Affiliation: University of Sofia, Bulgaria

Title: Observational signatures of exotic compact objects

Abstract: Recent advancements in observational techniques for supermassive black hole candidates have enabled resolving near horizon-scale features. This unlocks the possibility of directly probing gravitational physics in the strong field regime. I will present my PhD work, which steps on these new results and explores the possibility of directly observing non-Kerr geometries in future observations of supermassive black hole candidates like M87* and Sgr A*. I will frame the talk in the context of the current landscape of the field, argue the relevancy of my work, and show how more exotic spacetimes get encoded in the observational signatures that the current experiments can produce.

Affiliation: University of Lavras, Barzil

Title: Analytical study of field perturbations in black-hole configurations

Abstract: We analyze the sound perturbation of Unruh’s acoustic effective geometry in both (2+1) and (3+1) spacetime dimensions and present an exact analytical expression for the quasibound states of these idealized black-hole configurations by using a new approach recently developed, which uses the polynomial conditions of the hypergeometric functions. Our main goal is to discuss the effects of having an event horizon in such effective metrics. We also discuss the stability of the systems and present the radial eigenfunctions related to these quasibound state frequencies. These metrics assume just the form it has for a Schwarzschild black hole near the event horizon, and therefore may, in principle, shed some light into the underlying classical and quantum physics of astrophysical black holes through analog acoustic probes.

Affiliation: ICE, Bellaterra, Spain

Title: Relativistic turbulence: a covariant approach to large-eddy simulations

Abstract: Binary neutron star mergers offer a unique possibility to explore many extremes of physics. In particular, the large amount of numerical work carried out over the last decade has allowed us to obtain a robust but broad-brush picture of the merger dynamics. Current simulations are in fact far from resolving the full range of scales involved, particularly because of the development of turbulence in the merger remnant. This has motivated quite some recent effort towards adapting the large-eddy simulation strategy to the relativistic setting relevant for binary mergers. Despite the impressive successes of such efforts, however, all the practical implementations so far are problematic in that they break covariance. In this talk, I will discuss a proposed framework that allows us to overcome said limitations, and present some preliminary results on its practical implementation. Should time permit it, I will also briefly present a recent discussion on local magneto-shear instabilities that explicitly make contact with the large-eddy strategy and is well-suited to highly-dynamical environments such as mergers.

Affiliation: University of Athens, Greece

Title: The implications of orbital resonances in de-circularization of orbits in the Kerr-like (integrable) Johannsen metric

Abstract: The Johansen metric is a Kerr-like non-vacuum metric that is characterized by a number of extra parameters besides the mass and the spin parameter, while it still leads to integrable geodesic orbits with a Carter-like constant of motion. This metric could be suitably adjusted, through its parameters, to satisfy the Kennefick-Ori resonance condition. We have attempted to evolve numerically a set of initial orbital parameter in order to observe the resonance-induced eccentricity.

Affiliation: University of Athens, Greece

Title: The Kennefick-Ori argument about resonance-induced eccentricity and de-circularization of EMRI’s orbits

Abstract: Back in 1996 Dan Kennewick and Amos Ori presented an argument according to which an initially “circular” (spherical) orbit will remain circular, unless the resonance condition $\Omega_r/\Omega_{\theta}=2 n$ is met. For the Kerr metric this condition is never met, for all orbital parameters. Therefore we looked for a newtonian model and a relativistic one (the Johannsen metric with suitable parameters) where this condition could be present in the orbital parameter space of spherical orbits. These paradigms could be used as a testbed to check numerically how the Kennefick-Ori instability operates.

