In a study based on new observations from the Chandra X-ray Observatory, NuSTAR and Australia Telescope Compact Array, a team of researchers may have confirmed the presence of a black hole and a white dwarf star in the tightest orbit ever seen.

The system was observed in the 47 Tucanae globular cluster, is located about 16,700 light-years away in our own galaxy, and is the second-brightest globular cluster in the night sky. It’s also the site of the recent discovery of the first intermediate-mass black hole yet found. It’s a fertile ground for studying the role of black holes in globular clusters.

The system in question, 47 Tuc X9, is the brightest X-ray source in 47 Tuc’s core. Researchers initially thought its most massive constituent might be a so-called cataclysmic variable, a system whose brightness regularly increases and decreases. This occurs in a binary pair of stars in which material from the donor star (typically a white dwarf) falls onto the more massive (roughly Sun-mass) star, leading to the increased emissions.

The new study finds, based on the system’s X-ray and radio emissions, that it is something entirely different: X9 doesn't look like a cataclysmic variable should. “X9 would be extremely unusual among [cataclysmic variables] in its radio flux, its X-ray flux, and its X-ray to optical flux ratio. X9 would have the highest radio flux of any observed [cataclysmic variable],” the researchers write in their paper. There are scenarios that could still be contrived to produce the same observations with special kinds of cataclysmic variables, but the researchers conclude those are “extremely improbable."

Instead, the radio emissions suggest that its primary component is probably a stellar-mass black hole. That leaves the question of the identity of its companion, the donor star.

The companion

A 2015 study suggested that X9's less-massive body might make one full orbit of the black hole in roughly 25 minutes. The new study agrees with that result: the researchers notice a periodic variation in the system’s X-ray brightness that takes about 28 minutes to go through a cycle. While the researchers discuss other possible causes of this periodic behavior, they conclude that the most likely explanation is that it’s the donor star’s orbit around the black hole.

If so, that’s an absurdly quick orbit, meaning the star must be whipping around the hole at incredible speeds—on the order of 12 million km/h. This would require the star to be orbiting at a mere 2.5 times the distance between the Earth and the Moon. This donor star is almost certainly a white dwarf; anything more massive, such as a Sun-like star, couldn’t maintain an orbit so close to its black hole.

The researchers performed computer simulations to analyze the spectral lines in the X-ray light coming from the star. They concluded that some of the X-rays are being absorbed by a pocket of oxygen-rich material that envelops the black hole. That would be material that has been leeched off the donor star and is falling into the black hole. This provided further evidence that the hole’s companion is a white dwarf. Ordinary, Sun-like stars are composed of mostly hydrogen or helium, but white dwarves—which have long since burned through their hydrogen or helium fuel—are rich in heavier elements. The gas surrounding the hole is lacking in hydrogen but appears to have oxygen.

One way this system could have formed is if the black hole plowed into one of the globular cluster's red giant stars, rapidly consuming most of its material. The core of the red giant could survive as a white dwarf. Over time, the white dwarf may have spiraled closer and closer in toward the black hole until it got close enough for the black hole to begin sucking off some of its material.

Since the white dwarf is losing so much mass, it might eventually end up planet-sized. The mass loss might not stop there: it could ultimately evaporate completely, devoured by its black hole companion. Alternately, the mass loss could allow the companion to accelerate into a more distant orbit, where the feeding would stop.

Black holes and globular clusters

47 Tucanae is an example of a globular cluster, a blob of stars that orbits our galaxy. If X9 does turn out to be a black hole, it would be unusual. While many black holes have been discovered in binary systems throughout the galaxy, they tend not to show up in globular clusters.

In fact, the lack of black holes in globular clusters was surprising to scientists at first. Black holes form from massive stars that run out of fuel, and globular clusters have no shortage of massive stars. It was originally thought that clusters should each have something like 1,000 black holes. As evidence for that failed to turn up, researchers began to consider mechanisms that might prevent black holes from existing in globular clusters. They concluded that the holes’ strong gravity, combined with their interactions with other stars, might be shooting the black holes out of the clusters. That would explain the lack of observations, but does it mean that the clusters can’t have any black holes at all?

According to recent computer simulations (prior to the present study), the answer is no: some black holes should remain in their clusters up to the present day, maybe about a dozen per cluster. This suggestion prompted researchers to look for them observationally once again, and they did turn up a handful of candidates in globular clusters in the Milky Way and other galaxies.

One way to find black holes is through the X-rays they emit. The trouble is, a globular cluster such as 47 Tucanae contains many known X-ray sources, including several X-ray binaries (systems with two stars or black holes). Many of these have been investigated and so far have turned out to have neutron stars as their heavier components, rather than black holes.

Conclusions

The authors' conclusion that the binary consists of a black hole and white dwarf is not yet certain, but it's the likeliest explanation. It's also possible that it's a neutron star rather than a black hole, but that's a long shot: those have certain properties not evident in X9. The conclusion that the companion is a white dwarf, meanwhile, is much stronger.

"This was an exciting find, as we found compelling evidence that the companion is a white dwarf and furthermore could even show what the white dwarf is made of," wrote Arash Bahramian, the paper's lead author, in a blog post. "Also, if it indeed contains a black hole, maybe future gravitational wave observatories might be able to detect radiation from this fascinating binary."