Information about how photons move in the disk surrounding a black hole and how their interaction with the matter in these disks can affect the radiation we observe from Earth will reveal many more interesting things about the black hole itself. And such a search is the focus of the dissertation of Dmitriy Ovchinnikov, a student at the Institute of Physics in Opava.

Dmitriy Ovchinnikov, originally from Uzbekistan, has been researching radiation around black holes since 2019. "The aim of my PhD is to investigate how interactions between radiation and matter in accretion disks around black holes affect key properties of radiation observed from Earth. In retrospect, this helps us to better understand the properties of black holes themselves and their immediate surroundings. Together with other scientists, we are trying to understand the structures of accretion disks, and therefore the processes in the hot matter surrounding extreme objects such as black holes. To carry out this study, I am investigating how photons move around black holes and how radiation is transmitted in the accretion disks themselves. I analyse the different mechanisms using models of so-called multiple toroidal structures around different types of black holes," the student describes.

What are accretion disks?

Accretion disks are formations made of rapidly rotating gas that gradually spirals down and descends on a central body. Accretion disks are most interesting to us around black holes, but they are not the only types of bodies that surround disks. For astrophysicists, in any case, these disks are a virtually bottomless well of information, as they are governed by a combination of fascinating physical processes, including gravitational and electromagnetic interactions, fluid dynamics, and radiation transfer; studying and modeling these phenomena helps us better understand their structure. "The motion energy of matter rotating in the disk is converted into other forms, such as thermal energy, by various processes, which causes some of the matter in the inner part of the disk to fall into the black hole. Inside accretion disks, the matter is mostly made up of charged particles, which is why magnetic fields are generated. The friction in the inner part of the disk heats the matter considerably, producing radiation that can be registered by a distant observer. It is this radiation that I am interested in," the student explains, adding that it is the properties of the black hole that surrounds the disk that are behind the changes in intensity and other properties of the observed radiation. Studying it thus reveals not only the properties of the disk, but also of the black hole itself. It's actually a bit of detective work.

Scientifically very useful research

This research is of great scientific benefit, as understanding the physical processes in strong gravitational fields is one of the best ways to test Einstein's theory of gravity and any alternative theories of gravity. Moreover, the study of accretion disks has extensive applications in other areas of astrophysics. Where do astrophysicists encounter accretion disks most often?

"Accretion disks are not just surrounded by black holes. They form, for example, during star formation and around stars in tight binary systems. They are also found around neutron stars. But around black holes, they play a key role in observing the most energetic phenomena in the Universe. Accretion disk theory is used to study different sources of radiation in our Galaxy, over a wide range of wavelengths, as they produce different types of radiation. This includes, of course, the centre of our Galaxy, where the massive central black hole is located. Disks also help astronomers study the bright central regions of other galactic centres, where phenomena such as relativistic matter bursts or gamma-ray bursts originate. Investigating processes in accretion disks allows us to explain characteristic physical features, for example in spectra or light curves, and can tell us much more about the bodies they surround," Ovchinnikov gives examples.

Focus of our student on different projects

However, the young scientist is not only studying accretion disks in his work. "During my doctoral studies, I am also involved in other research projects that are partly related to the subject of my dissertation. These projects focus on investigating various optical phenomena occurring in the strong gravitational fields of various alternative black hole models, for example around so-called regular black holes that do not have a central singularity. Using specific methods, we and other astrophysicists are attempting to determine the characteristics of such black holes, including their mass, rotation and inclination. We also simulate potential observational results for such objects and compare them with well-known models proposed by famous physicists before us, such as Karl Schwarzschild and Roy Kerr," the student explains.

The research and mathematical solutions to the gravitational fields of black holes themselves are still the subject of theoretical research, based, for example, on a combination of Einstein's general theory of relativity with complex formulas for electrodynamics. Combining them gives scientists a better basis for investigating the motion of charged particles and photons around black holes, and ultimately such an extension helps to better understand the behaviour of matter and light in space-time.

Author´ contact details:

Dmitriy Ovchinnikov

Other contact details:

RNDr. Tomáš Gráf, Ph.D.

Mgr. Petr Horálek

prof. RNDr. Zdeněk Stuchlík, CSc.

doc. RNDr. Gabriel Török, Ph.D.

Scientifics papers: