Dr. Shafqat Riaz

**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.

Prof. Hajime Sotani

**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.

Mariachiara Celato

**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.

Prof. Galin Gyulchev

**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.

Dr. Sebastian Vökel

**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.

PhD cand. Llibert Aresté Saló

**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.

Prof. Jörg Frauendiener

**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.

Prof. Christos Charmousis

**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.

Dr. Laura Sberna

**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.

Dr. Arthur Suvorov

**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.

Dr. Hao-Jui Kuan

**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.

Saulo Soares

**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.

Prof. Hajime Sotani

**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.

Dr. Areti Eleni

**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.

Dr. Andrew Coates

**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.

Alex David Kerin

**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.

Dr. Roland Haas

**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.

Dr. Dina Traykova

**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.

Prof. Hajime Sotani

**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.

Prof. Marc Casals

**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.

Dr. Miguel Bezares

**Affiliation:** SISSA, Trieste, Italy

**Title:** K-dynamics: Dynamics of screening in the strong-field regime

**Abstract:** K-essence is a cosmologically relevant scalar-tensor theory that involves first-order derivative self-interactions, which pass all existing gravitational wave bounds and provides a screening mechanism. In this talk, I will present our results on the effect of this screening mechanism in non-linear stellar oscillations and gravitational collapse by using numerical relativity simulations. Towards the end of the talk, I will discuss the effects of this kinetic screening in binary neutron stars.

Prof. Lijing Shao

**Affiliation:** KIAA at Peking University, Beijing, China

**Title:** Testing gravity with pulsars

**Abstract:** Einstein’s general relativity has passed enormous tests with flying colours, ranging from the Solar System, binary pulsars, gravitational waves, to cosmology. Binary pulsars, being in a strong-field regime with quasi-stationary orbital motion, are excellent testbeds for multiple aspects of gravitation. I will introduce the pulsar-timing techniques that have enabled the various tests, and highlight a few cases where binary pulsars have played an essential role, in particular where strong gravity has significantly impacted the inner structure of neutron stars. Using binary pulsars to test the universality of free fall and long-range fifth force from dark matters will be briefly introduced as well.

Prof. Laura Bernard

**Affiliation:** LUTH, Meudon, France

**Title:** Some aspects of gravitational wave modelling in general relativity and scalar-tensor theories

**Abstract:** The next generation of gravitational wave detectors, such as the space-based interferometer LISA and the Einstein Telescope, will allow to test our gravitational paradigm with an unprecedented precision. One of the new challenge to analysis the forthcoming huge amount of data resides in the high precision modelling of GWs both in general relativity (GR) and beyond. In this talk, I will focus on the post-Newtonian method that is used to describe with very accurately the inspiral phase of the coalescence of compact binary systems. After reviewing the multipolar - post Minkowskian - post Newtonian formalism and the state-of-the-art in GR, I will explain how such a formalism can be simply applied to scalar-tensor theories of gravity. In particular, I will focus on the main phenomenological differences from GR and present the latest developments, including the contribution from the scalar dipolar tidal effect.

Dr. Jose Luis Blázquez-Salcedo

**Affiliation:** Complutense University of Madrid, Spain

**Title:** Quasinormal Modes of Neutron Stars in R^2 gravity

**Abstract:** In this talk we will discuss the quasinormal modes of neutron stars in R^2 gravity. In this theory, a (massive) scalar field modifies the properties of the compact objects. In particular, the spectrum of quasinormal modes. We analyze the influence of the scalar field on the radial, dipole and quadrupole modes under different equations of state, and we will discuss several universal relations.

