Recent observations of nearby low-luminosity active galactic nuclei (AGNs), in cases where the Bondi radius is resolved, have found accretion rates that are several orders of magnitude lower than those predicted by the classical Bondi model, as observed in M87, Sgr A*, and NGC 3115. Although supermassive black holes at galaxy centers are believed to evolve together with their host galaxies, how they interact across large scales remains a key challenge in understanding galaxy evolution.
Motivated by these observations, I present my recent research on low-angular-momentum accretion flows onto supermassive black holes within both the Newtonian and general-relativistic frameworks, using semi-analytical models and general-relativistic hydrodynamic (GRHD) simulations.
The study covers a wide range of spatial scales, extending from the event horizon to the Bondi radius (approximately (10^{5.7}) Schwarzschild radii).
Within the semi-analytical framework, I investigate the effects of outflows, outer boundary conditions, and the gravitational potential of the host galaxy, including both the stellar component and the dark matter halo, on the structure and dynamics of the accretion flow.
I also present GRHD simulations of quasi-spherical accretion onto a black hole embedded in a dark matter halo. These simulations explore how modifications to the spacetime geometry induced by the surrounding dark matter influence the properties and dynamics of the accretion flow. Finally, I will present a summary of my current research and discuss future directions. 

The speaker will give his talk in person.

Everybody is welcome to join us either in room 203 onsite, or from zoom:

Neutron stars are born in the aftermath of supernova stellar explosions. They contain the mass of one or two suns compressed within only ten kilometres, resulting in gravitational fields so strong that general relativistic effects become significant in their vicinity, affecting spacetime and the propagation of light near their surface. Due to their extreme compactness, the densities inside neutron stars reach values unattainable on Earth. Their matter may exist in superfluid and superconducting states, and free quarks may even appear in their cores.

When neutron stars orbit a companion star, they can accrete matter from the outer layers of the companion. This newly acquired matter may burn unstably on the neutron star surface, producing bright X-ray flashes known as Type I bursts.

In this talk, I will show how modelling the burning processes during Type I bursts provides insight into the physics of neutron stars, including their internal composition and magnetic fields, as well as into nuclear reactions and the binary systems in which the bursts occur, such as accretion processes.

The speaker will deliver the talk in person.

Everybody is welcome to join either onsite in room 203 or via Zoom.

https://us06web.zoom.us/j/81514948789?pwd=k4HQdJZ8bJzi4cFwlJUDIGytri1JIW.1

Meeting ID: 815 1494 8789
Passcode: 799936

The LIGO/Virgo/KAGRA (LVK) collaboration has discovered several dozen binary mergers since 2015; however, binary mergers are not the exclusive sources of gravitational waves.

Asymmetric rotating neutron stars and planetary or asteroid mass-primordial BH (PBH) binaries during their in-spiral phase also emit quasi-monochromatic, long-duration gravitational waves.

Detecting signals from these sources requires longer observation times due to their low amplitudes relative to detector sensitivities, and thus far, their detection has not been confirmed.

In the line of sight, a massive object is present; the signal can be gravitationally lensed, temporarily enhancing the source’s signal. This allows for the detection and investigation of the physical nature of both the lensing object and the source.

This presentation provides a brief overview of the gravitational wave lensing effect and explores the impact of lensing signatures on long-duration signals.

Accretion disks play a central role in shaping the dynamics around black holes. The magnetically arrested disk (MAD) state, in which magnetic flux near the event horizon saturates, has gained prominence following Event Horizon Telescope observations of M87* and Sagittarius A*, suggesting that many supermassive black holes may operate in this regime. Low-luminosity systems such as Sgr A*, however, are strongly affected by radiative cooling, which can modify the thermal, magnetic, and dynamical structure of the disk.In this talk, I examine how radiative cooling influences MADs at sub-Eddington accretion rates. Analytically, we identify a critical accretion rate above which synchrotron emission becomes the dominant cooling mechanism, shifting the thermal equilibrium and altering the MAD parameter. Using GRMHD simulations with our GPU-accelerated code cuHARM, I show how these cooling effects modify magnetic saturation, flux eruptions, force balance, and jet efficiency across a range of black hole spins and accretion rates. I also discuss how the traditional measure of disk height can be misleading in MAD systems, motivating the need for a revised definition. These results clarify how cooling regulates MAD dynamics in low-luminosity black holes and may help interpret future EHT observations.

