Polish astronomers discovered a free black hole in the Milky Way. There may be millions of them

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A black hole devoured a star the size of the sun. NASA has registered an extremely rare phenomenon

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A free black hole in the Milky Way

Modeling indicates that the invisible object causing gravitational microlensing has a mass 7 or 4.2 times that of the sun. These are the results of both research teams – the difference is due to a slightly different way of analyzing the data, but the masses indicate that the most likely object here is a black hole.

Black holesy are objects that have such gravity that nothing can get out of them, not even light. The first theoretical considerations about such objects appeared in the 18th century, but the proper development of research on them took place in the 20th century. For many years, they were only hypothetical objects, predicted by theory, but the development of observational techniques allowed to obtain observational evidence for their existence.

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The first object to detect a black hole is the Cygnus X-1 binary system. Currently, we already know several dozen other binary systems with black holes with masses of several solar masses. In recent years, thanks to the detection of gravitational waves, more have been confirmed cases the existence of black holes.

In addition to black holes with stellar masses, the existence of such objects is also known, but with masses of hundreds of millions or even billions of solar masses. Such objects are presumed to reside at the centers of galaxies and quasars. Scientists even managed to obtain images of two supermassive black holes (images of their shadow): in the galaxy M87 and in the center of the Milky Way.

These cases of black holes, however, are objects in binary systems with a star or surrounded by matter falling on them or circling stars and clouds, which makes it possible to detect the existence of an invisible object. The theory of stellar evolution predicts that black holes should form at the end of the evolution of massive stars. There should be a lot of such objects when traveling alone. But how to detect them, since the classical methods are not applicable here?

Astronomers from the OGLE project have the answer and have just shown that it is effective – by discovering the first free black hole. Non-luminous bodies can be detected using the technique of gravitational microlensing. When a massive object is in the path of light rays, it disturbs their course. Gravity acts like a giant lens and amplifies the light of a distant star. When observing from Earth, we will then see the brightening of a distant star, changing in a characteristic way. Microlensing depends on the weight of the lens, not its brightness, so it’s a great way to detect invisible objects, such as lonely black holes.

The greater the mass of the lens object, the longer gravitational microlensing takes. In the case of a lens with a mass of several solar masses (i.e. in the range of black hole masses), the phenomenon may last even several years.

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For massive lenses, you can observe an additional effect, the so-called astrometric microlensing. It is extremely difficult to detect, and consists in the fact that during the course of gravitational microlensing, the position of the light source (a distant star) on the celestial sphere changes very little. Simultaneous observations of microlensing photometrically and astrometrically allow to determine the mass of the lensing object, and thus to determine the type of this object.

It took 10 years to confirm that we are dealing with a black hole

The OGLE team regularly observed the discussed OGLE-2011-BLG-0462 gravitational microlensing for nine years, collecting 15,545 precise measurements of the object’s brightness, which were used to model the photometric effect of the phenomenon.

Once it was known that the time scale of the phenomenon is long (so the lens is massive), in the second half of 2011, a team of scientists led by Prof. Kailasha Sahu from the Space Telescope Science Institute (USA) began observing with the Cosmic Telescope Hubble by taking astrometric measurements. In the following years, further measurements were made and in 2021 they were supplemented with measurements from another team led by PhD student Casey Lam and prof. Jessica Lu from the University of California, Berkeley (USA).

– The modeling results clearly indicate that the OGLE-2011-BLG-0462 phenomenon was caused by a massive object with a mass of several solar masses. Since the light of an ordinary star with such a mass would be easily registered, the lens must be a non-luminous object – a free black hole with a stellar mass – explains Dr. Przemysław Mróz – a member of the OGLE team involved in modeling the phenomenon.

The discovery of the first free black hole in the Milky Way confirms the assumption that there should be a lot of such objects. It is also an opening new how to detect black holes. Scientists hope to find more cases of lonely black holes, including thanks to the Gaia space observatory, which will allow to estimate the population of these objects in our galaxy, as well as to determine the distribution of their masses.

Polish The co-authors of two publications in The Astrophysical Journal (arXiv: 2201.13296) and The Astrophysical Journal Letters (arXiv: 2202.01903) related to the discovery of a free black hole are: Andrzej Udalski, Przemysław Mróz, Szymon Kozłowski, Paweł Pietrukowicz, Radek Poleski, Jan Skowron, Igor Soszyński, Michał K. Szymański, Krzysztof Ulaczyk, Łukasz Wyrzykowski.

Source: PAP – Science in Poland

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