## Long term Magneto-thermal evolution of neutron stars: the roles of the Hall drift amb ambipolar diffusion

José A. Pons

It is generally accepted that the nonlinear, dynamical evolution of magnetic fields in the interior of neutron stars plays a key role in the explanation of the observed phenomenology (temperatures, luminosities, spin period and derivative). Understanding the transfer of energy between toroidal and poloidal components, or between different scales, is of particular relevance. In this talk I discuss the general aspects of the long term magnetic and thermal evolution, with particular emphasis in the role of the Hall drift and ambipolar diffusion for typical magnetar conditions

Meeting ID:  854 3210 3337        Password: 959078

## Bubble dynamics from holography

David Julián Mateos Solé (Universitat de Barcelona)

Cosmological phase transitions proceed via the nucleation of bubbles that subsequently expand and collide. The resulting gravitational wave spectrum depends crucially on the bubble wall velocity. Microscopic calculations of this velocity are challenging even in weakly coupled theories. I will show how to use holography to compute the wall velocity from first principles in strongly coupled, non-Abelian, four-dimensional gauge theories. No previous knowledge of holography or string theory required.

Meeting ID:  854 3210 3337        Password: 959078

## Comparison of Kerr and dilaton black hole shadows: Impact of non-thermal emission

Jan Röder (Institut für Theoretische Physik, Goethe -Universität Frankfurt)

With the Event Horizon Telescope, a very long baseline interferometry (VLBI) array, both temporal and spatial event horizon-scale resolutions needed to observe super-massive black holes were reached for the first time. Current open questions revolve around the type of compact object in the Galactic Center, plasma dynam- ics around it and emission processes at play. The main goal of this thesis is to assess whether it is possible to distinguish between two spacetimes by means of synthetic imaging, under the aspect of different emission models. Extending the studies conducted in the pioneering work of Mizuno et al. 2018, general relativis- tic radiative transfer (GRRT) calculations are carried out on general relativistic magneto-hydrodynamics (GRMHD) simulations of a Kerr and of a non-rotating dilaton black hole. The systems are matched at the innermost stable circular orbit, and both black holes are initially surrounded by a torus in hydrostatic equilibrium with a weak poloidal magnetic field. In order to investigate the plasma dynam- ics, GRMHD simulations were carried out using the “Black Hole Accretion Code” (BHAC). In the literature the ratio between the temperatures of simulated ions and radiating electrons is often taken to be a constant, while in reality it is ex- pected to depend on plasma properties. In radiative post-processing with the code “Black Hole Observations in Stationary Spacetimes” (BHOSS) the temperature ra- tio was therefore parametrized. Additionally, in the jet wall, electrons are believed to be accelerated and should therefore be modeled with non-thermal electrons. To this end, both thermal and non-thermal electron energy distribution functions were employed. Lastly, images were reconstructed from synthetic VLBI data with the “eht-imaging” Python package to study how the effects of the emission models carry over to an observational environment. The most impactful result is the effect of the parameter Rhigh in the temperature ratio parametrization, splitting source structures into torus– and jet dominated configurations. Non-thermal emission turns out to be negligible at the field of view used and for the region it is applied in. Hence, given the present observational capabilities, it is unlikely that it is possible to distinguish spacetimes in observations. The striking visual differences are due to the difference in rotation between the black holes. In synthetic VLBI images, even the difference in shadow size is lost for most configurations. The situation may be improved in the future by a better VLBI array.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## GR-Athena++: puncture evolutions on vertex-centered oct-tree AMR

Boris Daszuta (Friedrich-Schiller-Universität Jena)

GR-Athena++ is a general-relativistic, high-order, vertex-centered solver that extends the oct-tree, adaptive mesh refinement capabilities of the astrophysical (radiation) magnetohydrodynamics code Athena++. To simulate dynamical spacetimes GR-Athena++ uses the Z4c evolution scheme of numerical relativity coupled to the moving puncture gauge. Stable and accurate binary black hole merger evolutions are demonstrated in convergence testing, cross-code validation, and verification against state-of-the-art effective-one-body waveforms. GR-Athena++ leverages the task-based parallelism paradigm of Athena++ to achieve excellent scalability. Strong scaling efficiencies above 95% for up to 1.2×1e4 CPUs and excellent weak scaling up to 1e5 CPUs in a production binary black hole setup with adaptive mesh refinement are measured. GR-Athena++ thus allows for the robust simulation of compact binary coalescences and and offers a viable path towards numerical relativity at exascale.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## Maximum mass of compact stars from gravitational wave events with finite-temperature equations of state

