Category: Uncategorized

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

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

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

Luca Comisso (Columbia University)

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

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

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. 

Jan-Erik Christian (Goethe University)

Sergio Servidio (University of Calabria, Italy )

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.