Abbas Askar (CAMK, Warsaw)
Black holes with masses ranging from a few hundred to hundreds of thousands of solar masses, known as intermediate-mass black holes (IMBHs), remain elusive, and their formation pathways are still not fully understood. Dense star clusters have long been proposed as promising environments for both the formation and subsequent growth of IMBHs. In this talk, I will discuss the dynamical formation channels of IMBHs based on results from detailed Monte Carlo and direct N-body simulations of star clusters. I will describe how runaway stellar collisions, followed by tidal disruptions, can lead to the early formation and gradual growth of IMBHs, and how these processes depend sensitively on the initial properties of the host cluster. I will also discuss the observational implications of these growth channels, particularly for gravitational wave sources and electromagnetic transients. The long-term evolution of IMBHs within massive clusters will also be addressed, including their potential migration toward galactic centers and their role as seeds for the eventual formation of supermassive black holes.
Swadesh Chand (Inter-University Centre for Astronomy and Astrophysics, India)
Staszku, Podaję. Title: Accretion Geometry of Black Hole X-ray Binaries in the Hard State Abstract: Black hole X-ray binaries (BHXRBs) serve as ideal laboratories for studying the complex interplay between accretion and ejection processes in strong gravitational fields. Most of the low-mass BHXRBs are transient in nature and exhibit sporadic outbursts. During a typical outburst, these systems transition through various spectral states, from hard to soft, each associated with distinct spectral and timing properties. Despite decades of studies, the exact physical picture of these systems in the hard spectral state remains an open question, particularly regarding the geometry of the corona and extent of the inner accretion disk. While some studies argue for a disk that extends close to the innermost stable circular orbit (ISCO) in the hard state, others propose a significantly truncated disk scenario. In this talk, I will present findings from some of our recent broadband spectral and timing studies of BHXRBs in their hard state, which provide insights into the nature and evolution of the inner accretion flow.
Maciek Wielgus (Instituto de Astrofisica de Andalucia, Granada, Spain)
Very long baseline interferometry (VLBI) offers a way to study AGN jets with an exceptionally high angular resolution, zooming into the zone of the jet collimation and acceleration. In particular, a global array of the Event Horizon Telescope (EHT) offers unprecedented resolution with highest VLBI observing frequency of 230 GHz. During the first observing campaign of the EHT in 2017, observations of 16 AGN sources were carried out. Together with the previous measurements at lower radio frequencies, these new observations allow for a statistical study of the properties of AGN cores across the angular scales. We observe a persistent and statistically significant effect of brightness temperature increase with the distance from the central black hole, which indicates evolution of Doppler factors in accelerating jets and a gradual conversion of the magnetic energy into kinetic energy. I will describe these observations and discuss what they teach us about the AGN jets.
Sudhagar Suyamprakasam (CAMK, Warsaw)
Luke Dones (Southwest Research Institute, Boulder, Colorado)
