A rogue black hole wandering the spatial corridors of our galaxy, the Milky Way, could be the smallest black hole ever found, according to an estimate of its mass.
Earlier this year, astronomers led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, announced the discovery of the first known isolated stellar-mass black hole.
The black hole is 5,000 light-years away and was discovered through the power of its gravity to act as a gravitational lens, magnifying light from a background star 19,000 light-years away. It was initially spotted by two ground-based surveys, the Polish-led Optical Gravitational Lensing Experiment (OGLE), which primarily uses the Las Campanas Observatory in Chile, and the Microlensing Observations in Astrophysics (MOA) project at the observatory at Mount John University in New Zealand.
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Sahu’s team used the Hubble Space Telescope to track the discovery, and the degree of gravitational lensing allowed them to conclude that the black hole has a mass about 7.1 times the mass of the sun.
However, a second team has now come up with a different mass calculation. The group, led by Casey Lam of the University of California, Berkeley, concluded that the object had a mass between 1.6 and 4.4 times the mass of the sun. If correct, it could have intriguing implications.
Stellar-mass black holes are the product of supernovae from stars with a mass 20 times that of the Sun. On the other hand, when stars of 8 to 20 solar masses become supernovae, they leave behind a neutron star instead.
Neutron stars can theoretically have masses up to about 2.3 solar masses. Observations of detectable stellar-mass black holes in binary systems have found none with less than 5 solar masses, creating a gap between the most massive neutron stars and the least massive black holes. If the black hole is at the upper end of Lam’s mass range, it would help close that gap. (Several candidate gravitational wave events have also been detected involving objects that fall into this mass gap.)
“In any event, the object is the first dark stellar remnant discovered wandering the galaxy unaccompanied by another star,” Lam said in a NASA statement. (opens in a new tab).
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Even though stars over 20 solar masses are only 0.1% of all stars in the Milky Way, there are so many stars in the Milky Way (about 100-200 billion) and the Milky Way is so ancient (about 13 billion years old) that there should now be 100 million or more stellar-mass black holes in our galaxy.
Many of them are found in binary systems, where their presence is evident from their gravitational pull on their companion star and their accretion of matter from their neighbour. One of them was even discovered inside a star cluster, NGC 1850 in the Large Magellanic Cloud. However, many more will wander between the stars, going unnoticed until a random alignment with a background star means we spot them by creating a gravitational lens.
This discovery is just the tip of the iceberg. NASA’s Nancy Grace Roman Space Telescope, slated for launch in 2027, will survey large swathes of the Milky Way and is expected to identify several thousand microlensing events, many of which could be black holes.
The two articles of the Sahu team (opens in a new tab) and Lam’s team (opens in a new tab) are published online.
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