Dr. Alex Vano-Vinuales

**Affiliation:** CENTRA, Institute Superior Técnico, Portugal

**Title:** Current efforts on free evolution of the hyperboloidal initial value problem

**Abstract:** Gravitational wave radiation, our window for probing the strong field and dynamical regime of gravity, is unambiguously defined only at future infinity - the location in spacetime where light rays arrive and thus where signals and global properties of spacetimes can be measured. A convenient way to include it in numerical relativity simulations is by means of hyperboloidal foliations, which consist of smooth spacelike slices that reach future null infinity. I will review the current state of the art of two suitable free-evolution approaches. The first one uses conformal compactification, based on an idea by Nobel-laureate Roger Penrose, and implements it via the BSSN and Z4 formulations of the Einstein equations. It has provided some very promising spherically symmetric numerical evolutions of a massless scalar field coupled to gravity, which I will show. The second approach employs the dual-frame method to preserve the well-posedness of the Generalized Harmonic Gauge formulation while using coordinates adapted to the hyperboloidal slices. Its preliminary results in spherical symmetry also show the potential of this approach. Finally, I will comment on current steps to extend these results to 3D, as the final goal of this work is to provide a far-field numerical framework that includes null infinity for simulations of compact object mergers with accurate gravitational wave extraction.

Dr. Justin Ripley

**Affiliation:** DAMTP University of Cambridge, UK

**Title:** Evolution of binary black hole systems in scalar Gauss-Bonnet gravity

**Abstract:** Characterizing GW signals requires accurate template waveforms for the inspiral, merger, and ringdown of binary black hole systems. In this talk I will discuss recent work on numerically solving for the dynamics of binary black hole systems in Einstein scalar Gauss-Bonnet (ESGB) gravity. This modified gravity theory can be motivated by effective field theory reasoning, and admits scalar "hairy" black hole solutions. These two facts make it a promising theory to perform model-dependent tests of General Relativity with gravitational wave observations of binary black hole merger. I will discuss how recent advances in mathematical relativity--in particular, the development of the "modified harmonic formulation"--have opened up the possibility of constructing fully nonlinear solutions to the equations of motion of ESGB gravity (in addition to a wider class of scalar-tensor modified theories known as "Horndeski" theories) in numerical relativity. I will discuss recent progress in numerically solving for the dynamics of binary black hole dynamics in this theory, including the merger phase of binary evolution.

Mr. Valentin Boyanov

**Affiliation:** Complutense University of Madrid, Spain

**Title:** The nature of black holes in semiclassical gravity

**Abstract:** In this talk I will begin by presenting a brief overview of the standard picture of black hole evolution in semiclassical gravity, involving the formation of a trapped region and its evaporation through the emission of Hawking radiation. I will use a simple tool which captures the essential characteristics of this picture -- the Renormalised Stress-Energy Tensor in the Polyakov approximation. With it, I will then proceed to a more in-depth analysis of semiclassical effects related to trapped regions with different classical background dynamics. On the one hand, I will show that for a trapped region formed in the usual manner (from quickly collapsing matter), the dominant semiclassical contribution to the subsequent evolution of the system might not come from the Hawking effect if the whole structure, including the inner horizon, is considered. On the other hand, I will show that there are large semiclassical corrections in geometries which approach the formation of a trapped region sufficiently "slowly" (in terms of the radial velocity of the collapsing matter), leading to a possible formation mechanism for ultracompact horizonless objects.

Prof. Gopakumar Achamveedu

**Affiliation:** Tata Institute of Fundamental Research, India

**Title:** Promise of persistent multi-messenger GW astronomy with Blazar OJ287

**Abstract:** Recent coordinated observation and interpretation of disparate "messenger" signals from GW170817-GRB170817A-EM170817 have inaugurated the era of multi-messenger transient gravitational wave (GW) astronomy. I will argue that the bright blazar OJ 287 should allow us to pursue persistent multi-messenger GW astronomy during the era of Square Kilometer Array. This is mainly due to the several successful multi-wavelength observational campaigns, that allowed us to establish the presence of a spinning supermassive black hole binary that spirals in due to the emission of nano-Hertz GWs in the central engine of OJ287. Our on-going efforts, relevant to both the Event Horizon Telescope consortium and the International Pulsar Timing Array consortium which aims to detect GWs from such massive BH systems in the coming years,will also be discussed.

