Axial and polar ring down modes of a black hole in General Relativity and for a class of extensions

Peter O Hess (Instituto de Ciencias Nucleares-UNAM)

Axial and polar ring down modes of a Schwarzschild black hole are calculated within General Relativity and a class of extensions. The modification considered includes a correction to the metric of leading order 1/r^4. This results in a mass-function depending on r. As a mass function, we use a particular one, which barely avoids the event horizon.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Constructing solutions to the inverse problem in gravitation

Arthur-George Suvoroc (University of Tübingen)

A detection of non-Kerr features in astrophysical data concerning black holes would provide compelling evidence for the break-down of general relativity in the strong-field regime. While experiments have thus far validated the Kerr description, suppose we found that some object had a particular, non-Kerr structure; is there a clear way that this can be used to guide us towards the “true” theory of gravity? While a full answer to this problem is still far away, some recent progress has been made in that a recipe for constructing solutions to the inverse problem can be written down: given a metric (reconstructed from astrophysical data), it is shown how non-minimally coupled scalar-tensor and vector-tensor theories can be built around it. Some implications of this finding and other recent works are discussed.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Turbulent flares in magnetically-dominated astroplasmas

Joonas Naettilae (Columbia University)

Astrophysical compact objects like neutron stars and accreting black holes are luminous sources of nonthermal radiation. Their activity demonstrates efficient dissipation of macroscopic energy stored in magnetic fields. A possible way for the dissipation can be a macroscopic MHD instability that excites turbulent motions in the tangled magnetic fields which, in turn, enables a transfer of energy to small scales where it can be dissipated. We performed 2D and 3D fully-kinetic radiative simulations of such reconnection-mediated turbulent flares in magnetized weakly-collisional pair plasmas. Turbulence is excited on a macroscopic scale and we observe that it develops by forming thin, dynamic current sheets on various scales. This gives rise to highly variable nonthermal flares whose characteristics we model in detail. Our fully-kinetic simulations can also be used to study the energy transfer mechanism of turbulent cascades themselves since they capture the dissipation processes from first principles. 

Supermassive Neutron Stars Rule Out Twin Stars

Jan-Erik Christian (Goethe University)

The detection of gravitational waves from binary neutron star mergers has opened the possibility to constrain the equation of state (EoS) and probing for a phase transition of high-density QCD matter. The measurement of GW170817 points to a soft equation of state that feature more compact neutron stars. We find that a first order phase transition from hadronic matter to quark matter can generate neutron stars compact enough to be compatible with the GW170817 measurement, even for comparatively hard hadronic EOSs.  For GW190814 an unidentified compact obejct with a mass of 2.5 M was observed.  We investigate the implications of that object being a 2.5 M neutron star in regard to the possibility of a strong phase transition.  We use EOSs of varying stiffness provided by a parameterizable relativistic mean field model transitioning in a first order phase transition to quark matter with a constant speed of sound.  We find a strong connection between the discontinuity in energy density and the maximal mass generated by the EOS.  We demonstrate, that high maximal masses cannot be realized for large discontinuities in energy density, which are necessary for visible mass gaps in the mass radius relation, i.e. twin stars, especially for soft EOSs.  As a result we conclude that twin stars and maximal masses of Mmax > 2.2 M are mutually exclusive.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

A spectral method algorithm for numerical simulations of gravitational fields

Sergio Servidio (University of Calabria, Italy )

A numerical study of the Einstein field equations, based on the 3+1 foliation of the spacetime, is presented. A pseudo-spectral technique has been employed for simulations in vacuum, within two different formalisms, namely the Arnowitt-Deser-Misner (ADM) and the conformal Baumgarte-Shapiro-Shibata-Nakamura (BSSN) approach. The numerical code is based on the Fourier decomposition, accompanied by different filtering techniques. The role of the dealiasing, as well as the influence of the filter type, has been investigated. The algorithms have been stabilized via a novel procedure that controls self-consistently the regularity of the solutions. The accuracy of the model has been validated through standard testbeds, revealing that the filtered pseudo-spectral technique is very accurate. Finally, the procedure has been stressed via black hole dynamics and a new strategy, based on hyperviscous dissipation that suppresses spurious boundary problems, has been proposed. The model represents a valid tool of investigation, particularly suitable for the inspection of small scale nonlinear phenomena in gravitational dynamics.
Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Modeling gravitational waves from exotic compact objects

by Alexandre Toubiana

In the standard astrophysical paradigm, the only compact objects (with C>0.1) are black holes (BHs) and neutron stars (NSs). However, extensions of General Relativity (GR) and/or of the Standard Model can give rise to “exotic compact objects” (ECOs), which mimic the gravitational behavior of BHs and NSs to varying degrees. Thus, exotic compact objects can be difficult to distinguish from BHs and NSs in the inspiral phase of the binaries observed by gravitational-wave detectors, but significant differences may be present in the merger and post-merger signal.

