Astronomers discover a wandering black hole

Black holes are invisible because they absorb all the radiation. Accordingly, it is difficult to track down the inactive representatives of these gravitational giants floating solitary in space. But that’s exactly what two research teams might have done. They spotted a stellar black hole about 5,100 light years away, wandering alone. The invisible object was revealed by a gravitational lensing effect: the gravity of the black hole passing in the foreground enhanced and distorted the light of the background star. Astronomers were also able to estimate the approximate mass of a 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 the Milky Way itself. Some of them may even be racing through our galaxy at relatively high velocities because they have received an asymmetric supernova “nudge”. However, because black holes absorb 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 clearly detect an isolated, inactive stellar black hole.

Gravitational lensing reveals an invisible object

Now, however, for the first time, two teams of astronomers have succeeded. Independently of each other, they assessed the same data from the Hubble Space Telescope. The focus was on what are known as microlensing events – distortion and amplification of a distant star’s light due to the gravity of an object in the foreground. In principle, any massive object, from an exoplanet to an entire galaxy, can produce this gravitational lensing effect. However, a stellar black hole reveals itself, inducing a localized but more than 200-day lensing effect. Since the hole itself does not emit any light, the color of the background star’s light does not change; however, if a massive star were used as the lens, the light from both would mix.

In their research, both teams – one led by Casey Lam of the University of California at Berkeley and the other led by Kailash Sahu of the Space Telescope Science Institute in Baltimore – assessed several microlensing events detected by the Hubble Space Telescope and analyzed their duration. the light spectrum and the apparent shift of the background stars were analyzed. One event caught the attention of both teams because it had all the characteristics of a dark, free-floating black stellar 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 2,280 to 6,260 light-years away, according to estimates by Lam and her team, Sahu and his colleagues, determined a slightly more precise value of 5,153 light-years.

To determine if a foreground object was massive enough to form a black hole, astronomers had to make complex astrometric measurements that would tell them how much the foreground object shifted the apparent position of the background star. Thanks to the sharp “eyes” of the Hubble telescope and several years of repeated observations, they determined a displacement of about one millisecond for an object with the unwieldy double name MOA-2011-BLG-191 and OGLE-2011-BLG-0462. Based on this, Sahu and his team concluded that the object must have a mass of about 7.1 solar masses – and therefore a black hole.


(Video: NASA / Goddard)

First detection of a “dark” compact object

“We are announcing the first unequivocal discovery and determination of the mass of an isolated stellar black hole,” astronomers write. This is due to the large mass and the fact that the object does not emit its own light. Lam and her team are a bit more cautious: their measurements show a mass of 1.1 to 4.4 solar masses, and therefore cannot with certainty rule out that it is not a neutron star after all. “This is the first free-floating black hole or neutron star detected by gravitational lensing,” says Jessica Lu, Lama’s colleague. Both teams also do not fully agree on the speed at which the “dark” object is moving. Sahu and his colleagues set a relatively high speed of around 45 kilometers per second, while Lam and her team came up with a calmer speed of 30 kilometers per second. Where these deviations come from, now needs to be further investigated.

There is consensus, however, that this is an important discovery – and possibly just the tip of the “dark” iceberg. “Whatever it is, it was thanks to this object that we discovered the first dark stellar relic to travel without companionship through the galaxy,” says Lam. The find thus confirms the models according to which millions of black holes must circulate in the Milky Way. According to the calculations of Lam and her team, there could be about 200 million stellar black holes in the Milky Way – many of which would be isolated, invisible wanderers. The closest representative of these dark loners may be hiding just 80 light-years away. “Thanks to microlensing, we 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, approved, arXiv: 2201.13296; Casey Lam (University of California, Berkeley) et al., The Astrophysical Journal Letters, approved, arXiv: 2202.01903)

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