A celestial loner might be the first known rogue black hole

A solitary celestial object — more massive than the sun, yet far smaller — is wandering the galaxy a few thousand light-years from Earth. It might be the first isolated stellar-mass black hole to be detected in the Milky Way. Or it might be one of the heaviest neutron stars known.

The interstellar wanderer first revealed itself in 2011, when its gravity briefly magnified the light from a more distant star. But at the time, its true nature eluded researchers. Now, two teams of astronomers have analyzed Hubble Space Telescope images to unmask the traveler’s identity — and have come to somewhat different conclusions.

The mysterious rogue is a black hole roughly seven times as massive as the sun, one team reports in a study in press in the Astrophysical Journal. Or it’s a bit lighter — a mere two to four times the weight of our nearest star — and therefore either an unusually lightweight black hole or a curiously hefty neutron star, another group reports in a study in press in the Astrophysical Journal Letters.  

Neutron stars and stellar-mass black holes form when massive stars — at least several times the heft of the sun — collapse under their own gravity at the end of their lives. Astronomers believe that about a billion neutron stars and roughly 100 million stellar-mass black holes lurk in our galaxy (SN: 8/18/17). But these objects aren’t easy to spot. Neutron stars are so tiny — about the size of a city — that they don’t produce much light. And black holes emit no light at all.

To detect these kinds of objects, scientists typically observe how they affect their surroundings. “The only way that we can find them is if they influence something else,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore.

To date, scientists have detected nearly two dozen stellar-mass black holes. (These relatively lightweight black holes are puny compared to the supermassive behemoths that sit at the center of most galaxies, including our own (SN: 1/18/21).) To do so, researchers have watched how these objects interact with their nearby celestial neighbors. When a black hole is locked in a gravitational dance with another star, it rips away matter from its partner. As that material falls onto the black hole, it emits X-rays, which telescopes orbiting the Earth can detect.

But finding black holes in binary systems doesn’t paint a whole picture of the black hole kingdom. Because these objects are continually accreting matter, it’s challenging to determine the mass at which they formed. And since birthweight is a key characteristic of a black hole, that’s a significant drawback to looking at binary systems, Sahu says. “If we want to understand the properties of black holes, it’s best to find isolated ones.”

For more than a decade, researchers have been scanning the heavens for solitary black holes. The searches have hinged on Einstein’s theory of general relativity, which states that any massive object, even an unseen one, bends space in its vicinity (SN: 2/3/21). That bending causes light from background stars to be magnified and distorted, a phenomenon known as gravitational microlensing. By measuring changes in the brightness and apparent position of stars, scientists can calculate the mass of the intervening object that’s acting like a lens — a technique that’s rounded up a few extrasolar planets as well (SN: 7/24/17).

In 2011, researchers announced that they had spotted a star that suddenly had gotten more than 200 times brighter. But those initial observations, made using telescopes in Chile and New Zealand, were unable to reveal whether the star’s apparent position was also changing. And that information is key to pinning down the mass of the intervening object. If it’s a heavyweight, its gravity would distort space so much that the star would appear to move. But even a “big” shift in the star’s position would have been extremely small and hard to detect. And unfortunately fine details in astronomical images captured by ground-based telescopes tend to be blurred out because of our planet’s turbulent atmosphere (SN: 7/29/20).

To circumvent this Earthly limitation, two independent teams of astronomers turned to the Hubble Space Telescope. This observatory can capture extremely detailed images since it orbits above most of Earth’s atmosphere.  

Both groups found that the star’s location shifted over the course of several years. One of the teams, led by Sahu, concluded that the star’s apparent dance was caused by an object roughly seven times as hefty as the sun. A star of that mass would have been blazingly bright in the Hubble images, but the researchers saw nothing. Something that heavy and dark must be a black hole, the team reports.

But another group of researchers, led by astronomer Casey Lam at the University of California, Berkeley, found different results. Lam and her colleagues calculated that the mass of the lensing object was lower, only about two to four times the mass of the sun. It could therefore be either a neutron star or a black hole, the group concluded.

Whatever it is, it’s an intriguing object, says astronomer Jessica Lu, a member of Lam’s team also at UC Berkeley. That’s because it’s a bit of an oddball in terms of mass. It’s either one of the most massive neutron stars discovered to date, or it’s one of the least massive black holes known, Lu says. “It falls within this strange region we call the mass gap.”

Despite the disagreement, these are thrilling results, says Will M. Farr, an astrophysicist at Stony Brook University in New York not involved in either study. “To be working at the instrumental limit at the real forefront of what’s measurable is very exciting.”