Thanks to the observation of the jet of the supermassive black hole M87, the fact of its rotation has been established

Thanks to the observation of the jet of the supermassive black hole M87, the fact of its rotation has been established

An international team of researchers, including scientists from FIAN and MIPT, conducted an analysis of radio interferometric observations for 2022 of the M87 galaxy and established that the jet – a plasma jet that erupts from the black hole in its center – periodically changes its direction. According to scientists, this proves that the central supermassive black hole is rotating. The work was published in the journal Nature.

The giant galaxy M87 in the constellation Virgo, located 55 million light years from Earth, attracts astrophysicists with its relative proximity and supermassive black hole at its center, which is 6.5 billion times more massive than the Sun. Matter, falling on a black hole, makes the core of the galaxy an active source of radiation in the entire electromagnetic range. This process is accompanied by the release of a large amount of energy. Part of the surrounding infalling matter is ejected from the black hole and generates a jet in which the matter moves almost at the speed of light. The jet extends far beyond the galaxy, 4,900 light-years away. Due to its high brightness and proximity, the relativistic jet was first discovered in M87 back in 1918. More than a hundred years later, in 2019, the Event Horizon telescope detected a central radio source and an asymmetric ring structure consistent with the expectations of general relativity in the presence of a supermassive black hole. However, its rotation, which is of crucial importance in the formation and evolution of jets and galaxies, has not been directly observed.

Scientists noted the change in the positional angle of the jet direction since the first observations in the radio range with high angular resolution in 2000. However, there was no clarity about the origin of such structural changes. Manifestations of black hole activity and throwing matter into the jet or the development of plasma instabilities can affect this.

Figure 1. Top panel: the structure of the jet in M87 at 43 GHz from the two-year average of the observational data. White arrows indicate the corresponding direction of the jet. Bottom panel: change in jet direction over the entire observation period from 2000 to 2022. The red line represents the best fit of the precessing jet model with a period of 11 years. Source: Nature

To accurately trace the long-term morphological evolution of the jet near the supermassive black hole in M87, scientists analyzed 170 interferometric images obtained in 2000-2022 at frequencies of 22-24 and 43 GHz. It was these images that showed that, in addition to the well-known constant morphology of the jet confined to the edges, over the years it is possible to see a change in the positional angle of the jet direction. A large series of observations clearly shows systematic quasi-sinusoidal oscillations of the Jet stream on an annual scale with an amplitude range of about 10° (Figure 1).

To describe the evolution of the observed jet direction, the authors of the paper used a model in which the axis of rotation of the accretion disk is slightly tilted to the axis of rotation of the black hole (Figure 2). The rotation of the massive black hole affects the surrounding space-time, leading to the precession of the accretion disk, which extends to the jet due to the close connection between it and the accretion disk.

Figure 2. Schematic representation of the inclined accretion disk model. The axis of rotation of the black hole is directed vertically, the direction of the jet is almost perpendicular to the disk. Misalignment between the axis of rotation of the black hole and the axis of rotation of the disk leads to the precession of the disk and the Jet. Source: Yuzhu Cui et al. 2023, Intouchable Lab@Openverse and Zhejiang Lab

This precession is the result of the Lenze-Thirring effect, which is predicted by Einstein’s general theory of relativity and is observed near massive rotating bodies. The Lenze-Thirring effect is very small – about one part in several trillion. To detect it, it is necessary to study a very powerful object, and the active nucleus of the M87 galaxy is best suited for this.

“The detection of the precession of the M87 jet is conclusive evidence that the supermassive black hole is indeed rotating. A similar precession of jets can occur in other active galactic nuclei, but it is difficult to see due to the small size and long period of change. Our joint group of MIPT and FIAN is now actively involved in modeling precessing jets to explain the quasar observations,” commented Yevgenia Kravchenko, a senior researcher at the High Energy Physics Laboratory of MIPT.

Other observations of the jet of the M87 galaxy, conducted in 2009, allowed scientists to reproduce the heterogeneous structure of the jet, which resembles a tweed pattern in the form of a woven braid of spiral fibers. Their simulations demonstrated that the twisting of the central filaments is caused by instabilities developing in the plasma jet. They can develop when the velocity field across the jet is inhomogeneity. For example, it can be two different plasma flows, the interaction of which produces the observed phenomena. However, such a spiral-like structure of fibers can also be caused by physical processes in the immediate vicinity of the black hole. Probably, it is the Lenze-Thirring precession that leads to the development of these instabilities in the jet itself.

“Such observations allow us to study the phenomena of plasma physics and the life of the Cosmos in general. Currently, global positioning networks are actively developing – a satellite navigation system that provides distance measurement and determines the location of objects in the global coordinate system. They are based on monitoring stars that are constantly moving and not in one position. Thus, this system is not very stable, and currently the idea of ​​using distant quasars in its work is being actively promoted – the same active nuclei in which the jet is actually directed at us, which makes them the brightest points for the Earth. Unlike stars, their position is stable. But due to the fact that the jet is not uniform, which we once again proved in the research, there may be a displacement of the object in the sky, which affects the accuracy of the system. In this regard, a detailed study of quasars and jets allows us to perform the necessary correction and restore the absolute position of the objects,” Yevgenia Kravchenko concluded.

Scientists still have to answer many questions, for example, what is the structure of the disk and what is the exact value of the rotation of the supermassive black hole M87. These studies can only be performed using long-term observations with high angular resolution.

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