Although the proper oscillation frequencies of black holes (quasinormal modes) and the grey-body factors, which determine the scattering properties of black holes, represent two distinct spectral problems with different boundary conditions, a recent study has revealed an intrinsic connection between these quantities. We have shown that the correspondence between grey-body factors and quasinormal modes, previously established for spherically symmetric and asymptotically flat black holes, also extends to general parametrized axially symmetric black holes. This correspondence is limited to non-superradiant waves.

Czechia

R.A. Konoplya

Brazil

A. Zhidenko

Correspondence between grey-body factors and quasinormal frequencies for rotating black holes

Recently, magnetic reconnection has attracted considerable attention as a novel energy extraction mechanism, relying on the rapid reconnection of magnetic field lines within the ergosphere. We have investigated the properties of the energy extraction via magnetic reconnection in a Konoplya–Zhidenko rotating non-Kerr black hole spacetime with an extra deformation parameter. Our results show that the positive deformation parameter expands the possible region of energy extraction and improves the maximum power, maximum efficiency, and the maximum ratio of energy extraction between magnetic reconnection and the Blandford–Znajek process. This means that in the Konoplya–Zhidenko rotating non-Kerr black hole spacetime one can extract more energy via magnetic reconnection than in the Kerr black hole case. These effects of the deformation parameter may provide valuable clues for future astronomical observations of black holes and verification of gravity theories.

China

Fen Long

Shangyun Wang

Songbai Chen

Jiliang Jing

Magnetic reconnection and energy extraction from a Konoplya–Zhidenko rotating non-Kerr black hole

In this study, we investigate the parameterized Konoplya–Rezzolla–Zhidenko (KRZ) black hole (BH) spacetime in the presence of an external asymptotically uniform magnetic field. We first examine the innermost stable circular orbit (ISCO) radii for both neutral and charged test particles, demonstrating that the deformation parameters, $$\delta _1$$ and $$\delta _2$$ , reduce the ISCO values. Subsequently, we assess the energy efficiency of the magnetic Penrose process (MPP) for an axially symmetric parameterized BH, analyzing the effects of the deformation parameters and the magnetic field on the energy extraction process. Our findings indicate that the rotational deformation parameter $$\delta _2$$ is crucial for the efficiency of energy extraction from the BH. The synergy between the rotational deformation parameter and the magnetic field significantly boosts the energy extraction efficiency, with values exceeding $$100\%$$ . Interestingly, for extremal BHs with negative $$\delta _2$$ values, the energy efficiency increases, in contrast to Kerr BHs where the MPP effect diminishes. Additionally, we explore the astrophysical implications of the MPP by deriving the maximum energy of a proton escaping from the KRZ parameterized BH due to the beta decay of a free neutron near the horizon. Our results show that negative $$\delta _2$$ values require stronger magnetic fields to achieve equivalent energy levels for high-energy protons, providing deeper insights into high-energy astrophysical phenomena around the parameterized BH.

Uzbekistan

Tursunali Xamidov

Azerbaijan

Sanjar Shaymatov

India

Pankaj Sheoran

Bobomurat Ahmedov

Astrophysical insights into magnetic Penrose process around parameterized Konoplya–Rezzolla–Zhidenko black hole

Recently, in [1], it was demonstrated that various regular black hole metrics can be derived within a theory featuring an infinite number of higher curvature corrections to General Relativity. Moreover, truncating this infinite series at the first few orders already yields a reliable approximation of the observable characteristics of such black holes [2]. Here, we further establish the existence of another regular black hole solution, particularly the D-dimensional extension of the Dymnikova black hole, within the equations of motion incorporating an infinite tower of higher-curvature corrections. This solution is essentially nonperturbative in the coupling parameter, rendering the action, if it exists, incapable of being approximated by a finite number of powers of the curvature. In addition, we compute the dominant quasinormal frequencies of such black holes using both the Bernstein polynomial method and the 13th order WKB method with Padé approximants, obtaining a high degree of agreement between them.

Dymnikova black hole from an infinite tower of higher-curvature corrections

We demonstrate that the late time oscillatory tails of massive gravitons, present in both massive theories of gravity and effectively in extra-dimensional scenarios, could potentially contribute to gravitational waves with very long wavelengths. However, their impact on recent pulsar timing array observations might be relatively small, predominantly consisting of radiation emitted by black holes in our region of the Milky Way.

