Found: A rogue black hole
Stellar-mass black holes are formed when a massive star dies. To date, astronomers have only indirectly detected black holes by watching them devour material from a companion star.
But now, researchers think they’ve found an isolated stellar-mass black hole some 5,200 light-years away. The find has been submitted to The Astrophysical Journal.
The team combined two techniques to spot the black hole. The first, microlensing, is a form of gravitational lensing that occurs when a small celestial object crosses in front of a distant star. That object’s gravity bends and magnifies the starlight as it travels past. Astronomers can estimate the nearer object’s mass by how long the event lasts. A long lensing event caused by something invisible could be a black hole.
But a faint star moving slowly could create the same effect. So, the team also applied astrometry, the precise measurement of objects’ positions on the sky. By seeing how much the background star’s position appeared to shift during the lensing event, astronomers could better determine the nearer object’s mass: just over seven times the mass of the Sun. Such an object would be shining brightly if it was a star. That would mean it must be a black hole.
However, there’s another possibility: A separate team’s analysis of the same event puts the object between about 1.5 and 4 solar masses, which means it could be a neutron star instead. Astronomers have never detected an isolated neutron star either, so it would still be a remarkable discovery.