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

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.