The brightness of a black hole depends on its mass and on its feeding habits. Although the body itself traps light, the matter being drawn into it is often raised to energies where it emits copious amounts of light. In some cases, the mass of the black hole can be inferred by how much light is emitted by this matter. Some extremely bright X-ray emitting systems, for example, are thought to be powered by matter in a disk of plasma swirling around a black hole hundreds of times the mass of the Sun.

However, a bright X-ray source in the Pinwheel Galaxy could complicate that picture. Ji-Feng Liu and colleagues found a companion star locked in mutual orbit with the black hole and used the star to determine that the hole is much less massive than is suggested by X-ray emissions. These results could have profound implications for other luminous X-ray sources, which are currently thought to be powered by intermediate mass black holes.

Black holes fall into two major categories based on their mass. As their name suggests, the stellar-mass black holes have masses similar to those of stars; the largest known is about 16 times the mass of our Sun, though they could theoretically grow significantly larger. These black holes formed from the collapsed cores of very massive stars. Supermassive black holes, which reside at the centers of most large galaxies, are millions or billions of times the mass of the Sun; we're not currently certain how those form.

Between those two black hole types lies a vast desert. Some researchers have suggested a third type of black hole: the intermediate mass black holes (IMBH), with masses 100 to 1000 times that of the Sun. Astronomers have identified just a few intermediate mass black hole candidates, including the bright X-ray source M101 ULX-1. As the name indicates, it's located in the Pinwheel Galaxy, which is formally known as M101.

While it fluctuates in output quite a bit, M101 ULX-1 has emitted a stunning 3×1032 watts of X-ray light at its peak. (The Sun's luminosity is about 4×1026 watts, primarily in lower-energy visible light.) This data suggests that matter is falling onto an IMBH and forming a relatively low temperature accretion disk, such as those observed around other black holes.

Matter falling onto a black hole heats up and emits light, with the total amount of emission depending on the rate of infall. The maximum rate is known as the Eddington limit, which occurs when the matter falling inward achieves balance with pressure from light radiating back out. For a black hole to feed at anything close to the Eddington limit, it must have a companion object from which it can strip gas.

The Eddington limit also depends on the black hole's mass, so observations of the luminosity can provide an estimate of this value. Due to the nature of the light emission, many astronomers suspected that M101 ULX-1 and similar objects were intermediate mass black holes that were radiating far below the Eddington limit.

The new study found something strikingly different. Using archival visible and infrared data from the Gemini observatory, the researchers determined that M101 ULX-1 has a very massive companion star locked in mutual orbit. Using this gravitational dance, the researchers measured the black hole's mass to be in the range of 20 to 30 times that of the Sun. While larger than other known stellar mass black holes, that's far less massive than the previous estimates.

However, the new size estimate created a problem. For a relatively low-mass black hole to be as bright in X-rays as M101 ULX-1, it must be accreting matter at a rate much higher than the Eddington limit during its greatest outbursts. That's surprising, but a clue to the eventual solution could come from the identity of the companion star, which is a type known as a Wolf-Rayet star.

When they approach the end of their lives, very massive stars can begin shedding a lot of material—the Wolf-Rayet stage of evolution. In the case of M101 ULX-1, the shed material could be feeding the black hole at levels beyond those made possible by its gravity. While that's at odds with the accepted models for accretion, it's difficult to see how else this black hole could be so relatively low-mass yet also so luminous.

These results could have profound consequences for our view of other high-luminosity X-ray systems and IMBH candidates. If these other sources are accreting at a higher rate than the Eddington limit, then present mass estimates could be too high. The only way to be sure is to perform additional observations to look for companion objects—possibly other Wolf-Rayet stars—to see if systems like M101 ULX-1 are rare or typical.

Nature, 2013. DOI: 10.1038/nature12762 (About DOIs).