The fortuitous flare-up of a black hole last active in the mid-1990s solidifies an assumption sometimes used to estimate a black hole's spin, one of its two most important properties. (The other is mass.) As matter whips around a black hole, it radiates light perpendicular to its orbital radius, like a lighthouse. The mass and spin of the black hole can furrow a groove in spacetime that makes this orbit wobble in various ways, creating additional fluctuations in its radiation.

In 1996 a black hole system called GRO J1655-40 broadcast a pattern of x-ray oscillations suggestive of such a groove, then clammed up a few months later. In 2005 gas from a companion star got caught in the black hole's drain again, allowing researchers to observe the reenergized system over eight months. Sure enough, they discovered the identical pattern. "Detecting the same frequencies nine years later means what we are looking at here is really a fundamental property," not some gaseous mirage, says Jeroen Homan of the Massachusetts Institute of Technology. His team reported the results on January 9 at the annual meeting of the American Astronomical Society.

The group will now try to determine if factoring in these fundamental frequencies can give them better measurements of the black hole's spin.