Effects Of Strong Gravity Research Articles

Binary mergers in strong gravity background of Kerr black hole

ABSTRACT Binary-black-hole (BBH) mergers can take place close to a supermassive black hole (SMBH) while being in a bound orbit around the SMBH. In this paper, we study such bound triple systems and show that including the strong gravity effects of describing the SMBH with a Kerr metric can significantly modify the dynamics, as compared to a Newtonian point particle description of the SMBH. We extract the dynamics of the system, using a quadrupole approximation to the tidal forces due to the SMBH. We exhibit how the gyroscope precession is built into this dynamics, and find the secular Hamiltonian by both averaging over the inner and outer orbits, the latter being the orbit of the BBH around the SMBH. We study the long-time-scale dynamics, including the periastron precession and gravitational wave (GW) radiation reaction of the binary system, finding that the strong gravity effects of the SMBH can enhance the von Zeipel–Lidov–Kozai mechanism, resulting in more cycles, higher maximum eccentricity, and thereby a shorter merger time, particularly when the binary is close to, or at, the innermost stable orbit of the SMBH. We end with an analysis of the peak frequency of the GW emission from the binary system, highlighting possible observable signatures in the Einstein Telescope and Laser Interferometer Space Antenna frequency bands.

IXPE observation confirms a high spin in the accreting black hole 4U 1957+115

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The ∼571 ks observation reveals a linear polarisation degree of 1.9%±0.6% and a polarisation angle of −41.°8±7.°9 in the 2–8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than 50° are disfavoured.

CO2-brine interface evolution and corresponding overpressure in high-gravity CO2 storage complex

Technical and economic feasibility of large-scale CO2 geological storage projects (GCS) requires storing the maximum possible amounts of CO2 within a given pore space. However, one limitation is attributed to the overpressure that accompanies CO2 injection. Specifically, reliable estimation of the bottom-hole pressure during CO2 injection is essential to optimize the storage potential of the formation while ensuring the integrity of the rock. Several studies presented analytical/semi-analytical solutions to predict the overpressure accompanying CO2 injection. Nevertheless, they neglect the effects of gravity override on the temporal evolution of the plume and/or the bottom-hole pressure. Effects of gravity can be quantified by the dimensionless group “gravity number” which measures the relative importance of the gravitational-to-viscous forces within system. A lower gravity number expresses a more uniform/cylindrical displacement of CO2. Conversely, biasness of CO2 flow towards the top of the injection zone translates to a larger gravity number. The main objective of this work is to develop an analytical model able to predict the bottom-hole pressure during CO2 injection through a vertical well centered in the middle of a high-gravity thick saline aquifer. While accounting for the effects of gravity, the proposed solution will be developed assuming vertical equilibrium of pressure with a sharp interface separating the injected CO2 and the in-situ brine. First, we develop a closed-form analytical solution to estimate the evolution of CO2/brine interface considering strong gravity effects. The closed-form solution is based on extending the semi-analytical and/or the iterative expressions previously derived in the literature to predict the evolution of the plume during the injection period. Then, the interface model will be coupled with the assumption of the vertical equilibrium to obtain an analytical solution for the pressure field during injection. Next, the proposed solution for the evolution of pressure will be validated against numerical simulations for cases covering wide and practical ranges of gravity numbers and mobility ratios. The solution will be also validated against real field data to determine its robustness. Predictions of the overpressure from our analytical solution indicate a reasonable agreement with the simulations performed using a more complicated multi-phase flow simulator.

Spin Precession in the Gravity Wave Analogue Black Hole Spacetime

It was predicted that the spin precession frequency of a stationary gyroscope shows various anomalies in the strong gravity regime if its orbit shrinks, and eventually, its precession frequency becomes arbitrarily high very close to the horizon of a rotating black hole. Considering the gravity waves of a flowing fluid with a vortex in a shallow basin, which acts as a rotating analogue black hole, one can observe the predicted strong gravity effect on the spin precession in the laboratory. Attaching a thread with the buoyant particles and anchoring it to the bottom of the fluid container with a short-length miniature chain, one can construct a simple local test gyroscope to measure the spin precession frequency in the vicinity of the gravity wave analogue black hole. The thread acts as the axis of the gyroscope. By regulating the orbital frequency of the test gyroscope, one can also measure the strong gravity Lense–Thirring effect and geodetic/de-Sitter effect with this experimental set-up as the special cases. For example, to measure the Lense–Thirring effect, the length of the miniature chain can be set to zero, so that the gyroscope becomes static. One can also measure the geodetic precession with this system by orbiting the test gyroscope in the so-called Keplerian frequency around the non-rotating analogue black hole that can be constructed by making the rotation of the fluid/vortex negligible compared to its radial velocity.

Constraints on Cosmic Ray Acceleration Capabilities of Black Holes in X-ray Binaries and Active Galactic Nuclei

Rotating black holes (BHs) are likely the largest energy reservoirs in the Universe as predicted by BH thermodynamics, while cosmic rays (CRs) are the most energetic among particles detected on Earth. Magnetic fields surrounding BHs combined with strong gravity effects, thanks to the spacetime symmetries, turn the BHs into powerful accelerators of charged particles. At the same time, in the age of multi-wavelength and multi-messenger astronomy, BHs and their environments have not yet been probed with CR messengers, despite being observed across most of the electromagnetic spectrum, and neutrino and gravitational waves. In this paper, we probe the acceleration capabilities of BHs in 8 galactic X-ray binaries and 25 local active galactic nuclei (AGNs) within 100 Mpc, based on the ultra-efficient regime of the magnetic Penrose process of a BH energy extraction combined with observational data. We find that the maximum energy of the galactic BHs can reach only up to the knee of the CR spectrum, including supermassive BH Sgr A* at the Galactic Center. On the other hand, for supermassive BHs in AGNs, we find that the mean energy of primary CRs is of the order of 1019 eV. It is therefore likely that local supermassive BHs give sufficient contribution to the ankle—a sharp change in the slope of the cosmic ray spectrum around 1018.6 eV energy. We also discuss the energy losses of primary CRs close to the acceleration zones. In the galactic BH cases, it is likely dominated by synchrotron radiation losses.

