A solitary celestial object — more massive than the sun but far smaller — roams the galaxy several thousand light-years from Earth. This may be the first isolated black hole with a stellar mass found in the Milky Way. Or it could be one of the heaviest known neutron stars.
The Interstellar Wanderer was first discovered in 2011 when its gravity briefly magnified light from a more distant star. But at the time, his true nature eluded researchers. Now two teams of astronomers are analyzing images from the Hubble Space Telescope to reveal the identity of the passenger – and came to slightly different conclusions.
The mysterious scammer is a black hole approximately seven times more massive than the sun, according to a team in a study in the press in Astrophysical Journal. Either it is slightly lighter – only two to four times the weight of our nearest star – and therefore either an unusually light black hole or a strangely heavy neutron star, another group said in a study in the press in Astrophysical Journal Letters.
Neutron stars and black holes with stellar mass form when massive stars – at least several times larger than the sun’s – collapse under their own gravity at the end of their lives. Astronomers believe that about a billion neutron stars and about 100 million black holes with stellar mass are hidden in our galaxy (SN: 18.08.17). But these objects are not easy to spot. Neutron stars are so small – the size of a city – that they don’t produce much light. And black holes do not emit light at all.
To find these types of objects, scientists usually observe how they affect their environment. “The only way to find them is if they affect something else,” said Kailash Sahu, an astronomer at the Baltimore Space Telescope Science Institute.
To date, scientists have discovered nearly two dozen black holes with star mass. (These relatively light black holes are weak compared to the supermassive giants that are at the center of most galaxies, including ours)SN: 18.01.21To do this, researchers have observed how these objects interact with their close celestial neighbors. When a black hole is locked in a gravitational dance with another star, it detaches matter from its partner. As this material hits the black hole, it emits X-rays that telescopes in orbit around the Earth can detect.
But finding black holes in binary systems does not paint a complete picture of the realm of black holes. Because these objects are constantly accumulating matter, it is difficult to determine the mass at which they formed. And because birth weight is a key feature of the black hole, it’s a significant drawback when considering binary systems, Sahu said. “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 sky for lonely black holes. The search depends on Einstein’s theory of general relativity, which states that any massive object, even an invisible one, bends the space near it (SN: 2/3/21). This bending causes the background stars to increase and distort light, a phenomenon known as gravitational microlens. By measuring changes in the brightness and visible position of stars, scientists can calculate the mass of the intervening object, which acts as a lens – a technique that rounds out several extrasolar planets (SN: 24.07.17).
In 2011, researchers announced that they had spotted a star that suddenly became more than 200 times brighter. But these initial observations with telescopes in Chile and New Zealand have failed to reveal whether the star’s apparent position is also changing. And this information is crucial for determining the mass of the intervening object. If it is a heavy category, its gravity will distort space so much that the star will appear to be moving. But even the “big” shift in the position of the star would be extremely small and difficult to detect. And unfortunately, the fine details in astronomical images taken by ground-based telescopes tend to be blurred due to the turbulent atmosphere of our planet (SN: 29.07.20).
To circumvent this earthly constraint, two independent teams of astronomers turned to the Hubble Space Telescope. This observatory can capture extremely detailed images as it orbits over most of the Earth’s atmosphere.
Both groups found that the star’s location had changed over several years. One of the teams, led by Sahu, concluded that the star’s apparent dance was caused by an object approximately seven times heavier than the sun. A star with this mass would be incredibly bright in Hubble’s images, but researchers saw nothing. Something so heavy and dark must be a black hole, the team said.
But another group of researchers, led by astronomer Casey Lam of the University of California, Berkeley, found different results. Lam and her colleagues estimated that the mass of the lens was lower, only about two to four times the mass of the sun. Therefore, it could be either a neutron star or a black hole, the group concluded.
Whatever it is, it’s an intriguing object, says astronomer Jessica Lou, a member of Lam’s team at UC Berkeley. This is because it’s a little weird about the table. It’s either one of the most massive neutron stars ever discovered or one of the least massive known black holes, Lou said. “He falls into this strange region we call mass difference.”
Despite the disagreement, these are exciting results, said Will M. Farr, an astrophysicist at Stony Brook University in New York who is not involved in any of the studies. “Working on the instrumental frontier in the true foreground of what is measurable is very exciting.”