### Conveners

#### Theoretical and Observational Studies of Astrophysical Black Holes: Block 1

- Alexander Zakharov (ITEP, Moscow)

#### Theoretical and Observational Studies of Astrophysical Black Holes: Block 2

- Alexander Zakharov (ITEP, Moscow)

### Description

The session is devoted to theoretical and observational studies of astrophysical black holes. The session will include discussions of theoretical models for black hole solutions existing in GR and in alternative theories of gravity. Strong gravity effects and opportunities to test theoretical predictions with current and future observational facilities such as Keck, VLT(GRAVITY), The Event Horizon Telescope, JWST, E-ELT and TMT will be discussed. The main goal of the session is to establish an effective interaction between observers and theorists working in black hole astrophysics to test GR in a strong gravitational field limit and constrain alternative theories of gravity with current and future observations of astrophysical black holes.

Assuming only the existence of an apparent horizon and its regularity, we derive universal properties of the near-horizon geometry of spherically symmetric black holes. General relativity admits only two distinct classes of physical black holes, and both appear at different stages of the black hole formation. Using a self-consistent semiclassical approach, we find that the resulting...

We take on an extensive study of the rotating hairy Kerr black holes, which encompasses, in particular cases, the Kerr black hole ($\alpha=0$). We investigate ergosphere and shadows of the black holes to infer that their size and shape are affected due to the $l_0$ and are found to harbour a richer chaotic structure. In particular, the hairy Kerr black holes possess smaller size but more...

We provide a prescription to compute the gravitational multipole moments of compact objects for asymptotically de Sitter spacetimes. Our prescription builds upon a recent definition of the gravitational multipole moments in terms of Noether charges associated to specific vector fields, within the residual harmonic gauge, dubbed multipole symmetries. We first derive the multipole symmetries for...

The direct detection of gravitational wave (GW) from binary black hole (BBH) mergers has set a strong evidence for the general theory of relativity. These observations have enabled researchers to look for various aspects of black hole spacetimes; Gravitational wave memory (GW-memory) is one of such physical effects which has not been detected yet. The GW-memory manifests a permanent...

Accretion mechanism is one of the most efficient processes by which gravitational potential energy of matter can be converted into energy. This phenomenon provides us with an explanation regarding the huge amount of energy liberated and high luminosities observed in AGN's, X-ray binaries, etc. Therefore, modelling these accretion flows are necessary to obtain a proper picture of the system and...

The problem of calculating of redshift of electromagnetic spectrum of the star, moving in the vicinity of Schwarzschild black hole is solved in the framework of General Theory of Relativity. The inverse problem: determining of the parameters of motion of a star from observational data of redshift is considered. The approach that gives possibilities to solve the inverse problem is proposed. The...

When two black holes merge, the late stage of gravitational wave emission is a superposition of exponentially damped sinusoids. According to the black hole no-hair theorem, this ringdown spectrum depends only on the mass and angular momentum of the final black hole. An observation of more than one ringdown mode can test this fundamental prediction of general relativity. Here we provide strong...

Intrinsic Gravitational Modes (IGM) involving electromagnetic field fluctuations are found that are sustained by the time-dependent tridimensional gravitational field of Black Hole binaries when their collapse is approached. These “disk-rippling” modes, emerging from a plasma disk structure surrounding a binary, have ballooning amplitude profiles in the "vertical" direction (referring to the...

The GRAVITY instrument at the Very Large Telescope (VLT) has transformed the Galactic centre into a laboratory to test the strong field regime of gravity theories. The supermassive black hole in the center of the Milky Way (Sgr A*) is, at a distance of 8kpc, the closest of its kind and the largest in the sky. It is surrounded by a nuclear cluster of high velocity stars called S-stars, whose...

The occurence of infinite center of mass energies in particle collisions close to the horizon of an extremal Kerr black hole was first presented by Banados, Silk and West (BSW) in 2009. For their scenario, the rotation and the extremality of the black hole are key factors. Since their seminal paper, this phenomenon was studied for a large variety of spacetimes and for different particle...

If two particles collide near a rotating black hole, their energy in the centre of mass frame E_c.m. can become unbounded under certain conditions. In doing so, the Killing energy E of debris at infinity is, in general, remain restricted. If E is also unbounded, this is called the super-Penrose process. We elucidate when such a process is possible and give full classification of corresponding...

The gravitational three-body problem has a long history, extending all the way back to Sir Isaac Newton. In spite of hundreds of years of research, we still do not have a complete solution to the general case, where no restrictions are placed on the nature of the interaction. Historically, this has been attributed to the appearance of chaos in large regions of parameter space, implying that...

We reveal three new theorems on black hole rotation previously unexplored in the Hawking era. These results are based on the quasi-local energy investigation of the black hole in Kerr spacetime.

- The
*Horizon Mass Theorem*states that the mass at the event horizon of any

black hole is always twice its irreducible mass. In particular, the irreducible

mass does not contain...

By using a general relativistic approach to study Schwarzschild black hole (BH) rotation curves, we reveal the detection of the gravitational and the kinematic boosted redshifts in the strong gravitational regime of the Active Galactic Nucleus of NGC 4258, and estimate its BH mass-to-distance ratio in terms of astrophysical observable quantities.

The total relativistic redshift/blueshift...