Dr. Alejandro Cárdenas-Avendaño

**Affiliation:** Princeton University, USA

**Title:** Precision black hole photon ring tests

**Abstract:** The recent horizon-scale observations of black holes initiated another chapter of strong-field studies of general relativity. The never-stopping technological advances will improve these observations and require accurate theoretical predictions of the accretion physics and spacetime around black holes. In this talk, I will describe a numerical framework that exploits the integrability of the Kerr spacetime and solves the null geodesic equation by evaluating elliptic functions instead of directly integrating the geodesic equations. This framework uses a non-uniform resolution for computing black hole images suitable for detailed studies of the black hole photon ring (a narrow ring-shaped feature, predicted by general relativity but not yet observed). I will demonstrate its computational performance by presenting black hole high-resolution movies of non-stationary and non-axisymmetric source profiles around Kerr black holes that do not require high-performance computing resources. Our framework removes some of the current computational challenges and allows for various studies involving complex emission profiles and data variability.

Dr. Lia Medeiros

**Affiliation:** Institute for Advanced Study, Princeton University, USA

**Title:** Dynamical bosonic stars and gravitational waves

**Abstract:** Our current understanding of black holes assumes that they are described by the Kerr solution to Einstein’s equations. In April of 2019 the Event Horizon Telescope (EHT) published the first image of a supermassive black hole at the center of the M87 galaxy, resolved to event horizon scales. This image led to a null-hypothesis test of the Kerr metric and to the first constraint on the size of a black-hole shadow, the critical impact parameter between photons that fall into the black hole and those that escape to infinity. I will discuss recent results that used analytic calculations and numerical simulations to place constraints on regular, parametric, non-Kerr metrics. I will show that the shadow-size measurements can be used to place order unity constraints on deviation parameters that can be related to the second Post-Newtonian order and are, therefore, inaccessible to weak-field tests. I will show that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the size of predicted black-hole shadows that are inconsistent with even the current EHT measurements. Furthermore, I will briefly discuss how the same shadow size constraint can be used to constrain known solutions to gravitational theories and how these constraints may be improved with future EHT observations.

Dr. Nicolas Sanchis-Gual

**Affiliation:** Aveiro University, Portugal

**Title:** Dynamical bosonic stars and gravitational waves

**Abstract:** Bosonic stars are theoretical exotic compact objects made of ultralight bosonic particles that could explain part of dark matter. In this talk, I will review some recent numerical results on the stability and dynamical formation of these solutions. Then I will talk about bosonic star mergers, their gravitational-wave emission, and what we could learn about them from real gravitational-wave events, if these stars exist in the Universe.

Dr. Richard Brito

**Affiliation:** Instituto Superior Técnico, Portugal

**Title:** Probing ultralight bosons with black holes and gravitational-waves

**Abstract:** Ultralight bosonic fields are compelling dark-matter candidates and arise in a variety of beyond-Standard-Model scenarios. These fields can tap energy and angular momentum from spinning black holes (BHs) through superradiant instabilities, forming macroscopic bosonic condensates around astrophysical BHs. The formation of such condensates can lead to several striking signatures, such as the emission of a continuous gravitational-wave (GW) signal from the BH-boson cloud system, lack of highly spinning BH in particular BH mass ranges or signatures in binary BH systems. These signatures provide a unique portal to detect or constrain the existence of ultralight fields through the observation of BHs and GWs. In this talk I will give an overview of the status of this research program.

Prof. Masashi Kimura

**Affiliation:** Rikkyo University, Japan

**Title:** Metric Backreaction of the Blandford-Znajek Process

**Abstract:** We study the metric backreaction of mass and angular momentum accretion on black holes. We first develop the formalism of monopole and dipole linear gravitational perturbations around the Schwarzschild black holes in the Eddington-Finkelstein coordinates against the generic time-dependent matters. We derive the relation between the time dependence of the mass and angular momentum of the black hole and the energy-momentum tensors of accreting matters. As a concrete example, we apply our formalism to the Blandford-Znajek process around the slowly rotating black holes. We find that the time dependence of the monopole and dipole perturbations can be interpreted as the slowly rotating Kerr metric with time-dependent mass and spin parameters, which are determined from the energy and angular momentum extraction rates of the Blandford-Znajek process. Reference: arXiv:2105.05581

