine pretty brightly, thanks to both a white-hot accretion disk spinning around them and two gigantic jets of plasma blasting from each end. Why those jets shine was a mystery to scientists, but a new study presents a fascinating explanation.

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The leading theory for how these powerful jets form goes like this: gas and dust collect in the orbit of a black hole, forming a dense, spinning accretion disk that experiences enough friction to heat up into a magnetized plasma. Intense gravity from the black hole twists and warps that magnetic field with so much force that it eventually erupts in the form of two magnetic pillars that rocket out each end of the black hole.

Plasma shoots through these pillars, quickly gaining speed as they fly over a vast distance — sometimes further than an entire galaxy. At some point, the plasma begins to shine. If it's a supermassive black hole at the center of a galaxy, that forms a quasar (or, if it's pointing toward us, a blazar). Why and at what point this light appears been a matter of debate.

Mystery Solved

To find out, astronomers found two systems in the Milky Way called X-ray binaries, where a black hole is feeding off of a regular star. Then they called on two telescopes: NASA's NuSTAR space telescope to detect the X-ray light that would emit from all of the material in the jet, and the fast camera called ULTRACAM on the William Herschel Observatory in Spain to catch the moment the jet emitted optical light.

Despite the fact that these two black hole systems were very different — one, called V404 Cygni, experienced the brightest outburst ever seen in this century; the other, called GX 339-4, was at minimum brightness and had a star and black hole that was much closer together — the results were the same. There were similar time delays, about a tenth of a second, between when NuSTAR detected the X-ray light from material shooting out of the black hole and when ULTRACAM detected flares of optical light.

That gave astronomers a good explanation of how these jets shine. Plasma shooting out of the black hole creates X-rays almost immediately as strong magnetic fields accelerate it to incredibly high speeds, causing the particles to collide and creating a burst of optical light.

In the super-bright V404 Cygni, the cosmic onramp where the plasma feels the greatest acceleration and begins to shine is an expanse of about 19,000 miles (30,000 kilometers) from the black hole — about three times the diameter of Earth. Scientists have suspected that the time delays in more massive black holes are even greater, which suggests that the length of this onramp depends on the mass of the black hole.

That's exciting, because this study of smaller black holes seems to also explain the supermassive black holes that reside at the center of galaxies, including our own. For a place where light can't escape, black holes sure are bright.

For more on black holes, check out "A Black Hole is Not a Hole" by Carolyn Cinami DeCristofano. The audiobook is free with a 30-day trial of Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Radio Telescopes Capture Best-Ever Snapshot of a Black Hole's Jet

Written by Ashley Hamer December 7, 2017

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