Category: Uncategorized

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.

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-domain astronomy 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.

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.

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.

by Polychronis Koliogiannis

Neutron stars are the way for the Universe to manifest its densest objects with an internal structure. Their study requires the knowledge of the equation of state of the fluid in the interior of the star. While isolated neutron stars place constraints on the cold, catalysed matter, many dynamical phenomena depend sensitively on the equation of state of hot dense nuclear matter. The formation of proto-neutron stars, neutron stars mergers, as well as the aftermath remnant, depend on the equation of state at finite temperature, entropy per baryon, and a varying range of proton fraction. In this framework, we construct thermodynamically consistent equations of state to accurately describe thermal effects. Additionally, we study the thermal and rotation with the Kepler frequency effect on some of the most important quantities in neutron stars, including the mass and radius, the frequency, the Kerr parameter, the moment of inertia, etc. The extended study on these quantities and data from late observations of neutron stars, both isolated and in matter of merging, could provide useful insight and robust constraints on the equation of state of nuclear matter.

by Laura Sagunski

Longstanding anomalies in astrophysical observations on small scales suggest that dark matter might not be collisionless, as is commonly assumed, but could have sizable self-interactions. For the first time, we have probed the hypothesis of self-interacting dark matter (SIDM) at intermediate scales between galaxies and galaxy clusters. To model the SIDM halo density profiles, we have employed an observation-driven approach, the so-called Jeans model. So far, the limit on the self-interaction cross section from the Bullet Cluster is often cited as the strongest constraint on dark matter self-interactions. We show, however, that the halo density proles of relaxed systems like groups and clusters lead to much stronger bounds on the self-interaction cross section.

by Prof. Dr. Andreas Bauswein (Zoom Meeting ID:886 0167 6852   Password: 680019)

We describe generally the collapse behavior of neutron star mergers, which is quantitatively expressed by the threshold binary mass for prompt black formation. We discuss general dependencies of the threshold mass and sketch applications for instance for the equation of state constraints. Moreover, we describe the impact of the hadron-quark phase transition on the collapse behavior and postmerger gravitational wave emission.

Elias Most, ITP, Goethe Universität, Frankfurt

Fast Radio Bursts are short emissions of unknown extragalactic origin. This talk discusses, how to model them by collapsing neutron stars in resistive GRMHD simulations. Results on gravitational and electromagnetic emission will be presented. Furthermore, the possibility of forming charged black holes as the end states of gravitational collapses will be discussed.