Prof. Lavinia Heisenberg

**Affiliation:** ETH Zurich, Switzerland

**Title:** Cosmological footprints of gravity

**Abstract:** General Relativity and the Cosmological Principle are the fundamental pillars of Cosmology. After introducing them and their underlying properties I will discuss the successes and challenges of the Standard Model of Big Bang Cosmology. I will then discuss how we can test General Relativity using different cosmological observations, from its geometrical properties down to testing the involved propagating degrees of freedom. This analysis will also help us to classify the attempts of going beyond General Relativity together with their explicit implications.

Dr. Vishal Baibhav

**Affiliation:** CIERA Northwestern University, USA

**Title:** The Parents of LIGO Black Holes and Their Hometown

**Abstract:** Two of the dominant channels to produce black-hole binary mergers are believed to be the isolated evolution of stellar binaries in the field and dynamical formation in star clusters. Pair instabilities prevent stellar collapse from generating black holes more massive than about 45-60 solar mass. This “mass gap” only applies to the field formation scenario: repeated mergers in clusters can fill the gap. A similar reasoning applies to the binary’s spin parameters. If black holes are born slowly rotating, the high-spin portion of the parameter space (the “spin gap”) can only be filled by black-hole binaries that are assembled dynamically. I will discuss how such signatures are a smoking gun for the hierarchical origin, and how recent detections (GW190521 and GW190412) fit in this context. I will also talk about how we may be able to reconstruct the properties of progenitors of second-generation black holes.

Dr. Arthur Suvurov

**Affiliation:** Manly Astrophysics, Australia

**Title:** Picard-Lefschetz theory and (gravitational-) wave optics

**Abstract:** Rapidly rotating neutron stars that continuously emit gravitational waves in the ∼kHz band are promising sources for existing and upcoming ground-based interferometers. Owing to their faintness however, persistent monitoring over year-long timescales is necessary in many cases. If at some point during the observational window the line of sight comes to intersect with an individual star or cluster, transient though significant gravitational lensing may occur, modulating the observed waveform and ultimately affecting parameter inference concerning the equation of state or magnetic field structure. Furthermore, because the gravitational wavelength will exceed the Schwarzschild radius of any given microlens in this scenario, calculations relevant to the fully diffractive regime, where geometric optics does not apply, are necessary. The relevant mathematical object in this scenario is the Fresnel-Kirchhoff diffraction integral, which is notoriously difficult to calculate owing to its infinitely oscillatory nature. Using some recently developed techniques coming from complex Morse theory -- Picard-Lefschetz theory -- we show how these integrals might be calculated and what the implications are when connecting observations with theory in (hopefully soon-to-be detected) continuous wave measurements.

Dr. Adrian del Rio Vega

**Affiliation:** Penn State University, USA

**Title:** Quantum effects induced by gravitational waves

**Abstract:** Chiral anomalies in quantum field theory typically emerge whenever one introduces an interaction with a classical Yang-Mills or gravitational background, and their analysis have been traditionally connected to topological questions of this background and the study of instanton solutions. We show here how one can alternatively find situations of physical interest that only involve ordinary, but dynamical solutions of the background field equations. More precisely, we will show that solutions to the Einstein (Maxwell) equations are able to trigger these anomalies if and only if they admit a flux of gravitational (electromagnetic) radiation with net circular polarization. Using symmetry arguments we will then identify and discuss some examples in binary systems. As an application of this analysis, we will also discuss the potential that measurements of GW circular polarization could have in identifying precession effects in binary black hole systems.

