A new way to estimate the mass of supermassive black holes outside our galaxy was proposed by astronomers at Moscow State University.
Black holes are cosmic bodies in which the gravitational attraction is so great that even light cannot escape from them. Their existence follows from the modern theory of gravity - the general theory of relativity, the founder of which was Albert Einstein.
Despite the fact that the presence of black holes in nature has not been rigorously proven, astronomers have reason to believe that such objects do exist. For example, they have repeatedly observed phenomena accompanied by a huge release of energy, which can only be explained by the interaction of massive black holes with the surrounding matter falling on them. In this case, due to friction and heating of the substance, radiation arises, which allows indirectly "see" a black hole.
If there is no accretion - the fall of matter onto a cosmic body due to its attraction force - then it is extremely difficult to learn about the existence of a black hole. Researchers from the Sternberg Astronomical Institute of Moscow State University under the leadership of Elena Seifina became interested in such "dormant" supermassive black holes outside our galaxy and proposed a method for estimating their mass, even if they hardly manifest themselves. The research results are published in the journal Astronomy and Astrophysics.
It all started when astronomers noticed several flares from extragalactic sources. One of them, Swift J1644 + 57, was observed in 2011 by several space observatories (RXTE, Swift and Suzaku) in the X-ray and gamma range.
Initially, the researchers thought they saw a gamma-ray burst, which has already been observed in distant galaxies. However, usually the emission of such flares disappears after a day or two, while in this case, after two days, the burst became even brighter. In total, the outbreak was observed for two years, after which it went out.
Astronomers began to suspect that they were observing a tidal destruction of a star flying a short distance from a supermassive black hole (within three gravitational radii). The star collapses due to the fact that the forces of gravity differ significantly on the side of the star near and far to the black hole. At the same time, its matter does not immediately fall onto the hole, but forms a temporary accretion disk, which begins to glow brightly, as can be seen on Earth.
Previously, Elena Seifina observed similar flares involving known objects, which are considered black holes, both inside our galaxy and outside it, and found out how the slope of the X-ray spectrum (the graph of the dependence of radiation intensity on frequency) changes during an increase in luminosity.
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She identified features in the spectrum that clearly indicated the presence of black holes. Astronomers suggested that if the shapes of the spectra of similar flares are similar, then the processes taking place in them are also similar. Hence, they are also caused by the tearing apart of stars by black holes. This allows the discovery of new objects - candidates for black holes.
Astronomers determine the masses of already observed black holes by assessing the maximum luminosity of accretion disks formed around them from falling matter, assuming that an equilibrium has been achieved in the disk between the pressure of electromagnetic radiation and gravitational forces.
Comparing the tracks (the dependence of the spectrum slope on the accretion rate) of known objects with the tracks obtained for new extragalactic flares allowed the researchers to "weigh" invisible black holes. The new method of "weighing" dormant extragalactic black holes makes it possible to use data on known galactic objects, such as, for example, the well-known object Cygnus X-1 with a black hole in the center, which has long been studied by astronomers. Calculations have shown that in the outbreak Swift J1644 + 57 is indeed associated with a supermassive black hole with a mass of 7 × 106 solar masses.
If earlier observations in the ultraviolet were used to estimate the masses of black holes, then the new method allows us to limit ourselves to the X-ray range.
Astronomers hope that the versatility of the new method will help in assessing the many masses of various extragalactic objects, such as the nuclei of Seyfert galaxies and others, when traditional methods fundamentally do not work.
Based on materials from the press service of Moscow State University
Author: Alexey Ponyatov