Affiliation: Chinese University of Hong Kong, Hong Kong

Title: Probing neutron star astrophysics through numerical relativity simulations

Abstract: Neutron stars are natural laboratories for studying physics in extreme conditions, including behaviours in supra-nuclear density, high magnetic field, and strong gravity regimes. Detection of gravitational-wave signals from neutron stars provides a novel way of probing information inside the stars. The first signal detected was produced by the binary neutron star inspiral event GW170817. Aside from binary neutron star inspirals, excitation of neutron star oscillation modes in violent events, such as core-collapse supernovae, binary neutron star mergers, and phase-transition-induced collapses, are promising candidates for producing detectable gravitational-wave signals. These signals provide probes for the composition, structure, and dynamics of neutron stars in these scenarios. However, the formation and behaviour of strongly magnetized compact objects are highly complex and relativistic. Numerical relativity simulations are crucial tools to model them accurately. Here, we study the dynamical evolution, oscillations, and gravitational-wave signatures of neutron stars in different scenarios using a new multi-dimensional general-relativistic magnetohydrodynamics code Gmunu. In particular, we present universal relations in fundamental modes of rotating neutron stars with differential rotations, oscillations of highly magnetized non-rotating neutron stars, formation dynamics of a magnetized hybrid star from phase-transition-induced collapse and its corresponding gravitational-wave signatures.

Affiliation: SISSA, Italy

Title: Perturbation of slowly evolving black holes: quasi-normal modes and tidal response

Abstract: The mass of a black hole can dynamically evolve due to various physical processes such as accretion, Hawking radiation, or gravitational wave backreaction. This evolution can have a significant impact on astrophysical observables like the ringdown signal. An effective description of a spherically symmetric evolving black hole is provided by the Vaidya metric. In our investigation, we explore the dynamics of perturbations on this background, assuming a slow evolution. This approach enables us to expand relevant physical quantities around their static values and compute corrections at leading order in the mass derivative. In particular, we quantify how this phenomenon affects the spectrum of the quasi-normal modes and the tidal response of a black hole.

Affiliation: University of Tübingen, Germany

Title: Stability analysis and study of massive scalar perturbations on non-rotating and rotating blackholes in dynamical Chern-Simons gravity

Abstract: N/A

Affiliation: University of Tübingen, Germany

Title: Black holes in general relativity and beyond

Abstract: LIGO-Virgo collaborations detectors observed the first direct detection of gravitational waves, representing the first observation of a black hole merger. In addition, the Event Horizon Telescope released the first direct image of a black hole associated with the supermassive black hole that lies in the center of the Messier 87 galaxy. Therefore, despite black holes being initially considered purely theoretical objects, there is now substantial observational evidence supporting their existence. Black holes do not emit radiation directly, but there are some physical mechanisms that can serve as sources of radiation associated with a black hole. In particular, due to its powerful gravitational field, the accretion disk of a black hole is hot enough to emit X-rays just outside the event horizon. On the other hand, Hawking radiation is a theoretical idea suggesting that quantum effects near the event horizon can generate emission of of thermal black-body radiation. In this talk, I will address these aspects in the context of general relativity and discuss some basic topics of particular modified gravity theories as well as solutions describing black holes.

Affiliation: University of Tübingen, Germany

Title: Modeling bias in black hole spin measurements using X-ray reflection spectroscopy

Abstract: Blurred reflection features are commonly observed in the X-ray spectra of accreting black holes. These features arise from the illumination of the accretion disk by comptonized photons from the corona. The study of these features is known as X-ray reflection spectroscopy. With high-quality data and accurate astrophysical models, this technique can be a powerful tool for studying the morphology of accreting matter around the black hole, measuring its spin, and possibly testing General Relativity in the strong field limit. Over the past decade, there has been significant advancement in developing relativistic reflection models. However, these models are based on certain simplifications that diverge from actual astrophysical conditions, leading to potential systematic biases in estimating the parameters of accreting black holes. In this talk, I will focus on these systematic biases, particularly those affecting the measurement of black hole spin, due to the accretion disk's structure and the phenomenon of returning radiation.