Ms. Lucy Strang

**Affiliation:** University of Melbourne, Australia

**Title:** A model for X-ray plateaux in short Gamma Ray Bursts

**Abstract:** Many short Gamma-Ray Bursts (sGRBs) have a prolonged plateau in the X-ray afterglow lasting up to tens of thousands of seconds. A central engine injecting energy into the remnant may fuel the plateau. We develop a simple analytic model which naturally produces X-ray plateaux using a magnetar as the central engine. Our model leverages well-established descriptions of young supernova remnants and applies the underlying physics to sGRB remnants. We calculate analytically the energy distribution of a bubble of electrons powered by the magnetar wind to obtain both the light curve and the spectrum. Using data from the Swift X-Ray Telescope, we find our model aligns with observed data. We also produce spectra in X-ray plateaux which allow for parameter estimation. The plerion contribution is accompanied by an ejecta contribution which we do not model here. If combined with a gravitational wave signal, our model could provide insight into multimessenger astronomy and neutron star physics.

Dr. Guilherme Grams

**Affiliation:** Institut de Physique Nucléaire de Lyon, France

**Title:** Properties of neutron star crust within nuclear physics uncertainties

**Abstract:** It was recently shown that a broad class of gravity theories allows for spontaneous scalarization of black holes, letting these objects to grow “scalar hair” once certain conditions are met and to remain “bald” otherwise. Most works on the topic have focused on isolated black holes, but from an observational point of view it is important to study the two-body problem and explore how scalarization can affect gravitational waves emitted by coalescing compact binaries. In this seminar I will give a broad overview about black hole scalarization and also report on recent progress made in analysing this effect in binary black hole systems. A compressible liquid drop model (CLDM) is used to correlate uncertainties associated with the properties of the neutron star (NS) crust, with uncertainties associated with Chiral Effective Field theory (ChEFT) predictions for the properties of homogeneous neutron matter and nuclear matter. We also study the impact of the surface, curvature, and Coulomb energies on the crustal properties. Fits to experimental nuclear masses are employed to further constrain the CLDM, and we find that they disfavor some of the ChiEFT Hamiltonians. These fits also reveal how the curvature energy alters the correlation between the surface energy the bulk symmetry energy. Properties of the NS star crust against nuclear uncertainties are then analyzed, and we show that their impact vary from one observable to another: i) the finite size models impact the crust composition (A, Z) and have negligible influence on others quantities, ii) the largest uncertainties for the asymmetries I_cl and Y_e, as well as the volume fraction u, are induced by the Hamiltonians alone, iii) the largest uncertainties for the matter composition in the densest regions of the crust, as well as the precise location of the crust-core transition, are related to the Hamiltonians as well as by the surface energy isospin asymmetry parameter (p_surf). As a consequence, the crust moment of inertia is also largely impacted by the choice of Hamiltonian as well as by the parameter p_surf. The uncertainties induced by the loss function used for the fit to finite nuclei as well as by the in-medium nucleon mass m* are much smaller. Finally, we analyze the impact of these nuclear uncertainties on the NS mass-radius relations within a unified approach. In the analysis of the macroscopic NS properties, the Hamiltonians are the main source of uncertainties.

Dr. Hector O. da Silva

**Affiliation:** Albert Einstein Institute, Potsdam, Germany

**Title:** Black hole dynamics in compact binaries beyond-general relativity

**Abstract:** It was recently shown that a broad class of gravity theories allows for spontaneous scalarization of black holes, letting these objects to grow “scalar hair” once certain conditions are met and to remain “bald” otherwise. Most works on the topic have focused on isolated black holes, but from an observational point of view it is important to study the two-body problem and explore how scalarization can affect gravitational waves emitted by coalescing compact binaries. In this seminar I will give a broad overview about black hole scalarization and also report on recent progress made in analysing this effect in binary black hole systems.

**Affiliation:** Aristotle University of Thessaloniki, Greece

**Title:** A look into binary neutron star merger remnants via equilibrium models

**Abstract:** In this talk, I will summarize recent results obtained using equilibrium modelling to describe binary neutron star (BNS) merger remnants. Focusing on a recently proposed differential rotation law producing realistic post-merger rotational profiles, the properties of remnant models are explored. In addition, using specific equations of state, the threshold mass for prompt collapse to a black hole is deduced and crucial predictions of BNS coalescence simulations are replicated. Finally, a possible correlation is conjectured between the compactness of quasi-equilibrium remnant models at the threshold mass and the compactness of maximum mass non-rotating models.

