A massive rogue roaming our galaxy could be a black hole

A lonely and massive celestial object wanders in our galaxy several thousand light years from Earth. It’s not too big, but its mass is greater than that of our sun. Astronomers suspect that this may be the first discovered solitary black hole in the Milky Way with a mass similar to that of our sun. Or it could turn out to be one of the heaviest known neutron stars.

This vagrant first appeared in 2011. It was not seen. Instead, astronomers discovered it when its gravity briefly amplified light from a more distant star. At the time, no one was sure what it could be. Now two teams of astronomers have analyzed images from the Hubble Space Telescope. They’re still not entirely sure what the heavy object is, but they’ve narrowed down the list of candidates.

One group suspects that this mysterious rogue is a black hole roughly seven times more massive than the sun. Make no mistake, its 94 authors say: “We report the first unambiguous detection and measurement of the mass of an isolated stellar-mass black hole.” They describe it in a paper due out soon in Astrophysical Journal.

Not so fast, says another team of 45 scientists. They think it’s a bit lighter – just two to four times the weight of our nearest star. If true, that would make it an unusually light black hole — or a curiously heavy neutron star. This group will share their findings in an upcoming issue of Astrophysical Journal Letters.

Both neutron stars and stellar-mass black holes can form when massive stars—those with at least several times the mass of our sun—collapse under their own gravity. This happens at the end of the life of these stars. Astronomers now estimate that about one billion neutron stars and approximately 100 million stellar-mass black holes lurk in our galaxy.

Neither of these types of objects are easy to spot. Neutron stars are small – about the size of a city. They also produce some light. Black holes, regardless of their size, do not emit any light. To find these objects, scientists usually observe how they affect what’s around them.

“The only way we can detect them is if they affect something else,” explains Kailash Sahu. He is an astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

The great mystery

To date, scientists have discovered nearly two dozen stellar-mass black holes. (They are faint compared to their supermassive cousins, which are found at the center of most galaxies, including our own.) Researchers discovered these relatively small black holes by observing changes in some of their neighbors. Sometimes a black hole and a normal star will be caught in a spiral. Think of it as a dance.

But it’s a dangerous dance, as the black hole rips matter away from this companion star. As the star’s material falls onto the black hole, it emits X-rays. Telescopes in Earth orbit can detect this radiation. But scientists will have a hard time figuring out how big a black hole was before it started feeding on the star. And since birth weight is a key characteristic of a black hole, looking at black holes eating stars can confound the picture. That’s why Sahu says, “If we want to understand the properties of black holes, it’s best to find isolated ones” — like the new loner.

For more than a decade, researchers have been scanning the heavens for such isolated black holes. Hoping to spot these rogues, scientists have been looking for distorted starlight.

Einstein’s theory of general relativity states that the gravity associated with any massive object – even an invisible one – will bend the space around it. This bending magnifies and distorts the light of the background stars. Astronomers call this gravitational lensing. By measuring the changes in brightness and apparent position of the stars, scientists can calculate the mass of a traveling object that acts as a lens. This technique has already discovered several exoplanets.

In 2011, researchers announced that they had spotted a star that had suddenly become more than 200 times brighter. These observations, made using telescopes in Chile and New Zealand, could not determine whether the apparent position of the star also changes. And this information would be key to determining the mass of the object that acted as a lens. If it is a heavyweight, its gravity will distort space so much that the star will appear to be moving. However, even a “large” change in the star’s position would be very small and difficult to detect. And it’s hard to see fine details in images taken by telescopes on Earth’s surface. (Our planet’s turbulent atmosphere simply obscures them.)

To overcome this problem, two independent teams of astronomers turned to Hubble. Orbiting high above the pesky atmosphere, this telescope can capture extremely detailed images.

Both groups found that the star’s location had changed over the course of several years.

The team led by Sahu now believes that the star’s apparent motion is caused by an object roughly seven times the mass of the sun. A star this massive should be lightning bright in Hubble images. But the researchers saw nothing. To be so heavy and dark, the mysterious object must be a black hole, the team concluded.

Astronomer Casey Lam led a group of researchers who came to a different conclusion. Lam works at the University of California, Berkeley. She and her colleagues calculated that the mass of the lensed object is smaller. It was probably two to four times closer to our sun. In that case, they said, it could be a black hole or a neutron star.

In any case, it’s an intriguing object, says astronomer Jessica Lu of the University of California, Berkeley. She is a member of Lam’s team. The mystery object, Lu says, is either one of the most massive neutron stars ever discovered or one of the most massive black holes. “It falls into this strange region we call the ‘table chasm.'”

Any way you look at it, the new results are exciting, says Will M. Farr. He is an astrophysicist at Stony Brook University in New York who was not involved in the two new analyses. He says that working “on the real front of what’s measurable is very exciting.”

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