October 01, 2021
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.

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September 24, 2021
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.


July 30, 2021
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.

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July 23, 2021: Dr. Panagiotis Iosif

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.

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July 16, 2021: Prof. Paolo Pani

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.

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July 09, 2021: Prof. Caio Macedo

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.

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July 02, 2021: Ms. Elisa Maggio

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.

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June 25, 2021: Prof. Eugen Radu

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.

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June 18, 2021: Prof. Theocharis Apostolatos

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.

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June 11, 2021: Prof. Maria Giovanna Dainotti

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.

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May 21, 2021: Dr. Nicholas Loutrel

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.

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May 07, 2021: Dr. Marius Oltean

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].

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Apr 30, 2021: Dr. Roman Konoplya

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.

Apr 16, 2021: Prof. Carlos Herdeiro

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.

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Apr 09, 2021: Dr. Rodrigo Panosso Macedo

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.


Mar 26, 2021: Dr. David Hilditch

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.

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Mar 19, 2021: Dr. Stoytcho Yazadijev

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.


Jan 29, 2021: Dr. Giovanni Camelio

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.