A nearby black-hole unexpectedly burst into a gleaming, flickering flare last June. For months, the black hole known as V404 Cygni became the brightest X-ray object you could see from Earth as it sputtered in several minute- to hour-long flashes. Even with just a dinky backyard telescope a person could watch it flicker in visible light.

But this awesome outburst had the scientific community befuddled. What was behind the black-hole's weird, irregular pulsing?

Today that question has been answered. A team of more than 60 astrophysicists and astronomers—led by Mariko Kimura, an astronomer at Kyoto University in Japan—announced the cause of the strange fluctuations of light seen from V404 Cygni and other gleaming black-holes like it. The new theory, which is backed up by dozens of observations from telescopes across the world, is published in the journal Nature.

A Black Hole Mystery, Illuminated

Kimura's idea is relatively straightforward, but it helps to first understand how a black hole, a point from which no light can escape, can actually shine. This particular black hole, V404 Cygni, is continually fed by a nearby star slightly smaller than our sun. As these objects orbit each another, the star's gas is slowly siphoned in a steady stream. This gas forms a bright disk around the black hole before it gets sucked into the unknown. It is that disk of surrounding gas, not the black-hole itself, that can shine by erupting in x-ray and visible light.

What was behind the black-hole's weird, irregular pulsing?

Scientists had long suspected that the strange flickering of light they see from a black hole's disk during an intense flare-up might be caused by the irregular speed at which the star's gas is siphoned. The basic idea here is that the faster gas comes in, the brighter the disk gets. The slower it comes in, the dimmer.

Not so, says Kimura's team. Thanks to V404 Cygni's relative proximity to Earth (a mere 8,000 light years) the astronomers saw that the black-hole's pulsations of light were seemingly unrelated to the flow of incoming gas. Rather, Kimura says, the flickering light is almost entirely the result of the size of the gas disk, not the incoming flow. The size of that disk depends on how far away a black-hole's source of fuel lies. The closer the star, the smaller the disk.

Here's how the theory works in action. As gas from the inner part of the disk—the stuff that's right up next to the black hole's edge—is sucked into the black hole, glowing hot gas from the outer rim moves in to replace it. But if the disk is large enough, that motion is irregular, causing a sort of sloshing in the gas that erratically flashes light. This is why V404 Cygni flickered with such extreme levels of light. The star orbits a black hole 10 times its massive ever six and a half days. Do the math, the scientists say, and that allows for a disk with 55 quintillion tons of gas.

How To Spot It

Kimura says that 10 amateur astronomers using personal telescopes as small as 20 centimeters, "provided valuable data of V404 Cygni's repetitive violent fluctuations" which was instrumental for forming this new understanding of black-holes. In fact, with a bit more powerful backyard equipment, you can still spy V404 Cygni today.

"Observations over the past few nights show that... under good conditions and dark skies... a [roughly] 30 cm telescope should be able to catch it," says Poshak Gandhi, an astronomer at the University of Southampton who did not collaborate with Kimura's team, via email.

Still, there are two challenges in seeing this phenomenon. For one thing, he says, V404 Cygni "is strongly variable on timescales of hours or less, and sporadically fades. The second problem is that, unfortunately, V404 Cygni "also happens to lie close to the Sun as seen from Earth (because of Earth's orbit, the position of the Sun relative to background stars changes," he writes. "So V404 Cygni is only visible near the local horizon for about an hour or so at sunset, and there is only a short window of opportunity to try and observe it now before its proximity to the Sun renders it invisible until March."

If you want to find it now, before it temporarily falls behind the sun for the next few months, "a rough guide is that the object lies on the leading edge of southern wing of the swan [constellation] represented by Cygnus, about 4.6 degrees west of the bright naked eye star epsilon Cygni. It lies approximately along the line connecting epsilon and eta Cygni, two of the bright stars of the Cygnus constellation."

Happy hunting, black hole-gazers.

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