Dynamics of processes around compact stars

The goal of my project is to study the cosmic surroundings of compact stars: black holes and neutron stars, which are sources of extreme gravitational potential.

These stars interact with their cosmic environment by attracting nearby matter. If such matter has some non-zero angular momentum, then at a certain distance from the centre (greater than the size of the event horizon of a black hole or the radius of a neutron star), it can move in a circular orbit, and in order to fall below the horizon, it should lose this angular momentum. This is driven by mechanisms related to friction or turbulence, and in the case of accretion disks (consisting of hundreds of rings of matter located in such subsequent circular orbits), the presence of a magnetic field is an important factor. A neutron star, on the other hand, which has a hard surface, helps the matter lose angular momentum in the so-called boundary layer. As a result of this process, huge amounts of energy are released, which we observe in the form of radiation. If the magnetic field is very strong, it can also push material away from the horizon. Then we are dealing with the effect of the so-called magnetically arrested disk (MAD).

The study of the accretion process in MAD mode requires the solution of complex differential equations describing the magnetohydrodynamics of plasma in a strong gravitational field, which forces us to take into account the effects of General Relativity (GR). In our research, we use computer simulations for this purpose, because these equations cannot be solved analytically on a piece of paper. However, there are now accurate and reliable numerical methods to study the phenomena occurring in strongly magnetized plasma in the vicinity of the black hole horizon. The computer programs used and developed by our team allow us to simulate the phenomena occurring in the cosmic plasma, to determine what the image of the magnetosphere surrounding the accretion disk and its model radiation spectrum will look like. We also examine how the emission of photons in different wavelength ranges will change over time.

prof. Agnieszka Janiuk, photo Łukasz Bera From observations available by radio interferometry (Event Horizon Telescope – EHT), it was possible to obtain an image of a ring of light surrounding the black hole in the M87 galaxy, as well as in our Galaxy, where the centre is known as Sgr A*. This ring glows due to the emission of synchrotron radiation, and the shape of this emission agrees well with what a MAD-structured disk can emit. In one of our most recent papers, we show an accurate fit of the spectrum of radiation emitted by such a disk to observational data obtained by X-ray and optical telescopes for the galaxy M87. In this work, we confront for the first time the temperature range in the inner region of the disk derived from dynamical simulations with what is required by observations of the supermassive black hole object imaged by the EHT telescope.

In another series of papers, two of which have already been published in the prestigious journal Nature, we, together with an international team of researchers, analyse the phenomena known as quasi-periodic eruptions (QPE). These short-lived radiation bursts, lasting from several dozen hours to a few days, occur in the vicinity of massive black holes in distant galaxies. The exceptional time scale of the phenomenon and its spectacular nature has led researchers to put forward various hypotheses about the causes of such eruptions. One possibility is instability in the accretion process, in which the accreted matter in a short period of time comes from the burst of a star in the vicinity of the black hole horizon. Another possibility is the periodic variability of the jet ejected during this process, caused by the precession phenomenon. Our theoretical calculations were used to verify the hypotheses.