The existence of a free black hole in the Milky Way was recognized thanks to many years of observations made as part of the OGLE project, implemented by the University of Warsaw. This project uses what is called a microgravitational lens, which is an effect of general relativity. The phenomenon of microlensing occurs when light coming from a distant source is bent by a closer object called a lens.

Scientists from the OGLE Project observed the phenomenon of microgravity on June 2, 2011, designated as OGLE-2011-BLG-0462. At the same time, it was also registered in the New Zealand-Japan Ministry of Agriculture project.

At the time of the discovery of the brightness due to the fine lens, we did not think that this object would prove to be so interesting and pioneering in astrophysics. A has become another gem among the 22,000 microlensing phenomena we discovered

– The professor indicated. Andrzej Udalski from the Astronomical Observatory from the University of WarsawQuoted in a press release.

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Free black hole in the Milky Way

Modeling suggests that the invisible object causing the micro-lensing of gravity has a mass of 7 or 4.2 times the mass 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 blocks suggest that the most likely object here is a black hole.

black holesy are objects that have such a gravity that nothing can get out of them, not even light. The first theoretical considerations about such objects appeared in the eighteenth century, but the appropriate development of the search for them occurred in the twentieth century. For many years, they were just hypothetical things, predicted by theory, but the development of observational techniques allowed to obtain observational evidence of their existence.

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The first thing that discovers a black hole is the binary system Cygnus X-1. Currently, we already know of dozens of other solar-mass black hole binary systems. In recent years, thanks to the discovery of gravitational waves, more cases Existence of black holes.

Apart from stellar-mass black holes, the existence of such objects is also known, but their masses are hundreds of millions or even billions of solar masses. It is assumed that such objects are located in the centers of galaxies and quasars. Scientists were even able to obtain images of two supermassive black holes (images of their shadows): in galaxy M87 is in the center of the Milky Way.

However, these states of black holes are objects in binary systems with a star or surrounded by matter falling on them or orbiting stars and clouds, allowing the detection 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 these things when traveling alone. But how do we discover it since the classical methods are not applicable here?

Astronomy scientists From the OGLE project you have the answer and it just proved effective – by discovering the first free black hole. Non-luminous objects can be detected using micro-gravity technology. When a huge object is in the path of the rays of light, it disturbs its path. Gravity acts like a giant lens and magnifies the light of a distant star. When observing from Earth, we will then see the brightness of a distant star, changing in a characteristic way. Microlensing is based on the weight of the lens, not its brightness, so it’s a great way to detect invisible objects, such as lone black holes.

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The greater the mass of the lens body, the longer the microgravity. In the case of a multi-solar-mass lens (that is, in the range of black hole masses), this phenomenon may persist for several years.

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For massive lenses, you can notice an additional effect called the micro-astronomical lens. It is very difficult to detect, and it consists in the fact that during the course of microgravity, the position of the light source (distant star) on the celestial sphere changes very little. Simultaneous observations of a micro-lens with photometry and astrometry allow the determination of the mass of the lens body, and thus the determination of the type of that object.

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

The OGLE team regularly monitored the discussed OGLE-2011-BLG-0462 micro-gravitational lens for nine years, and collected 15,545 minute measurements of object brightness, which were used to model the photovoltaic effect of the phenomenon.

Once it was known that the time scale of the phenomenon is long (and therefore the lens is huge), in the second half of 2011, a team of scientists led by Prof. Kailasha Sahu of the Space Telescope Science Institute (USA) began observing with Cosmic telescope Hubble by taking astronomical measurements. In the following years, more measurements were made and in 2021 they were supplemented by measurements from another team led by doctoral student Casey Lamm and the professor. Jessica Low from University of California, Berkeley (USA).

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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 of this mass can be easily recorded, the lens must be a non-luminous object – a free black hole with stellar mass – explains Dr. Przemyslav Mroz – a member of the OGLE team involved in the apparent modeling.

The discovery of the first free black hole in the Milky Way confirms assumptions that there must be many such objects. It’s also open the new How to discover black holes. Scientists hope to find more cases of isolated black holes, including thanks to the Gaia space observatory, which will allow to estimate the number of such objects in our galaxy, as well as determine the distribution of their masses.

polishing The co-authors of two books in The Astrophysical Journal (arXiv: 2201.13296) and The Astrophysical Journal Letters (arXiv: 2202.01903) associated with the discovery of a free black hole are: Andrzej Udalski, Przemysław Mróz, Szymon Kozłowski, Pawe, Pietrukowic Polski, Igor Soskornski, Michael K.

source: PAP – Science in Poland