Affiliation: RIKEN, Japan

Title: Elastic oscillations in neutron stars and crust properties

Abstract: We carefully examine the shear and interface modes, which are excited due to the presence of crust elasticity, in neutron stars with pasta structures, adopting the relativistic Cowling approximation. We find that the shear modes are independent of the presence of the cylindrical-hole and spherical-hole nuclei at least up to a few kilohertz, while the interface modes strongly depend on the presence of the cylindrical-hole and spherical-hole nuclei. In addition, we find empirical relations for the interface mode frequencies multiplied by the stellar mass and for the shear mode frequencies multiplied by the stellar radius. These relations are expressed as a function of the stellar compactness almost independently of the stiffness in a higher-density region inside the neutron star, once one selects the crust equation of state. Thus, if one would simultaneously observe the shear and interface modes from a neutron star, one might extract the neutron star mass and radius with the help of the constraint on the crust stiffness obtained from terrestrial experiments.

Affiliation: University of Tübingen, Germany

Title: Oscillations and Universal Relations for Compact Objects Composed of Dark and Nuclear Matter

Abstract: Some compact objects have been observed that cannot be composed of pure nuclear matter. Instead, these observations might be explained by the presence of exotic matter fractions, or even dark matter. In this talk, we postulate the existence of non-rotating compact objects composed of self-interacting dark matter, we assess their stability concerning adiabatic radial oscillations, and we determine the oscillation frequencies and damping times of f- and p-modes, as they are crucial for gravitational wave emission. We explore universal relations from neutron star models, evaluate their suitability for dark stars, and suggest improved expressions tailored to our models. These relations are finally used to estimate mass, radius and moment of inertia from observed gravitational waves.

Affiliation: University of Sofia, Bulgaria

Title: Image of the shadow and thin accretion disk in gravity with a minimally coupled scalar field

Abstract: This talk will discuss the possible observable relativistic images of a black hole and naked singularity appearing in a rotating geometry in Einstein's gravity, minimally coupled to a scalar field. Particular attention will be paid to a Kerr-like (KL) alternative to the rotating Fisher-Janis-Newman-Winicour solution. The study includes analytical and numerical calculations of equatorial circular orbits, shadow images, and radiation from thin accretion disks for various values of the object's angular momentum and scalar charge. It is found that the KL solution cannot be ruled out by the observations for small values of the scalar charge either. As the scalar charge increases, the optical properties change dramatically. The photon region does not hide the singularity, so it should be classified as a strong singularity. The compact object's shadow and accretion disk images can become multiply connected and strongly oblate. This qualitatively new feature can be used to distinguish observationally black holes from naked singularities via contemporary Very Long Baseline Interferometry experiments at short wavelengths, possible for implementation via the global radio telescope array such as Event Horizon Telescope.

Affiliation: AEI Potsdam, Germany

Title: Confronting Black Hole Spectroscopy with the Ringdown

Abstract: Black hole spectroscopy has been widely considered to be a powerful tool for probing the Kerr hypothesis and thus our understanding of astrophysical black holes and general relativity itself. The standard working hypothesis is that one is able to measure a part of the quasi-normal modes spectrum during the ringdown of binary black hole mergers. This requires the validity of linear perturbation theory at sufficiently late times and that the extraction of quasi-normal modes is possible. However, recent studies have questioned some of the main assumptions by analyzing linear and non-linear simulations, in order to quantify the robustness of the quasi-normal mode extraction. In this talk, we first outline the standard problem, before reviewing some of the novel developments. By considering a simplified problem, we demonstrate as a proof of principle that some of the key problems of the non-linear analysis, are very similar in the linear case. We conclude that the practical application of black hole spectroscopy is conceptually more involved than commonly anticipated.

Affiliation: Queen Mary, University of London, UK

Title: Non-linear dynamics in modified gravity

Abstract: In this talk I will present our 3+1 formulation of the Four-Derivative scalar-tensor theory of gravity with a modified gauge that proves to be strongly hyperbolic. Then I will show the results from Binary Black Hole evolutions that we have obtained from the implementation of these equations with GRChombo, a numerical general relativity code with fully adaptive mesh refinement.