Armen Sedrakian ( Frankfurt Institute for Advanced Studies)

We conjecture and verify a set of universal relations between global parameters of hot and fast-rotating compact stars, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to the GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars 2.15+0.100.07MTOV2.24+0.120.10 for the typical range of entropy per baryon 2S/A3 and electron fraction Ye=0.1 characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## Black hole scalarization

Stoytcho Yazadjiev ( Eberhard Karls University of T ̈ubingen & Sofia University)

In extended scalar-tensor theories, such as
scalar-Gauss-Bonnet gravity, the black holes can undergo spontaneous
scalarization – a strong gravity phase transition triggered by a
tachyonic instability due to the non-minimal coupling between the
scalar field(s) and the spacetime curvature. This very interesting
phenomenon is the only known dynamical mechanism for endowing black
holes (and other compact objects) with scalar hair without altering
the predictions in the weak-field limit. In this talk, I will present
the basic theoretical ideas behind the spontaneous scalarization and
will review some of the recent achievements in the field.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## Black holes with scalar hair and astrophysical implications

Daniela Doneva ( Eberhard-Karls-Universitat Tuebingen)

Even though the Kerr black hole fits very well in the
interpretation of various astrophysical observations, there is a
number of yet untested modifications of general relativity that can
endow it with hair. The rapid advance of observational astrophysics
gives us the unique opportunity to test the existence of beyond-Kerr
black holes and eventually to constraint the strong-field regime of
gravity. A particular widely studied case is a Kerr-like black hole
endowed with scalar hair that can form for example in the presence of
time-varying complex scalar field or in the more general context of
tensor-multi-scalar theories. We will discuss these solutions and
their astrophysical manifestations. We will put a special emphasis on
the accretion discs around such objects that can have fundamentally
different properties compared to pure GR.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## Instabilities in neutron-star post-merger remnants

##### Xiaoyi Xie (University of Southampton)

Using nonlinear, fully relativistic, simulations we investigate the dynamics and gravitational-wave signature associated with instabilities in neutron star post-merger remnants. For simplified models of the remnant, we establish the presence of instability in stars with moderate T/|W|, the ratio between the kinetic and the gravitational potential energies. Detailed analysis of the density oscillation pattern reveals a local instability in the inner region of the more realistic differential rotation profile. We apply Rayleigh’s inflection theorem and Fjørtoft’s theorem to analyze the stability criteria concluding that this inner local instability originates from a shear instability close to the peak of the angular velocity profile and that it later evolves into a fast-rotating m 1⁄4 2 oscillation pattern. We discuss the importance of the presence of a corotation point in the fluid, its connection with the shear instability, and comparisons to the Rossby wave and Papaloizou-Pringle instabilities considered in the wider literature.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## Extraction of Black Hole Energy via Magnetic Reconnection

#### Luca Comisso (Columbia University)

Different mechanisms of black hole energy extraction have been carefully analyzed over the years, most notably the Penrose and Blandford-Znajek processes, providing us major insights on the mechanisms that might play a role in a number of highly energetic astrophysical phenomena, from active galactic nuclei to gamma-ray bursts to ultraluminous X-ray binaries. On the other hand, the possibility of extracting black hole rotational energy as a result of rapid reconnection of magnetic field lines has been generally overlooked. In this talk, we will analyze the mechanism of black hole energy extraction via fast magnetic reconnection as a function of the key parameters that regulate the process: black hole spin, reconnection location, orientation of the reconnecting magnetic field, and plasma magnetization. We will obtain the conditions under which black hole energy extraction occurs and we will quantify the rate of energy extraction and the reconnection efficiency in order to evaluate whether magnetic reconnection is an effective energy extraction mechanism for astrophysical purposes. In particular, we will see that magnetic reconnection in the ergosphere of a rapidly spinning black hole expels energized plasma that can exceed the energy originally stored in the magnetic field and might be responsible for black hole flares.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

## 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