In this talk, I will briefly review some models of ECOs and numerical simulations of non-BH binaries. Based on these results, I will present a toy model aiming to capture the main features of the full GW signal emitted by ECO binaries and use it to assess the detectability of such exotic signals with current and future detectors, and whether they can be distinguished from black hole binaries.

Testing the Kerr hypothesis: the examples of synchronisation and scalarisation

by Carlos A.R. Herdeiro

The Kerr hypothesis is that astrophysical black hole candidates are very special objects, with only two degrees of freedom and well described by the Kerr metric. Theoretically, this hypothesis is based on the uniqueness theorems for electro-vacuum. But in the presence of other types of matter or modified gravity are there any viable alternatives? In this talk I will illustrate some examples of black holes with “hair” that could co-exist with Kerr black holes, but emerge dynamically (and be preferred) at particular scales, either in General Relativity with ultralight bosonic matter or in modified gravity with higher curvature corrections, commenting on their theoretical and phenomenological differences (e.g. shadows) and on their phenomenological viability.

Magnetic fields, jets, and turbulence in the multimessenger era

by Philipp Moesta.  Meeting ID: 896 2772 7941 Password: 809669

Magnetic fields, turbulence, and jet-driven outflows play a critical role in core-collapse supernovae and compact-object mergers. These transients belong to the most luminous and energetic events observed in the universe and are key targets for time-domaiastronomy surveys. I will discuss the unique challenges in both input physics and computational modeling for these systems involving all four fundamental forces and highlight recent breakthroughs in full 3D simulations. I will pay particular attention to how these simulations can be used to reveal the engines driving these events and conclude by discussing what remains to be done in order to maximize what we can learn from current and future time-domain transient surveys.

Lattice Boltzmann Method for relativistic hydrodynamics

By Alessandro Gabbana   Zoom ID: 896 2772 7941     Passcode: 809669

The study of relativistic hydrodynamics has received renewed interest in recent years, as it has been realized that phenomena in diverse areas of physics, such as astrophysics, quark gluon plasma and even condensed matter physics can be studied via a hydrodynamic approach  in the relativistic regime. For example, quark-gluon plasmas (QGP) created in heavy-ion collisions at RHIC or LHC or electron flow in 2D graphene sheets can be considered, in some cases, as relativistic fluids.  For a long time, relativistic fluid dynamics has been hampered by several theoretical and computational shortcomings,  since relativistic versions of Navier-Stokes equations suffer from causality problems linked to the order of the derivatives appearing in the dissipative terms. Some of these problems can be avoided by employing a lattice kinetic approach, that treats space and time on the same footing (i.e., via first-order derivatives).
This is one of the main reasons why Relativistic Lattice Boltzmann Methods (RLBMs), that discretize in coordinate and momentum space the Boltzmann equation, and yet ensure that the resulting synthetic dynamics retains all its hydrodynamic properties, have been recently proposed as effective computational tools to study relativistic flows.
This talk presents an overview of the formal algorithmic derivation of a RLBM capable of handling a wide range of physics parameters and kinematic regimes, from ultra-relativistic to mildly relativistic and eventually to the non-relativistic limit. Moreover, a few examples of applications will be discussed, including cross-comparisons with other numerical methods in the study of relativistic shock waves in QGP.

Large-EDDY-simulations in binary neutron star  mergers

by Carlos Palenzuela.  Meeting ID: 886 0167 6852   Password: 680019

One of the most important open issues in the theoretical understanding of binary neutron star collisions is the amplification of magnetic fields after the merger. This happens first in a turbulent way at small scales via  Kelvin-Helmholtz instability, followed by a large-scale ordering via winding and magneto-rotational instability. However, the highest numerical resolution achieved in full GRMHD simulations O(10m) are extremely expensive (tens of millions of CPU hours) and are still far from capturing all the scales at play, possibly of a meter or less. Here we present how large-eddy-simulations with the gradient sub-grid-scale model can reproduce the magnetic amplification up to local values of 10^17 G during the first 10 milliseconds after the merger, but at a much lower computational cost. This results anticipates a more accurate simulations in the near future with reachable current resources.