**Affiliation:** Sapienza University of Rome, Italy

**Title:** New physics on the horizon? Recent developments and challenges in tests of dark compact objects

**Abstract:** Gravitational-wave astronomy and new electromagnetic facilities allow us for unprecedented tests of the nature of dark compact objects and provide a novel way to search for new physics. I will give an overview of the many recent results in this area (including shadows, constraints on the multipolar structure, ringdown tests, gravitational-wave echoes, and tidal effects in binaries) and discuss the outstanding challenges ahead.

**Affiliation:** Federal University of Pará, Salinópolis, Brazil

**Title:** Binary evolution in accretion disks and other media

**Abstract:** Binary systems, such as those discovered by the LIGO-VIRGO collaboration, have opened a new window for testing astrophysical systems. Allied with electromagnetic detections, we have unprecedented ways of looking at the physics of these systems. Usually, binary systems are considered to be in a vacuum, where they tend to lose eccentricity due to the emission of gravitational waves. However, astrophysical environments can be extremely rich and thus generate detectable signatures through binary systems. In this seminar, we revisit the physics of binary systems considering that the binary system is immersed in a medium. We show the binary evolution can considerably be affected by dynamic frictional and accretion forces generated by the medium. Furthermore, in asymmetric binary systems, we show that the center of mass can acquire considerable velocities, capable of reaching the escape velocity of some galaxies.

**Affiliation:** Sapienza University of Rome, Italy

**Title:** Testing the nature of dark compact objects with gravitational waves

**Abstract:** Black holes are the most compact objects in the Universe. According to General Relativity, black holes are endowed with an event horizon that hides a singularity where Einstein’s theory breaks down. Recently, gravitational waves opened the possibility to probe the existence of horizons and investigate the nature of compact objects. This is of particular interest in view of some quantum-gravity models which predict the existence of horizonless dark compact objects that overcome the paradoxes associated to black holes. Such dark compact objects can emit a modified gravitational wave signal with respect to the black hole case and late-time gravitational wave echoes as characteristic fingerprints. In this talk, I overview the phenomenology of dark compact objects and their observational evidence with current and future gravitational-wave detectors.

**Affiliation:** Aveiro University, Portugal

**Title:** Static black holes with non-spherical horizons

**Abstract:** Static (single) black holes (BHs) in electrovacuum can only have an electric monopole, and they are necessarily spherically symmetric. We discuss two different mechanisms to circumvent this result. In the first case, we show that static BHs in AdS-electrovacuum can have an arbitrary electric multipole structure, and present explicit examples of static BHs with no continuous (spatial) symmetries. The second example consists in asymptotically flat scalarized BHs in a class of Einstein-Maxwell-scalar (EMS) models. The corresponding BHs bifurcate from the Reissner-Nordstrom BH trunk, forming an infinite (countable) number of branches, and possess a large freedom in their multipole structure. Unlike the case of electrovacuum, the EMS model admits static, asymptotically flat, regular on and outside the horizon BHs without spherical symmetry and even without any spatial isometries, which are thermodynamically preferred over the electrovacuum state.

**Affiliation:** University of Athens, Greece

**Title:** Could we discern a pure relativistic black hole from gravitational wave signals?

**Abstract:** Black holes are extremely simple astrophysical objects arising from the mathematical theory of general relativity. Although the recent detections of gravitational waves have made us more confident about the existence of black-hole-like objects, further observations are needed to exclude other more exotic objects. The method presented in this talk to address the issue of how the observed objects are related to real black-hole objects, is based on the distinct dynamics followed by integrable systems, such us the ones describing the orbits around black holes.

