2022 Fall Schedule

Title: High-precision optical polarimetry as a tool to learn about orbital orientation and nature of optical emission of black hole X-ray binaries

Abstract: Polarimetry provides an avenue for probing the geometry of the emission region and physical mechanisms producing radiation in many astrophysical objects, including stellar-mass X-ray binary systems. I present the results of multiwavelength (BVR) polarimetric studies of a sample of historical black hole X-ray binaries, observed during their outbursts or in the quiescent state. As a highlight I will concentrate on the analysis and interpretation of the data on black hole X-ray binary MAXI J1820+070 obtained with high-precision polarimeter DIPol-UF at the Nordic Optical Telescope, from which we constrain the position angle of the binary orbital axis. Combining this with previous determinations of the relativistic jet orientation, which traces the black hole spin, and the inclination of the orbit, we determine a lower limit of 40 deg on the black hole spin-orbital angular momentum misalignment angle. The misalignment must originate from either the binary evolution or black hole formation stages. If other X-ray binaries have similarly large misalignments, these would bias measurements of black hole masses and spins from X-ray observations. A high misalignment adds complexity to the models of the X-ray and optical quasi-periodic oscillations observed from black hole X-ray binaries in their hard state.

Title: Cosmology and Gravity in the New Era of Multi-Messenger Astronomy: Are we Approaching New Physics?

Abstract: We review the Standard Model of Cosmology and Gravity, that has been established through intense theoretical and observational investigation. Additionally, we provide the motivation of extending it in various ways. Then we present the new window that has opened in science, namely multi-messenger astronomy, in order to study the Universe and constrain theoretical models and scenarios. Finally, we summarize the famous tensions between various datasets and theoretical predictions, that could be a sign that we are approaching New Physics.

Title: Space is the New Black

Abstract: We are entering an age, where increasing numbers of players reach towards space. In addition to the old players – countries and space agencies, the new kids on the block are commercial companies, which build and launch small satellites to harness the economic potential of space. What does this mean in terms of utilisation of space? At least the new satellites will eventually contribute to the more than 8000 tons of debris existing in space already. Once on orbit, those new spacecraft will encounter a dynamic environment, controlled by external drivers and internal processes. This talk reviews the rapid change that space has experienced over the last couple of years, and discuss the consequences and potential scientific benefits of this change.

Title: Episodic Mass Loss in Evolved Massive Stars: Key to Understanding the Explosive Early Universe

Abstract: Episodic mass loss is not understood theoretically, neither accounted for in state-of-the-art models of stellar evolution, which has far-reaching consequences for many areas of astronomy. I will introduce the ERC-funded ASSESS project (2018-2024), which aims to determine whether episodic mass loss is a dominant process in the evolution of the most massive stars, by conducting the first extensive, multi-wavelength survey of evolved massive stars in the nearby Universe. It hinges on the fact that mass-losing stars form dust and are bright in the mid-infrared. We aim to derive physical parameters of ~1000 dusty, evolved massive stars in ~25 nearby galaxies and estimate the amount of ejected mass, which will constrain evolutionary models, and quantify the duration and frequency of episodic mass loss as a function of metallicity. The approach involves applying machine-learning algorithms to select dusty, luminous targets from existing multi-band photometry of nearby galaxies. I will present the results of the project so far, including the machine-learning methodology for target selection, results from the analysis of our spectroscopic observations and the derived mass-loss rates of red supergiants. The emerging trend for the ubiquity of episodic mass loss, if confirmed, will be key to understanding the explosive early Universe and will have profound consequences for low-metallicity stars, reionization, and the chemical evolution of galaxies.

Title: Massive black hole pairs and binaries in the cosmos

Abstract: Massive black holes weighing from a few thousands to tens of billions of solar masses inhabit the centers of most galaxies. During their cosmic evolution the galaxies hosting massive black holes interact and merge: this gives rise to pairs of massive black holes that can be observed as dual active galactic nuclei. As the dynamical evolution of the pairs proceeds, eventually binaries are formed, whose coalescence driven by emission of gravitational waves can be detected with ESA’s planned satellite LISA and with Pulsar Timing Arrays. I will discuss the physical processes through which massive black holes pair and bind and present the analysis of cosmological simulations to investigate the properties of dual active galactic nuclei, merging massive black holes and their host galaxies.

Title: A New Statistical Solution to the Three-Body Problem

Abstract: TBA

Title: Dynamo Generation of Astrophysical Magnetic Fields

Abstract: Astrophysical Magnetic fields generated by dynamo action, such as those found in planets, stars and galaxies, often exhibit a large degree of order, even though they are generated in a turbulent environment. One such example is the solar cycle, where a coherent spatio-temporal magnetic field arises to give the famous eleven year solar cycle. Astrophysical dynamo theory is a technically challenging problem involving the interaction of turbulence, rotation and magnetic fields. Here I shall give some observational evidence for dynamos and also give some pointers to contentious issues within the theory. I shall finish by examining a new method for solving for astrophysical fluids (and dynamos) termed Direct Statistical Simulation.

Title: Ultraluminous X-ray sources - our window onto super-Eddington accretion

Abstract: Super-Eddington accretion is a key ingredient needed to understand the observed and evolving universe. One of the most accessible sets of objects where super-Eddington accretion is known to occur are the Ultraluminous X-ray sources (ULXs). In the last 10 years, our understanding has undergone a revolution following the confirmation that many ULXs likely harbour neutron stars, where strong magnetic fields and surface effects complicate the picture. In this talk I will present some of the most critical phenomenology, open questions and efforts to understand these important systems.