[On June 15, NASA’s Swift caught the onset of a rare X-ray outburst from a stellar-mass black hole in the binary system V404 Cygni. Astronomers around the world are watching the event. In this system, a stream of gas from a star much like the sun flows toward a 10 solar mass black hole. Instead of spiraling toward the black hole, the gas accumulates for decades in an accretion disk around it. Every couple of decades, the disk switches into a state that sends the gas rushing inward, starting a new outburst. Credit: NASA’s Goddard Space Flight Center]
A black hole that has been quiet since 1989 has been caught burping.
A NASA satellite monitored and controlled by Penn State University detected a black hole erupting high-energy flares called an X-ray nova from a black hole 8,000 light-years away from Earth in a system named V404 Cygni. This black hole has been known to burp up once in a while, but it had been dormant since 1989. Until NASA’s Swift Gamma-ray Burst Explorer caught it belching again on June 15.
This system is in the constellation Cygnus. V404 Cygni has a star slightly smaller than the Sun orbiting a black hole 10 times its mass in 6.5 days. Astronomers classify this system as a low-mass X-ray binary. The tight orbit and strong gravity of the black hole pulls a stream of gas from its companion star. The gas travels to a storage disk around the black hole and heats up to millions of degrees. When this super hot gas falls inward into the black hole, it produces a stream of X-rays causing the flares.
Relative to the lifetime of space observatories, these black-hole eruptions are quite rare,” said Neil Gehrels, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “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.”
V404 Cygni has flared many times since the eruption began. “It repeatedly becomes the brightest object in the X-ray sky — up to 50 times brighter than the Crab Nebula, which is normally one of the brightest sources,” said Erik Kuulkers, the INTEGRAL project scientist at ESA’s European Space Astronomy Centre in Madrid. “It is definitely a ‘once in a professional lifetime’ opportunity.”