**Affiliation:** NAOJ, Sokendai, Japan and Space Science Institute, Colorado

**Title:** The Optical Luminosity-Time Correlation for More than 100 Gamma-Ray Burst Afterglows and future perspectives with the KISO observatory

**Abstract:** Gamma-ray bursts (GRBs) are fascinating events due to their panchromatic nature. Their afterglow emission is observed from sub-TeV energies to radio wavelengths. We investigate GRBs that present an optical plateau, leveraging on the resemblance with the X-ray plateau shown in many GRB light curves (LCs). We comprehensively analyze all published GRBs with known redshifts and optical plateau observed mostly by the Neil Gehrels Swift Observatory (Swift). We fit 267 optical LCs and show the existence of the plateau in 102 cases, which is the largest compilation so far of optical plateaus. For 56 Swift GRBs with optical and X-ray plateaus, we compare the rest-frame end time at both wavelengths ( Topt, TX ), and conclude that the plateau is achromatic between Topt and TX . We also confirm the existence of the two-dimensional relations between Topt and the optical luminosity at the end of the plateau emission, which resembles the same luminosity-time correlation in X-rays (Dainotti et al. 2013). The existence of this optical correlation has been demonstrated for the largest sample of optical plateaus in the literature to date. The squared scatter in this optical correlation is smallest for the subset of the Gold GRBs with a decrease in the scatter equivalent to 52.4% when compared to the scatter of the entire GRB sample. Currently, we are investigating GRBs observed in high energies by the Fermi-LAT and we are looking for the existence of the plateau emission in these high energy GRBs. Thus, a joint analysis of GRBs observed by Fermi, Swift BAT+XRT X-rays and optical (Swift UVOT + SUBARU and other optical facilities) will provide additional insight on the nature of the plateau emission. A joint follow-up of optical afterglow of GRBs will be performed by the KISO observatory operated by Tokyo University and in correspondence with the telescopes DDOTI and RATIR operated by UNAM in Mexico. This will allow a better coverage of the lightcurves and the hunt for short GRBs.

**Affiliation:** Princeton University, USA

**Title:** Neutron Stars in the Effective Fly-By Framework: f-Mode Resummation

**Abstract:** Compact object binaries whose gravitational wave emission is within the frequency band of ground based detectors can be formed in dense stellar environments through dynamical interactions. Such binaries will necessarily have large, close to unity, orbital eccentricity. Recently, a new framework called the effective fly-by framework was developed to model the gravitational wave emission from such binaries. Here, I will discuss how to extend this framework to include the dynamical tidal interactions present if at least one of the binary components is a neutrons star. This results in an analytic model of the f-modes excited during pericenter passage, with the amplitude and phase determined by the orbital parameters of the binary and the properties of the neutron star. I will further discuss the accuracy of this model and its prospects for the detection of f-modes.

**Affiliation:** ESADE Business School, Barcelona, Spain

**Title:** Perturbative applications of quasilocal conservation laws in general relativity: gravitational self-force and cosmology

**Abstract:** Quasilocal definitions of stress-energy-momentum---that is, as boundary densities (rather than local volume densities)---have proven generally very useful in formulating and applying conservation laws in general relativity. In this talk, I will discuss recent advances following such an approach (in particular, using the Brown-York quasilocal stress-energy-momentum tensor, for matter and gravity combined) in the contexts of the gravitational self-force [1907.03012] and cosmology [2006.10068].

**Affiliation:** Silesian Univesity, Opava, Czech Republic, and RUDN University, Moscow, Russia

**Title:** General parametrization of wormhole spacetimes and its application to shadows and quasinormal modes

**Abstract:** The general parametrization for spacetimes of spherically symmetric Lorentzian, traversable wormholes in an arbitrary metric theory of gravity is presented. The parametrization is similar in spirit to the post-Newtonian parametrized formalism, but with validity that extends beyond the weak field region and covers the whole space. Our method is based on a continued-fraction expansion in terms of a compactified radial coordinate. Calculations of shadows and quasinormal modes for various examples of parametrization of known wormhole metrics that we have performed show that, for most cases, the parametrization provides excellent accuracy already at the first order. Therefore, only a few parameters are dominant and important for finding potentially observable quantities in a wormhole background. We have also extended the analysis to the regime of slow rotation.

