Wandering singularity discovered through microlensing as it warps distant starlight
The Milky Way is filled with the ghosts of dead stars. Our Galaxy is thought to be home to some 100 million black holes, the final chapter in the lives of large stars, but tracking down these elusive objects is difficult as they emit no light and can only be found through their effect on the surrounding environment.
However, after six years of painstaking observations, the Hubble Space Telescope has uncovered what appears to be a lone black hole floating through space and has measured its mass, giving the first ever direct evidence of a dark stellar remnant that’s unaccompanied by stars or an accretion disc.
The black hole was found using a technique known as microlensing, when light from a distant background star is deflected by the gravity of a foreground object. Because of this, chance alignments cause the background star’s brightness to fluctuate. These alignments are rare, but if a telescope is pointed at enough stars for a long time, it should eventually see a tell-tale wobble in the background star’s brightness, revealing an otherwise invisible object.
“Astrometric microlensing is conceptually simple but observationally very tough,” said Kailash Sahu from the Space Telescope Science Institute, who led one of two teams investigating the discovery. “Microlensing is the only technique available for identifying isolated black holes.”
Sahu’s team used the pattern of changing brightness to estimate the black hole is around seven solar masses.
However, another team led by Casey Lam from the University of California, Berkeley, measured it to be between 1.6 to 4.4 solar masses. While the top end of that range would indeed be a black hole, the lower end would be a neutron star.
“Whatever it is, the object is the first dark stellar remnant discovered wandering through the Galaxy unaccompanied by another star,” says Lam.
The discovery proves the validity of using microlensing to find black holes and helps provide insight into how common they are within our Galaxy. Using the data from this investigation, the team predicted that the nearest lone black hole to Earth is probably around 80 lightyears away.
In the coming years, new telescopes such as the Nancy Grace Roman Space Telescope will discover hundreds more black holes in the same way. Our journey to understanding these mysterious objects is just beginning. hubblesite.org