But on June 15, it woke up.
Now, scientists around the world are using highly sophisticated instruments to learn as much as they can about this mysterious beast of nature before the black hole returns to its slumber, which will be soon.
Black holes are very dense, massive objects in space that have an immensely powerful gravitational field that traps anything and everything that comes too close, including light. But on occasion they'll spit out material as well as suck it in.
On June 15, one of NASA's satellites picked up a torrent of x-rays all coming from a single source: the black hole.
"Relative to the lifetime of space observatories, these black hole eruptions are quite rare," said Neil Gehrels, the principal investigator for Swift, the NASA satellite that first identified the eruption in a NASA press release. "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."
A deadly companion
This black hole is just one half of a two-body system called V404 Cygni. Its partner is a star slightly smaller than our sun, and it's been nourishing the black hole for at least 77 years. The x-rays that astronomers observed on June 15 were the heated guts from the companion star that had spiraled into the mouth of the black hole.
When black holes in binary systems, like V404 Cygni, feed, they do so by gravitationally attracting a single thread of gas from the star. The black hole is 12 times more massive than its companion and therefore has a much stronger gravitational grip which slowly pulls gas from the star as the star orbits around it, like in the animation below:
As the gas gets pulled in, it orbits around the black hole, forming a disc. The closer the gas gets to the black hole, the stronger gravitational force it feels and so the faster it moves, heating up to searing-hot temperatures. When the gas reaches temperatures of more than 1.7 million degrees Fahrenheit, it emits a jet of high-energy particles, which satellites like NASA's Swift instrument then detect - albeit 8,000 years later because of the time it takes light to travel from the V404 Cygni to Earth.
But there isn't always a steady stream of gas falling into the black hole, which is why it takes such long naps in between feedings.
That disc has two regions: and inner, hot region, and an outer cool region. You need a lot of gas to provide enough pressure and push to cross this barrier, which takes time to generate. Once the black hole has consumed all of the gas in the inner region, which it does in a few days, it has to wait for more.
A once-in-a-professional-lifetime opportunity
V404 Cygni's black hole has erupted before.
Astronomers first took interest in this black hole more than 77 years ago, when they initially detected a strong burst of light from it in 1938. It came again in 1956, and then again in 1989.
While the eruption of 1989 was studied with a handful of instruments, the outburst wasn't studied in half the detail compared to this year's event.
Outbursts like this usually only last for a few weeks to months, so astronomers have culminated a total of nine instruments in space and on the ground to study the black hole in all wavelengths, from very low energy like radio waves to the most energetic like gamma rays, before time runs out.
Some of the instruments they're using include the Chandra X-ray Observatory, the European Space Agency's INTEGRAL satellite, Japan's MAXI, and the 10.4-meter Gran Telescope Canarias operated by Spain in the Canary Islands.
"It is definitely a 'once in a professional lifetime' opportunity," said Erik Kuulkers, the INTEGRAL project scientist in the NASA press release.