**Affiliation:** Aveiro University, Portugal

**Title:** Testing the Kerr hypothesis: dynamically robust non-Kerr black holes and horizonless imitators

**Abstract:** I will entertain the possibility if (and under which circumstances) astrophysical black hole candidates could be described by something else rather than the Kerr geometry; that is to test the “Kerr hypothesis”, both in view of theoretical consistency, and in particular dynamics, as well as in view of the current observational developments. Explicit examples of dynamically robust non-Kerr black holes and horizonless imitators that mimic current data will be discussed, as illustrations.

**Affiliation:** Queen Mary University of London, England

**Title:** Revisiting black-hole perturbation theory: the hyperboloidal slice approach

**Abstract:** After reviewing the well-stablished notion of black-hole perturbation theory and the concept of quasinormal modes, I will discuss an alternative geometric framework to treat problems in the field: the hyperboloidal framework. In this context, I will summarise some results from the last past years in which the hyperboloidal framework has provided new tools to re-asses and expand on open questions in black-hole perturbation theory such as the spectral representation of solutions to relativistic wave equations and the stability of the quasinormal modes spectra. Not only do the results play an important role on our understanding of fundamental aspects of the theory, but also they have a potential high impact in gravitational waves astronomy and black-hole spectroscopy.

**Affiliation:** CENTRA, Instituto Superior Técnico, Lisbon, Portugal

**Title:** Putting Infinity On the Grid

**Abstract:** One of the key deliverables of numerical relativity are
gravitational waveforms from compact binaries. In my talk, I will
describe an ongoing research program relying on a 'dual frame'
approach to the field equations of GR (in generalized harmonic gauge)
on compactified hyperboloidal slices. These slices terminate at
future-null infinity, and the hope is to eventually extract
gravitational waves from simulations there directly. The main obstacle
to their use is the presence of ’infinities’ coming from the
compactified coordinates, which have to somehow interact well with the
assumption of asymptotic flatness so that we may arrive at regular
equations for regular unknowns. I will explain our broad strategy for
how this may be achieved, and relate this to the state-of-the-art in
the field.

**Affiliation:** Sofia University and the Bulgarian Academy of Sciences, Bulgaria

**Title:** Black holes with scalar-hair in extended scalar-tensor theories of gravity- equilibrium solutions and dynamical formation

**Abstract:** In this talk I will briefly review the extended scalar-tensor theories of gravity. I will mainly focus on the Gauss-Bonnet theory and the dynamical Chern-Simons gravity. Then I will present some representative black hole solutions with scalar hair and will discuss some of their properties. Special emphasis will be given to the theories with scalar coupling allowing for tachyonic instability of the Schwarzschild and the Kerr black hole and their phase transition to scalarized black holes. The very dynamics of the scalar hair formation will be also discussed.

**Affiliation:** Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

**Title:** Modeling binary neutron star merger remnants

**Abstract:** Being able to determine the stationary structure of a neutron star allows to study its properties and how they depend on its physical condition (eg, rotation, temperature, and magnetic field) and on the microphysical equation of state. Moreover, this stationary configuration can be used as initial condition for more resource demanding hydrodynamic simulations. This is particularly important for the hot (T ≲ 1e12 K) and differentially rotating compact remnant of a binary neutron star merger, because the theoretical predictions can be confirmed with the observed gravitational, electromagnetic, and (potentially) neutrino radiation. A key approximation made for computing the stationary structure of hot and rotating neutron stars is that of barotropicity, namely that all thermodynamic quantities are in a one-to-one relationship. However, this is a poor approximation for the remnant of a binary neutron star merger or of a core-collapse supernova. In this talk I describe how we developed a new method to determine the structure of neutron stars without the barotropic approximation, used it to model a binary neutron star merger remnant, and performed an extensive study of its parameter space.

Design: TEMPLATED Images: G. Andre Oliva.