For the first time, astronomers have used a quirk of gravity's effect on starlight to spot a "free-floating" black hole.
Until now, the only way to spy on the mysterious cosmic objects was by looking for the light reflected by matter on a black hole's perimeter.
Astronomers may have done the seemingly impossible and spotted a wandering black hole for the first time.
Black holes themselves are invisible by definition because not even light can escape their intense gravitational pull. In just the past few years, the international collaboration behind the Event Horizon Telescope managed to photograph black holes for the first time. But when we look at these images, the light that we see is actually the disk of hot gas and material circling around the edge of the black hole itself.
Sometimes black holes are apparent because one or many stars are orbiting them, as is the case with the supermassive black hole at the center of the Milky Way. But scientists expect that there are hundreds of millions of black holes drifting through the more isolated corners of the cosmos.
Now teams of astronomers have documented what could be either a neutron star or a vagabonding lone wolf of a black hole cloaked in the inescapable power of its own gravity. This was done for the first time by observing how the same force distorts the light from a more distant star, a phenomenon called gravitational microlensing.
"This is the first free-floating black hole or neutron star discovered with gravitational microlensing," said University of California, Berkeley, astronomy professor Jessica Lu, in a statement. "With microlensing, we're able to probe these lonely, compact objects and weigh them. I think we have opened a new window onto these dark objects, which can't be seen any other way."
Lu helped lead one of two teams that analyzed the same data of the microlensing event observed by NASA's Hubble Space Telescope. Their analysis has been accepted for an upcoming issue of The Astrophysical Journal Letters.
Another team from the Space Telescope Science Institute in Baltimore calculated a slightly different mass for the object and concluded with a higher degree of confidence that it is, in fact, a black hole. That paper will be published in The Astrophysical Journal.
"As much as we would like to say it is definitively a black hole, we must report all allowed solutions. This includes both lower-mass black holes and possibly even a neutron star," Lu said.
It is between 1.6 and 7.1 solar masses, according to the competing estimates. The lower mass allows for the possibility that the object might be a neutron star. If it's at the higher end of the range, it becomes more indisputable that the object is a black hole.
Whatever it is, the object goes by the labels MOA-2011-BLG-191 and OGLE-2011-BLG-0462 (OB110462, for short) and is 5,000 light-years from Earth, so there's little worry of it sneaking up on us anytime soon.
The debate over exactly what type of cosmic character is bending the light from stars behind it may soon be settled. The Hubble Space Telescope is set to make more observations and collect more data on the object in the second half of 2022.