Affiliation: University of Otago, New Zealand

Title: How to kick a black hole

Abstract: It is well known that gravitational waves interact in a non-linear way. This makes it difficult to describe them rigorously. The cleanest description is based on certain conformal properties of the Einstein equations — first discovered by R. Penrose they were rigorously developed and used by H. Friedrich to prove several important global results for general relativistic space-times. The conformal field equations which implement this conformal framework provide various well-posed initial (boundary) value problems for use in many different situations. The talk will give a computational perspective on one particular application, the non-linear interaction of gravitational waves with an initially static (and spherically symmetric) black hole. We will show how to 'kick' the black hole and possibly how to spin it up.

Affiliation: LPT Orsay, Université Paris-Saclay, France

Title: A survey of compact objects in scalar tensor theories

Abstract: I will discuss some explicit solutions of higher order scalar tensor theories. We will start by reviewing classical GR solutions and some of their key properties such as integrability of geodesics in Kerr spacetime. We will then construct stealth solutions, ie solutions that are still Einstein metrics but with a non trivial scalar field. We will construct solutions which are distinct from GR, static and stationary by employing transformations and using the key works of Carter on Kerr geodesics. We will briefly discuss vacuum regular wormhole solutions as well as examples of static neutron star metrics that allow significantly higher compacity than GR.

Affiliation: AEI Potsdam, Germany

Title: The orthogonality of quasinormal (and other) modes

Abstract: Quasinormal modes are mode solutions for perturbations of black hole spacetimes, and are a useful tool to model the “ringing" of a black hole resulting from a binary merger. I will introduce a conserved product for perturbations of Kerr black holes, and show that quasinormal modes with different frequencies are orthogonal under this product. This so-far overlooked property is not a straightforward consequence of standard properties of the Teukolsky equation and relies on the t-ϕ reflection isometry of Kerr. I will also show that quasinormal mode excitation coefficients are given precisely by the projection of initial data with respect to this product. Our product could be useful to set up a framework for nonlinear quasinormal mode coupling in Kerr, and can be extended to other black hole modes associated with massive fields.

Affiliation: Manly Astrophysics, Australia

Title: LS I +61°303: a magnetar in a binary?

Abstract: LS I +61°303 is an X-ray binary that is also catalogued as a gamma-ray binary as a result of frequent outbursts at ~TeV photon energies. Since the discovery of the (Be star) companion in the 50s, a debate has been held regarding the nature of the primary: is there a neutron star there, or is it a microquasar (black hole)? As detailed in a Nature paper published last year, FAST managed to relieve us of our ignorance, as radio pulsations spaced ~0.27s apart were detected. Aside from TeV outbursts, all manner of multiwavelength emissions have been observed in the last 60 years, including two soft-gamma flares in 2008 and 2012. Even prior to the spin period discovery, this SGR-like behaviour was taken as evidence favouring a magnetar interpretation by some. I will present a brief history of the source in the time we have known it, to see how the FAST discovery impacts on previous theoretical models related to its accretion dynamics, spin evolution, age limits, gamma-ray emissions, and radio switch-on. We will try and move towards an answer of whether there really is a magnetar there -- which would make this the first known magnetar in a binary -- or a more "ordinary" neutron star.

Video

Affiliation: AEI Potsdam, Germany

Title: Dynamical Scalarization during Neutron Star Mergers in scalar-Gauss-Bonnet Theory

Abstract: In certain classes of the scalar-Gauss-Bonnet theory, strong spacetime curvature in the vicinity of neutron stars and black holes can spontaneously trigger scalarization in the compact object if the coupling strength of the scalar field to the Gauss-Bonnet invariant exceeds a critical value. For isolated neutron stars, this threshold depends on the mass and equation of state. We discover a universal relation between the critical coupling strength and the stellar compactness. In synergy with such relations, one can, at least in principle, constrain the theory parameters regardless of the uncertainty in the equation of state. On the other hand, the presence of a companion will enhance the curvature between the centres of the two stars, and thus a stable hair can be installed at a lower magnitude of coupling for those neutron stars as members of binaries. Focusing on binary neutron star mergers, we investigate this latter dynamically-driven scalarization, and find that the reduction in the threshold coupling strength seems to be more profound for symmetric binaries, while the threshold is only marginally reduced for rather asymmetric binaries. The associated scalar radiation is also discussed.

