Observation of the invisible

Black holes are extremely massive cosmic objects that are formed by the collapse of stellar material into a small and extremely dense body. As a result, they exert such a strong gravitational field on their surroundings that they can bend light significantly and even trap particles of light - photons - in their close proximity. For this reason, direct observation of black holes is quite impossible, and astrophysicists have to look at the behaviour of the material around them to determine the properties of a given black hole. Black holes interact with this material by emitting very specific X-rays, and physicists have now figured out how to estimate the mass of a black hole from its properties.

Unknown plot, known results

Observations of radiation from material near black holes have revealed a very interesting phenomenon: the precise periodicity of X-ray "bursts". The periodicity is literally cosmically fast - on the order of tens to hundreds of hertz. For the sake of understanding: imagine that this radiation is emitted by a rotating lighthouse. In that case, it would have to rotate hundreds of times in one second. Astrophysicists have compared observations of this phenomenon in several localised black holes in our Galaxy and found that the process that causes the X-ray "dose" to change takes place very close to the black hole, in regions with very strong gravitational fields. By measuring these doses, the properties of the holes themselves can then be determined. However, the actual cause of the periodic X-ray bursts near black holes remains a mystery.

Measuring the most material

Based on the observation of this interesting phenomenon, a team of scientists led by Prof. Marek Abramović, a researcher at the Institute of Physics in Opava, tried to measure the values of periodic changes in X-rays around significantly more massive black holes - those located in the centres of large galaxies. For these black holes, the frequency was much lower - on the order of minutes to hours. From observations of this phenomenon in different black holes, scientists have devised a way to determine their masses.

A supermassive black hole with the mass of billions of suns

Astrophysicists have used this method to estimate the masses of several black holes, typically ranging from hundreds of thousands to millions of solar masses. The analysis of one black hole with an almost unforgettably long name was carried out by a student of the Institute of Physics in Opava, Mgr. Katerina Klimovicova who found out a key finding. "We have measured the frequency of X-ray emission changes in an active galactic nucleus with the unpleasant name XMMU J134736.6+173403 in the constellation Shepherd and we were really surprised. The mass estimate, whose inaccuracy is due to the unknown distance of the observed X-ray emission from the black hole, is somewhere between 10 million and 1 billion solar masses!"

Supported by the Grant Agency of the Czech Republic

Other interesting objects in the crosshairs of the Opava physicists are so-called microquasars, i.e. black holes in the centres of small galaxies. It is known that these types of exotic objects leak irregular doses of high-energy radiation during hitherto incomprehensible phenomena. "In recent years, we have already contributed to solving this important open astronomical problem. We have calculated and verified that many phenomena occurring in the vicinity of these black holes, in so-called accretion disks, are behind it. That is, rings of hot matter rotating around black holes. After all, the fact that we observe a black hole is only made possible by their interaction with the matter around them. The accretion disks themselves glow, reflect light and behave according to rules that are not yet fully understood. The speed of rotation of these disks is close to the speed of light in some places, and we necessarily consider very strong so-called relativistic phenomena similar to the weaker ones that have been known for more than 100 years, such as the rotation of the perihelion of the planet Mercury. Much remains unexplained - in particular the periodic changes in the radiation of these disks. This is what we want to focus on next ," says Assoc. Gabriel Török, a project leader.

Last year, the results of the project were recognized as excellent by the Grant Agency of the Czech Republic (GA CR). In cooperation with scientists from the Astronomical Institute of the Academy of Sciences, the physicists from Opava are, within the framework of the newly approved project of the EXPRO competition for excellence in basic research, this year they are looking for further answers to open questions concerning objects that are known to most of us only from the field of science fiction. It is possible, however, that the latest interesting facts about black holes and similar objects in the upcoming documentary series or popularization articles will once again be due to the unceasing inventiveness of scientists from the Institute of Physics in Opava.

Contact deatails and other information:

Mgr. Katerina Klimovicova

Prof. Marek Abramowicz, Ph.D. (english, polish)

Mgr. Petr Horalek

Mgr. Debora Lancova

doc. RNDr. Gabriel Török, Ph.D.