Lightweight and heavyweight black holes are pretty run of the mill while middleweight contenders have remained somewhat elusive, but now astronomers have found an X-ray source emanating from a galaxy that represents one of the best examples of an intermediate candidate.

Intermediate black holes contain between 100 and 10,000 times the Sun's mass. “We observe the heavyweight black holes in the centres of galaxies and the lightweight ones orbiting stars in our own Galaxy,” says Goddard Space Flight Center astronomer Tod Strohmayer. “But finding the ‘tweeners’ remains a challenge.”

Ultraluminous X-ray sources (ULXs) seen in nearby galaxies emit more energy than any known process powered by stars, but less energy than the centres of massive active galaxies, which are known to contain black holes with the equivalent masses of millions of Suns.

“ULXs are good candidates for intermediate-mass black holes, and the one in galaxy NGC 5408 is especially interesting,” says Richard Mushotzky of the University of Maryland.

Strohmayer and Mushotzky used ESA's XMM-Newton space telescope to study the 15.8 million light year distant source, known as NGC 5408 X-1, in 2006 and 2008. The astronomers observed a regular flickering caused by the pile-up of hot gas deep within the accretion disc that forms around a massive object.

Since the rate of flickering was about 100 times slower than that seen from stellar-mass black holes, but in X-rays NGC 5408 X-1 outshines these systems by about the same factor, the astronomers concluded that the source must contain between 1,000 and 9,000 solar masses. “For this mass range, a black hole’s event horizon – the part beyond which we cannot see – is between 3,800 and 34,000 miles across, or less than half of Earth’s diameter to about four times its size,” says Strohmayer.

The astronomers also suggest that if NGC 5408 X-1 is indeed feeding on gas to fuel its powerful X-ray emission, the material likely flows to the black hole from an orbiting star, typical for stellar-mass black holes in our Galaxy.

Using NASA's Swift space telescope to search for the companion star yielded a rise and fall of X-rays every 115.5 days. “If this is indeed the orbital period of a stellar companion then it’s likely a giant or supergiant star between three and five times the Sun’s mass,” says Strohmayer. Since the Swift observations only cover about four orbital cycles further observation is needed to confirm the findings.

NGC 5408 X-1 has long been suspected to host an intermediate black hole, and these new results, presented in the 1 October issue of The Astrophysical Journal, take a step closer in demonstrating that it is unusually massive.