Affiliation: University of Paraiba, Brazil

Title: Phenomenology of Quantum Gravity with Black holes

Abstract: At the dawn of the multi-messenger era of astronomy and astrophysics, new techniques and experimental methods enlightened the way for us to address some of the most important open problems in theoretical physics. One of those problems is the problem of quantum gravity/quantum spacetime. Currently, there are several different theories describing the gravity or the spacetime within a quantization scheme. All of those theories share the same difficulty: testing their predictions. The energy scales involved in experiments where quantum gravity effects could manifest themselves are usually of the order of the Planck energy scale, which is far beyond the energy scales involved in our current particle accelerators. The Quantum Gravity Phenomenology has emerged in search for alternatives to approach this problem. The idea of this talk is to present the general state of the art of that problem, and how the Group of Quantum Gravity Phenomenology from the Federal University of Paraiba has been approaching this problem. One of the ideas of the group is to use black holes' observables, such as their ringdown frequencies, to detach the signature of quantum gravity modifications from future observational data. For this goal, we make use of quantum spacetime's effective models.

Video: Unfortunately, no video available due to technical issues.

Affiliation: RIKEN, Japan

Title: Constraint on the NS model using high-frequency QPOs in magnetar

Abstract: Recently, quasi-periodic oscillations (QPOs) are newly observed in GRB 200415A. To theoretically explain this observation, we systematically examine the crustal torsional oscillations and find that the observed QPOs can be identified with several overtones. Since crustal oscillations strongly depend on crustal properties, we can constrain the EOS parameters through the identification of the observations. Furthermore, such a constraint tells us the corresponding neutron star mass and radius.

Video: Unfortunately, no video available due to technical issues.

Affiliation: University of Athens, Greece

Title: A Newtonian problem as a tool to study complicated orbital phenomena around Kerr BH

Abstract: We show that there is a Newtonian gravitational analogue that shares a lot of the special characteristics of Kerr. This similarity could be used as a tool to study qualitatively the odd behavior of adiabatic evolution of Kerr orbits whenever they cross a resonance by evolving the Newtonian analogue due to an artificial self-force that resembles the relativistic self-force in Kerr. Also, we study a non-integrable analogue of a perturbed Kerr metric and found that the passage of an orbit through a resonance is further prolonged when an artificial dissipative force is used to evolve the orbit, instead of the average losses of energy and angular momentum caused by the same force.

Video: Unfortunately, no video available due to technical issues.

Affiliation: Koç University, Turkey

Title: End of time evolution in self-interacting vector theories

Abstract: Non-linear extensions of Proca theory are pathological at large field amplitudes. I will explain why. I will also discuss a separate criterion for breakdown proposed in the literature, and why it does not correspond to a real physical problem.

Affiliation: University of Melbourne, Australia

Title: Neutron star mountain creation via stochastic local crustal failure

Abstract: Knowledge of the material properties of the neutron star crust allows for the calculation of the size of the largest mountains the crust can physically support. However the largest mountains that can form are not necessarily the same as the mountains that do form. The crust of an isolated neutron star builds up mechanical strain as the star spins down. We present a formalism to describe the local failure-induced deformations of the crust of an isolated neutron star over the course of spin-down. We model the crust at macroscopic scales using a cellular automaton with nearest-neighbour interactions which redistribute and dissipate mechanical stress-energy and deform the crust locally. Using this model we are able to make predictions of the statistics of failure events, such as their size and waiting-times, and the rate of failure over the star’s life. We find that while most failures occur early in the star’s life they persist until the star is near totally spun down, ≈ 1% of the initial frequency. Additionally as the automaton describes the crust’s shape the mass ellipticity and gravitational wave strain can be calculated.

