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 / Updated  / Source: Space.com
By Elizabeth Howell, Space.com

After taking a 26-year nap, a waking black hole released a burst of X-rays that lit up astronomical observatories on June 15 — and it's still making a ruckus today.

Astronomers identified the revived black hole as an "X-ray nova" — a sudden increase in star luminosity — coming from a binary system in the constellation Cygnus. The outburst may have been caused by material falling into a black hole.

The burst was first caught by NASA's Swift satellite, and then by a Japanese experiment on the International Space Station known as Monitor of All-sky X-ray Image, or MAXI. [Video: Black Hole Wakes Up With A Bang]

"Relative to the lifetime of space observatories, these black-hole eruptions are quite rare," Neil Gehrels, Swift's principal investigator at NASA's Goddard Space Flight Center, said in a statement. "So, when we see one of them flare up, we try to throw everything we have at it, monitoring across the spectrum, from radio waves to gamma-rays."

The binary system responsible for the eruption is called V404 Cygni, according to the statement from NASA. It's made up of a star slightly smaller than the sun that orbits a black hole 10 times its mass. The orbital period is just 6.5 days, which means it orbits than 10 times faster than Mercury goes around our own sun.

Because the star orbits so closely to the black hole, the massive body pulls a stream of gas away from the star. Over time, the gas forms into a disc around the black hole.

When the gas is cooler, it's able to resist the black hole's pull. But as more gas gathers and it warms, eventually, the dam bursts, and gas is pulled toward the black hole. The rapidly moving, hot gas radiates an outburst of X-rays as it falls toward the gaping black maw, according to NASA.

This stellar duo has been active before, but only sporadically. The system was caught fluctuating in visible light in 1938 and 1956, and then in X-rays in 1989. The latter outburst was observed by instruments aboard Russia's Mir space station and a Japanese X-ray satellite called Ginga.

This is a condensed version of a report from Space.com. Read the full report. Follow Elizabeth Howell on Twitter. Follow Space.com on Twitter, Facebook and Google+.