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A lone, massive celestial object roams our galaxy a few thousand light-years from Earth. It’s not too big, but its mass is greater than that of our sun. Astronomers suspect it could be the first solitary black hole in the Milky Way found with a mass similar to that of our sun. Or it could turn out to be one of the heaviest known neutron stars.
This wanderer first revealed himself in 2011. He was not seen. Instead, astronomers found 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 still don’t know exactly what the weight object is, but they’ve narrowed down the list of candidates.
A group suspects that this mysterious thief is a black hole about seven times more massive than the sun. Make no mistake, its 94 authors say, “We report the first detection and unambiguous mass measurement of an isolated stellar-mass black hole.” They describe it in an article that will soon appear in the 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 its findings in an upcoming issue of Astrophysical Journal Letters.
Neutron stars and stellar-mass black holes can form when massive stars — those that are at least several times the weight of our sun — collapse under their own gravity. This happens at the end of the lives of these stars. Astronomers now believe that about a billion neutron stars and about 100 million stellar-mass black holes lurk in our galaxy.
None of these types of objects are easy to spot. Neutron stars are tiny – barely the size of a city. They also produce little light. Black holes, regardless of their size, emit no light. To detect these objects, scientists typically observe how they affect their surroundings.
“The only way to find them is if they influence something else,” says Kailash Sahu. He is an astronomer at the Space Telescope Science Institute in Baltimore, Maryland.
The great mystery
To date, scientists have detected nearly two dozen stellar-mass black holes. (These are puny compared to their supermassive cousins that sit at the center of most galaxies, including our own.) Researchers discovered these relatively tiny 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 material from that companion star. When material from the star falls on the black hole, it emits X-rays. Telescopes orbiting Earth can detect this radiation. But scientists will struggle to know how big a black hole was before it started eating on the star. And since birth weight is a key characteristic of a black hole, looking at black holes eating stars can confuse the picture. That’s why, says Sahu, “if we want to understand the properties of black holes, it’s better to find isolated ones” – like the new solitaire.
For more than a decade, researchers have been scanning the skies for these isolated black holes. Hoping to spot these thieves, scientists searched for distorted starlight.
Einstein’s theory of general relativity states that the gravity associated with any massive object – even invisible ones – will bend nearby space. This curvature amplifies and distorts the background starlight. Astronomers call this gravitational lensing. By measuring changes in the brightness and apparent position of stars, scientists can calculate the mass of a moving object that acts like a lens. This technique has already made it possible to discover several exoplanets.
In 2011, researchers announced that they had spotted a star that suddenly became 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 was also changing. And this information would be essential in determining the mass of the object that acted as a lens. If it is a heavy weight, its gravity would distort space so much that the star would appear to be moving. However, even a “big” change in the star’s position would have been very small and difficult to detect. And it’s hard to see fine detail in images captured by telescopes on Earth’s surface. (Our planet’s turbulent atmosphere simply blurs them.)
To circumvent this problem, two independent teams of astronomers turned to Hubble. Orbiting above the pesky atmosphere, this telescope can capture extremely detailed images.
Both groups discovered that the location of the star had changed over several years.
The Sahu-led team now believe the star’s apparent motion was caused by an object about seven times heavier than the sun. Such a massive star should have been extremely bright in Hubble images. But the researchers saw nothing. To be so heavy and dark, the mystery object must be a black hole, the team now concludes.
Astronomer Casey Lam led a group of researchers who came to a different conclusion. Lam works at the University of California at Berkeley. She and her colleagues calculated that the mass of the lens object was smaller. It was probably closer to two to four times that of our sun. In this case, they said, it could be a black hole – or a neutron star.
Either way, it’s an intriguing object, says astronomer Jessica Lu of the University of California at Berkeley. She is part of Lam’s team. The mysterious object, Lu says, is either one of the most massive neutron stars ever discovered or one of the least massive black holes. “It falls into this strange region that we call the ‘mass divide’.”
Either way, the new results are exciting, says Will M. Farr. He is an astrophysicist from Stony Brook University in New York who was not involved in any of the new analyses. He says working “at the cutting edge of what is measurable is very exciting.”