Location: room 203, also available via Zoom

Zoom details:
https://us06web.zoom.us/j/81514948789?pwd=k4HQdJZ8bJzi4cFwlJUDIGytri1JIW.1
Meeting ID: 815 1494 8789
Passcode: 799936

Rapidly rotating, strongly magnetized neutron stars („millisecond proto-magnetars”) formed in stellar core-collapse, neutron star mergers, and white dwarf accretion-induced collapse have long been proposed as central engines of gamma-ray bursts (GRB) and accompanying supernovae/kilonovae. However, during the first few seconds after birth, neutrino heating drives baryon-rich winds from the neutron star surface, potentially limiting the magnetization and achievable Lorentz factors of the outflow and casting doubt on whether proto-magnetars can launch ultra-relativistic jets at early times, as needed to power short-duration GRB.  I will present three-dimensional general-relativistic magnetohydrodynamic simulations of neutrino-heated proto-magnetar winds that incorporates neutrino transport. While the global wind properties broadly agree with previous analytic estimates calibrated to one-dimensional models, our simulations reveal essential multidimensional effects. For rapidly rotating models with spin periods P = 1 ms, centrifugal forces strongly enhance mass loss near the rotational equator, producing a dense, sub-relativistic outflow (v ~ 0.1c). This equatorial wind naturally confines and collimates less baryon-loaded outflows emerging from higher latitudes, leading to the formation of a structured bipolar jet with a peak magnetization along the pole up to sigma ~30-100, sufficient to reach bulk Lorentz factors ~100 on larger scales. The resulting angular stratification of the outflow energy into ultra-relativistic polar and sub-relativistic equatorial components is also broadly consistent with the observed partition between beaming-corrected GRB energies and supernova/kilonova ejecta. Our results demonstrate that millisecond proto-magnetars can launch relativistic jets within seconds of formation and highlight their potential role in powering the diverse electromagnetic counterparts of compact-object explosions.

Zoom: 

https://us06web.zoom.us/j/81514948789pwd=k4HQdJZ8bJzi4cFwlJUDIGytri1JIW.1

Meeting ID: 815 1494 8789
Passcode: 799936

Location:

room 203, Al. Lotników / Zoom

The horizon-scale images of supermassive black holes (BHs) by the Event Horizon Telescope Collaboration have provided new opportunities to test general relativity and other theories of gravity. In view of future projects, such as the next-generation Event Horizon Telescope and the Black Hole Explorer, which have the potential to enhance our ability to probe extreme gravity, it is natural to ask how much two BH images can differ. To address this question and assess the ability of these projects to test theories of gravity with BH shadows, we use general-relativistic magnetohydrodynamic and radiative-transfer simulations to investigate the images of a wide class of accreting BHs deviating from the Kerr solution. By measuring the mismatch between images of different BHs, we show that future missions will be able to distinguish a large class of BH solutions from the Kerr solution when the mismatch in the images exceeds values between 2% and 5% depending on the image-comparison metric considered. These results indicate that future horizon-scale imaging with percent-level image fidelity can place meaningful observational constraints on deviations from the Kerr metric and thereby test strong-field predictions of general relativity.

Zoom:
https://us06web.zoom.us/j/81514948789pwd=k4HQdJZ8bJzi4cFwlJUDIGytri1JIW.1

Meeting ID: 815 1494 8789
Passcode: 799936

Location: 
room 203, Al. Lotników / Zoom