Another way to detect a black hole is to find one distorting the image of a celestial object behind it.
In our previous chapter on Gravitational Lensing, the lenses were mostly galaxy clusters and galaxy superclusters. But even a single planet can act as a lens – bending and magnifying light from objects behind it. These are micro-lenses and analyzing their lensing effects is referred to as microlensing. The greater the mass density of the micro-lens, the larger the lensing impact. This opens up the possibility of finding a free roaming black hole by detecting its microlensing effects.
It’s estimated that there are over 100 million free roaming black holes in the Milky Way, representing almost 1% of the galaxy’s total mass. The few dozen stellar mass black holes discovered so far have been found in x-ray binary systems. Astronomers had not identified an ‘isolated black hole’ until Hubble found one drifting through interstellar space in 2022.
In this image, we see a star that measurably brightened, as first captured by Hubble beginning in August, 2011. This brightening was caused by a dark lens identified as OB110462 that drifted in front of the star. The background star both brightened and shifted in its apparent position. After over 200 days, it faded back to its normal brightness and position.
This long lensing event duration, combined with the lens dynamics associated with the amount of background star brightening, combined with the Hubble measurements on the amount of deflection of the background star’s image provided the data to calculate the distance, velocity, and mass of the micro-lens.
The results showed that the star’s image was offset from where it normally would be by just over a milliarcsecond. This amount of deflection indicated that the lens is 5 to 6 thousand light years away, is traveling at around 24 km/s (that’s 15 mi/s), with a mass of 3.7 suns. This mass makes it a stellar mass black hole given that the mass separation between a neutron star and a black hole is around 3 solar masses.
It should be noted that there are other models of this event that indicate the there is a non-trivial chance that the lens is a neutron star. Further examinations should be able to confirm or refute this Black Hole claim.