Affiliation: University of Illinois at Urbana-Champaign

Title: Present and future astrophysics simulations using the Einstein Toolkit

Abstract: General relativistic numerical mangeto-hydrodynamics simulations are used to simulate the merger of neutron star and black hole binaries, disks around black holes, as well as supernovae. Driven by multi-messenger detections of gravitianal waves and electromagnetic signals new groups that did not in past engage in these simulations are joining the simulation effort. These simulations however combine demanding hydrodyanimcs simulations using high-resolution shock capturing schemes with a fully general relativistic treatment of gravity cannot be developed by single researchers anymore. Instead, the Einstein Toolkit is a community driven framework for numerical these astrophysics simulations that is used by many group throughout the world.
I will first present an overview of current work using the Einstein Toolkit to simulate neutron star mergers, accretion disks, and binary black hole mergers, focusing on the aspects of the toolkit that facilitate these simulations. I will highlight the scientific results obtained using the toolkit by different researchers around the world.
In a second part I will introduce the next generation mesh refinement driver for the Einstein Toolkit: CarpetX. CarpetX leverages the AMReX mesh refinement library that is being developed as part the Exascale project. It improves on the current mesh refinement driver, Carpet, by supporting fine grained mesh refinement to track features in the simulations and native support for GPU accelerated computing. I will present an outline of CarpetX's design and current capabilities.

Affiliation: Albert-Einstein-Institut, Potsdam, Germany

Title: Dynamical friction from scalar dark matter

Abstract: Despite the plethora of evidence of the existence and abundance of dark matter we have from large scale cosmological observations, there is still little we know of its properties or its behaviour on small scales. A promising way to test this is through the effects it may have on the gravitational wave signal from black hole binary mergers. In particular, the presence of dark matter around black holes may lead to a distinctive dephasing of the signal due to dynamical friction. During the last stages of an extreme mass ratio inspiral, when the satellite object starts to move at relativistic speeds, this effect could be detectable by LISA. In this talk I will present the results from numerically calculating of the dynamical friction force on a black hole generated by a cloud of ultra-light scalar field. This has been studied before in the nonrelativistic case, and I will discuss how our results extend this description to the relativistic case, relevant on small scales.

Video: Unfortunately, no video available due to technical issues.

Affiliation: Kyoto University, Japan

Title: Asteroseismology in supernova gravitational waves

Abstract: The supernova, which is the event at the last moment of the massive star's life, is the next promising candidate as the gravitational wave source. Up to now, gravitational waves from supernova explosions have been mainly discussed via numerical simulation. These results tell us the existence of the gravitational waves whose frequencies increase from a few hundred hertz up to kHz within a second. However, the physics behind this signal has been unclear. In this talk, we discuss the supernova gravitational waves from the approach with asteroseismology and we show the empirical relation in the supernova gravitational waves.

Affiliation: Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil; Institut für Theoretische Physik, Universtität Leipzig, Germany

Title: Stabilities and Instabilities of Rotating Black Holes and Loss of Predictability in their Inside

Abstract: Black holes in the Universe do not exist in isolation but, rather, they are surrounded by matter. It is therefore important to study the stability properties of black holes under matter field perturbations. In this talk we will discuss the stability properties under classical field perturbations of several rotating (Kerr) black hole spacetimes. In particular, we will present recent results on some of the mode stability properties of the following: (i) a maximally-rotating (extremal) Kerr black hole, whose event horizon suffers from an instability; (ii) Kerr-de Sitter spacetime, representing a Kerr black hole in a Universe with accelerated expansion, and so possessing a cosmological horizon as well as an event horizon; and (iii) the inner (Cauchy) horizon of Kerr-Newman-de Sitter spacetime, representing a charged Kerr-de Sitter black hole, where we find evidence for violation of Penrose’s strong Cosmic Censorship conjecture, i.e., evidence for the loss of predictability of the Einstein field equations inside the black hole.