SS 433 is a ravenous black hole that sucks the matter off its companion supergiant star like some sort of cosmic vampire—and it’s a messy eater. To date, SS 433 has been the only confirmed instance of a phenomenon known as “supercritical accretion” in which the black hole’s gluttonous stardust scarfing results in a hail of crumbs being thrust out into space. The viewpoint from Earth relative to SS 433 shows the object through a disk of material spiraling toward the black hole, so we do not see the its powerful x-rays in all their glory. But if the view were not obstructed by accreting material, SS 433 would appear as the brightest x-ray emitter in the galaxy.

Because it inhales material so ravenously, SS 433 has attained a singular status as an oddball in the Milky Way. Now, observations of exceedingly bright black hole binaries in nearby galaxies—other stars partnered with voracious black holes—suggest that these astronomical pairs may be up to the same thing. The extragalactic binaries are known as ultraluminous x-ray sources (ULXs) and this new work reveals that they are probably also superaccreting objects.

The ULX enigma

ULXs are the brightest sources of x-ray radiation in nearby galaxies, other than the supermassive black holes in galactic centers. Typically, galaxies harbor but one or two ULXs at most. Like the black hole binaries in the Milky Way, ULXs are comprise a black hole—the compact remnant of an exploded star—siphoning material off a partner star. The in-falling material forms an accretion disk around the black hole that heats to millions of degrees and flares x-ray radiation. But ULXs burn hundreds or thousands of times brighter than the black hole binaries in our galaxy, and scientists are still trying to figure out how.

As a rule, the faster black holes eat, the brighter they shine—but there is an upper limit on their luminosity. At a certain point, the pressure of the radiation blazing off the accretion disk is strong enough to counteract the black hole’s gravitational pull, so any excess in-falling material is blasted back out into space. This turning point is called the Eddington limit, which states the more massive a black hole is the stronger its gravity and the brighter it can get before it reaches its Eddington limit.

If ULX black holes were only as big as the black holes in the Milky Way, they should have reached their Eddington limits before getting as bright as they are. So one explanation for ULX luminosity might be that these objects contain “intermediate-mass black holes” at least a thousand times the mass of the sun—not as enormous as the supermassive black holes in the cores of galaxies, but much bigger than the stellar-mass black holes that pepper the Milky Way.

The alternate explanation for ULX brightness is that they are relatively small black holes, upward of 200 solar masses, which ingest material so voraciously that they have reached “supercritical accretion.” Basically, they cheated the Eddington limit and kept getting brighter. “Now, this is not as daft an idea as it might seem,” says Tim Roberts, an astrophysicist at Durham University in England, who was not involved with this study. The existence of the Eddington limit hinges on the assumption that material falls toward the black hole from all directions. But since real-life black holes wrap material around themselves in disks, it is possible for them to keep gobbling matter even after the outward push of radiation should have stemmed their intake. As black holes reach their Eddington limits, their disks become hot and bloated. The matter on the interior of the doughnut-shaped accretion disk is not blasted away by the radiation emanating from the disk’s surface, so it gets sucked into the black hole. Meanwhile radiation pressure blows away outer layers of the disk in a powerful wind. Scientists see this kind of outflow from SS 433.

Since the turn of the millennium astronomers have observed ULXs in hopes of determining which of the two theories describes the true nature of these strange objects, but x-ray observations alone have not settled the debate. So a group of scientists from the Special Astrophysical Observatory in Russia and Kyoto University in Japan investigated the visible light emitted by ULXs, hoping these alternate wavelengths might provide new insight. The team used the Subaru telescope in Hawaii to get optical spectra of the four nearest ULXs. Their work was published in Nature Physics on June 1. (Scientific American is part of Nature Publishing Group.)

SS 433 joins an eccentric family

The scientists found that all of their subjects emitted visible light with similar qualities, meaning that these ULXs probably constitute their own homogenous family of objects. The researchers also saw that key features of the ULX spectra were inconsistent with those expected of a standard accretion disk, ruling out the possibility that ULXs are intermediate-mass black holes gorging on matter in the usual way. Instead, the scientists found that the ULX spectra bore an uncanny similarity to that of SS 433, which, it is thought, harbors a black hole of only about 10 solar masses.

The scientists determined that ULXs are similar to SS 433 by examining features of their visible light called “emission lines.” Such data give insight into the chemical composition of celestial objects and how these particles are moving. The scientists found that the relative strengths and breadth of the hydrogen and helium emission lines in ULX spectra could be explained as originating from different regions in a dense, outward flowing wind—a telltale sign of superaccretion. SS 433 exhibits similar features because of its own disk wind. The absolute brightness of SS 433 is also in the same range as the ULXs observed in this study, which lends further support to the idea of their kinship. The scientists concluded that ULXs are most likely small black holes that have surpassed the Eddington limit, like SS 433.

Lingering uncertainties

“This is not the whole story on ULXs,” says Roberts, who is not convinced that they definitively make up their own homogeneous class of objects. “While the majority of objects may be these supercritical black holes, a minority may be equally exotic and maybe more extreme objects.” That means that the alternate theory for ULXs may also be correct. According to Roberts, there are still a handful of ULXs that appear to be good candidates for hosting intermediate-mass black holes.

Astrophysicist David Cseh at Radboud University Nijmegen in the Netherlands, who was also not involved with this study, agrees with Roberts. Cseh points out that astronomers have spotted at least one ULX that pulsates—that is, it periodically increases and decreases its brightness—which can only be explained by the presence of a neutron star rather than a black hole. “So, for those of us that work in ULXs, there is still plenty of mystery left,” Roberts says. The longstanding debate over the nature of ULXs might not be over yet, but it looks like one weird black hole binary might finally fit in somewhere.