Black holes are invisible because they absorb all the radiation. It is correspondingly difficult to track down inactive representatives of these gravitational giants floating isolated in space. However, this is exactly what two research teams may have succeeded in doing. They have spotted a stellar black hole about 5100 light-years away, wandering on its own. The invisible object was given away by the gravitational lensing effect: the gravity of the black hole passing in the foreground amplified and distorted the light of a background star. The astronomers were also able to estimate the approximate mass of the black hole from the duration and strength of this effect. It could therefore weigh a good seven solar masses.
When a massive star explodes in a supernova, above a certain mass, its core collapses into a black hole. Given the large number of massive stars that have formed and died over time, there should be millions of such stellar black holes in our Milky Way alone. Some of them might even be hurtling through our galaxy at relatively high speeds because they got an asymmetric “nudge” at the supernova. But because black holes swallow all radiation, they remain dark and invisible unless they interact with other celestial bodies, for example by sucking matter from a partner star or releasing gravitational waves during collisions. So far, astronomers have not been able to unequivocally detect an isolated, inactive stellar black hole.
Gravitational lensing reveals invisible object
Now, however, two teams of astronomers may have succeeded for the first time. Independently of each other, they had evaluated the same data from the Hubble Space Telescope. The focus was on so-called microlensing events – the distortion and amplification of the light from a distant star due to the gravity of a foreground object. Such a gravitational lensing effect can in principle be caused by any massive object – from the exoplanet to the entire galaxy. However, a stellar black hole gives itself away by triggering a localized but lasting more than 200 days lensing effect. Because the hole itself does not emit any light, the light color of the background star is not changed – with a massive star as the lens, on the other hand, the light from both would mix.
For their study, the two teams – one led by Casey Lam from the University of California at Berkeley and one led by Kailash Sahu from the Space Telescope Science Institute in Baltimore – evaluated several microlensing events detected by the Hubble Space Telescope and analyzed their duration, light spectrum and the apparent Background star shift analyzed. One event caught the eye of both teams because it had all the hallmarks of a dark, free-floating stellar black hole: the light from the background star, some 19,000 light-years away, was amplified and distorted for more than 270 days. The foreground object responsible for this – the gravitational lens – is 2280 to 6260 light-years away, according to estimates by Lam and her team, Sahu and his colleagues determined a slightly more precise value of 5153 light-years.
To determine whether the foreground object was massive enough to form a black hole, the astronomers had to perform elaborate astrometric measurements that would tell them how much the foreground object was shifting the apparent position of the background star. Thanks to the keen “eyes” of the Hubble telescope and several years of repeated observations, they determined a displacement of around one milliarcsecond for the object with the cumbersome double name MOA-2011-BLG-191 and OGLE-2011-BLG-0462. From this, Sahu and his team conclude that the object must have a mass of around 7.1 solar masses – and therefore a black hole.
(Video: NASA/ Goddard)
First detection of a “dark” compact object
“We report the first unequivocal discovery and mass determination of an isolated stellar black hole,” the astronomers write. This is supported by the large mass and the fact that the object does not emit any light of its own. Lam and her team are a bit more cautious: their measurements show a mass of 1.1 to 4.4 solar masses and therefore they cannot rule out with certainty that it is not a neutron star after all. “This is the first free-floating black hole or neutron star to be detected using gravitational lensing,” says Lam’s colleague Jessica Lu. The two teams also do not quite agree on the speed at which the “dark” object is moving. Sahu and his colleagues determined a relatively high speed of around 45 kilometers per second, while Lam and her team came up with a more leisurely 30 kilometers per second. Where these deviations come from must now be further investigated.
However, there is agreement that this is an important discovery – and possibly just the tip of a “dark” iceberg. “Whatever it is, with this object we have discovered the first dark stellar relic to wander unaccompanied through the galaxy,” says Lam. The find thus confirms the models according to which there must be millions of black holes wandering around in isolation in the Milky Way. According to calculations by Lam and her team, there could be around 200 million stellar black holes in the Milky Way – a large proportion of which would be isolated, invisible wanderers. The closest representative of these dark loners could be hiding as little as 80 light-years away. “With microlensing, we’ve opened a new window to these dark objects that are otherwise undetectable,” says Lu.
Source: Kailash Sahu (Space Telescope Science Institute, Baltimore) et al., The Astrophysical Journal, accepted, arXiv:2201.13296; Casey Lam (University of California, Berkeley) et al., The Astrophysical Journal Letters, accepted, arXiv:2202.01903)