Wednesday Colloquium


"Studying Magnetic-driven Outflows from Magnetized Neutrino-cooled Accretion Disks"

Fatemeh Hossein-Nouri (Center for Theoretical Physics, Warsaw)

Neutrino-cooled accretion flow around a spinning black hole, produced by a black hole-neutron star (BHNS) or a binary neutron star (BNS) merger is a promising scenario for jet formation and magnetic-driven outflows. Based on GW170817 gravitational wave detection by LIGO and Virgo observatories followed by electromagnetic counterparts, this model can explain the central engine of the short duration gamma ray bursts (GRB) and kilonova radiations. Using the open-source GRMHD HARM_COOL code, we evolve several magnetized accretion disk-black hole models with realistic equation of state in the fixed curved space-time background. The disk and black hole’s initial parameters are chosen in a way to represent different possible post-merger scenarios of the merging compact objects. We identify the effects of disk’s mass and black hole’s spin on the disk’s evolution, paying particular attention to measuring the properties of the ejected outflows. These results are used to estimate the luminosity and light curves of possible radioactively powered transients emitted by such systems.


"Dynamical double black holes in star clusters"

Debatri Chattopadhyay (Cardiff University)

In this talk, I will present an investigation on the relationship between the global properties of star clusters and their double black hole (DBH) populations. For this study, the code NBODY6 is used to evolve a suite of star cluster models with an initial mass of ~O(1e4)Msun and varying initial parameters. This work concluded that cluster metallicity plays the most significant role in determining the lifespan of a cluster, while the initial half-mass radius is dominant in setting the rate of BH exchange interactions in the central cluster regions. It is also observed that the mass of interacting BHs, rather than how frequently their interactions with other BHs occur, is more crucial in the thermal expansion and eventual evaporation of the cluster. We formulate a novel approach to easily quantify the degree of BH-BH dynamical activity in each model. 12 in-cluster and three out-of-cluster (after ejection from the cluster) DBH mergers of different types across the eleven models are presented. It is noted that cluster initial density plays the most crucial role in determining the number of DBH mergers, with the potential hint of too-high stellar density preventing in-cluster BH mergers.


"Variability and evolution of Active Galactic Nuclei"

Marzena Śniegowska (CAMK, Warsaw)

The time-dependent phenomena in Active Galactic Nuclei (AGN) are not yet well-understood. Those sources are massive, with a broad range of physical properties and observations across the entire electromagnetic spectrum to explore. I explore different timescales, from hours/days to the chemical evolution of the galaxy over millions of years. Short-term variations are observed in the form of Quasi-Periodic Eruptions (QPE) and changing look (CL) active galactic nuclei - CL AGN. I discuss the model of radiation pressure instability taking into account the presence of inner advection-dominated accretion flow (ADAF) or the presence of the magnetic field. The long-term evolution shows through the correlation between the observed accretion rate and the chemical composition. I address this issue by determining the metallicity of high accretion rate sources.


"Neutron stars: thermodynamics and hydrodynamics"

Giovanni Camelio (CAMK, Warsaw)

In this seminar I will present the research I carried on in CAMK in the last two years. First, I will talk about the modeling of the binary neutron star merger remnant. If the remnant is not massive enough to immediately collapse to a black hole, after approximately 30 ms from the merger it can be described as a stationary neutron star in differential rotation and with a hot ring on the equatorial plane. This configuration is nonbarotropic, namely the thermodynamic variables cannot be put in a one-to-one relationship. We developed a new method to solve the force balance equation for nonbarotropic stars and performed an extensive parameter study of the remnant. Then, I will talk about bulk viscosity in neutron star codes. Bulk viscosity is a dissipative process that occurs in out-of-equilibrium systems, as for example a multi-component fluid with reacting particle species. We developed a new one-dimensional general relativistic hydrodynamic code for comparing different approaches to bulk viscosity in neutron stars. In particular, we consider the `exact' approach of tracking the different particle species of the multi-component fluid, and two Mueller-Israel-Stewart models where bulk viscosity is approximately included in the stress energy tensor with the bulk stress. We found that the Mueller-Israel-Stewart approaches are good approximation but that the multi-component fluid is easier to implement and more accurate.