Asymptotic tails of massive gravitons in light of pulsar timing array observations

The well-known correspondence between quasinormal modes of any stationary, spherically symmetric and asymptotically flat or de Sitter black hole and parameters of the circular null geodesic was initially claimed for gravitational and test field perturbations. According to this correspondence the real and imaginary parts of the <math><mi>ℓ</mi><mo>≫</mo><mi>n</mi></math> quasinormal mode (where ℓ and n are multipole and overtone numbers respectively) are multiples of the frequency and instability timescale of the circular null geodesics respectively. Later it was shown that the correspondence is guaranteed only for test fields and may be broken for gravitational and other non-minimally coupled fields. Here, we further specify the correspondence and prove that even when it is guaranteed, it may not represent the full spectrum of the <math><mi>ℓ</mi><mo>≫</mo><mi>n</mi></math> modes, missing the quasinormal frequencies which cannot be found by the standard WKB method. In particular we show that this always happens for an arbitrary asymptotically de Sitter black holes and further argue that, in general, this might be related to sensitivity of the quasinormal spectrum to geometry deformations near the boundaries.

Further clarification on quasinormal modes/circular null geodesics correspondence

Recently, Cardoso et al. [1] found an exact solution describing the black hole immersed in a galactic-like distribution of matter. There, the properties of gravitational radiation were studied. Here we continue analysis of properties of this geometry via consideration of the electromagnetic radiation. We calculate quasinormal modes, asymptotic tails and grey-body factors for electromagnetic radiation. In addition, we discuss the Unruh temperature for this spacetime. Estimations made in the regime which is best fitting the galaxies behavior show that influence of the environment on classical and quantum radiation around such black holes must be relatively small.

Russia

Black holes in galactic centers: Quasinormal ringing, grey-body factors and Unruh temperature

The spherically symmetric deformation of the Schwarzschild solution owing to the quantum corrections to gravity is known as Kazakov-Solodukhin black-hole metric. Neglecting non-spherical deformations of the background the problem was solved non-perturbatively. Here we analyze the basic characteristics of this geometry, such as: quasinormal modes and grey-body factors of fields of various spin and shadow cast by this black hole. The WKB approach and time-domain integration method, which we used for calculation of quasinormal modes, are in a good concordance. The analytical formula for quasinormal modes is deduced in the eikonal regime. The radius of shadow is decreasing when the quantum deformation is turned on.

Quantum corrected black holes: Quasinormal modes, scattering, shadows

The Einstein-Lovelock theory contains an infinite series of corrections to the Einstein term with an increasing power of the curvature. It is well-known that for large black holes the lowest (Gauss-Bonnet) term is the dominant one, while for smaller black holes higher curvature corrections become important. We will show that if one is limited by positive values of the coupling constants, then the dynamical instability of black holes serves as an effective cut-off of influence of higher curvature corrections in the 4D Einstein-Lovelock approach: the higher is the order of the Lovelock term, the smaller is the maximal value of the coupling constant allowing for stability, so that effectively only a first few orders can deform the observable values seemingly. For negative values of coupling constants this is not so, and, despite some suppression of higher order terms also occurs due to the decreasing threshold values of the coupling constant, this does not lead to an noticeable opportunity to neglect higher order corrections. In the case a lot of orders of Lovelock theory are taken into account, so that the black-hole solution depends on a great number of coupling constants, we propose a compact description of it in terms of only two or three parameters encoding all the observable values.

4 Einstein-Lovelock black holes: Hierarchy of orders in curvature

Arbitrarily long lived modes, called quasi-resonances, are known to exist in the spectrum of massive fields for a number of black-hole backgrounds at some discrete values of mass of the field. Here we show that these modes also exist in the background of wormholes, unless a wormhole has a constant red-shift function, that is, tideless in the radial direction. The evidence of quasi-resonances is supported by calculations in the frequency and time domains, which are in a good concordance. At large masses of the field, time-domain profiles of the absolute value of the wave function have peculiar behavior: the long-lived modes dominate in the signal after a long period of power-law tails.