Electromagnetic signatures of strong-field gravity from accreting black-holes

Observations of galactic nuclei help us to test General Relativity. Whereas the No-hair Theorem states that classical, isolated black holes eventually settle to a stationary state that can be characterized by a small number of parameters, cosmic black holes are neither isolated nor steady. Instead, they interact with the environment and evolve on vastly different time-scales. Therefore, the astrophysically realistic models require more parameters, and their values likely change in time. New techniques are needed in order to allow us to obtain independent constraints on these additional parameters. In this context, non-electromagnetic messengers have emerged and a variety of novel electromagnetic observations is going to supplement traditional techniques in the near future. In this outline, we summarize several fruitful aspects of electromagnetic signatures from accretion disks in strong-gravity regime in the outlook of upcoming satellite missions and ground-based telescopes. As an interesting example, we mention a purely geometrical effect of polarization angle changes upon light propagation, which occurs near the black hole event horizon. Despite that only numerical simulations can capture the accretion process in a realistic manner, simplified toy-models and semi-analytical estimates are useful to understand complicated effects of strong gravity near the event horizon of a rotating black hole, and especially within the plunging region below the innermost stable circular orbit.

Tidal deformability of neutron stars with exotic particles within a density dependent relativistic mean field model in R-squared gravity

There is a growing interest in investigating modified theories of gravity, primarily, with the aim of explaining the universe's accelerated expansion, which has been confirmed by several independent observations. Compact objects, like neutron stars, exhibit strong gravity effects and therefore are used to study modified gravity theories. We use the f(R)=R+aR 2 model, where R is the Ricci scalar and a is a free parameter. This model has been studied both perturbatively and non-perturbatively. However, it was found that perturbative methods results in nonphysical solutions for the neutron stars. In this paper, we examine neutron star properties, such as mass, radius, tidal deformability in non-perturbative f(R) gravity model with density dependent relativistic equation of state with different particle compositions. The strange particles in the core of NS in the form of Λ hyperons, K - condensate, and quarks are considered. We have observed that while the mass-radius relation allows for a wide range of parameter a, when tidal deformability is considered, the parameter a is constrained down by one order.

Tidal excitation as mixing in thermal CFT

We use mixed correlators in thermal CFT as clean probes of the strong gravity effects in their holographic duals. The dual interpretation of mixing is an inelastic conversion of one field to another field, induced by gravity: tidal excitation. We find an enhanced mixing at high temperatures, corresponding to large AdS black holes, concentrated to small boundary momenta, dual to the deep bulk, where strong gravitational fields are expected. We also find large mathcal{O}left(1/{G}_Nright) tidal conversion in the low temperature phase of the U(N) vector model, strengthening suspicions that the bulk dual of this phase also houses extremely compact objects.

Nonlinear gravitational-wave memory from cusps and kinks on cosmic strings

The nonlinear memory effect is a fascinating prediction of general relativity (GR), in which oscillatory gravitational-wave (GW) signals are generically accompanied by a monotonically-increasing strain which persists in the detector long after the signal has passed. This effect is directly accessible to GW observatories, and presents a unique opportunity to test GR in the dynamical and nonlinear regime. In this article we calculate, for the first time, the nonlinear memory signal associated with GW bursts from cusps and kinks on cosmic string loops, which are an important target for current and future GW observatories. We obtain analytical waveforms for the GW memory from cusps and kinks, and use these to calculate the ‘memory of the memory’ and other higher-order memory effects. These are among the first memory observables computed for a cosmological source of GWs, with previous literature having focused almost entirely on astrophysical sources. Surprisingly, we find that the cusp GW signal diverges for sufficiently large loops, and argue that the most plausible explanation for this divergence is a breakdown in the weak-field treatment of GW emission from the cusp. This shows that previously-neglected strong gravity effects must play an important rôle near cusps, although the exact mechanism by which they cure the divergence is not currently understood. We show that one possible resolution is for these cusps to collapse to form primordial black holes (PBHs); the kink memory signal does not diverge, in agreement with the fact that kinks are not predicted to form PBHs. Finally, we investigate the prospects for detecting memory from cusps and kinks with current and future GW observatories, considering both individual memory bursts and the contribution of many such bursts to the stochastic GW background. We find that in the scenario where the cusp memory divergence is cured by PBH formation, the memory signal is strongly suppressed and is not likely to be detected. However, alternative resolutions of the cusp divergence may in principle lead to much more favourable observational prospects.

The high energy Universe at ultra-high resolution: the power and promise of X-ray interferometry

We propose the development of X-ray interferometry (XRI), to reveal the Universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 μ as resolution at 10 Å (1.2 keV) and 20 μ as at 2 Å (6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH ‘shadow’; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within sim 100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable Universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft ‘constellation’ interferometer would resolve well below 1 μ as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole.

Strong gravity effects of charged Q-clouds and inflating black holes

In this paper, we re-examine charged Q-clouds around spherically symmetric, static black holes. In particular, we demonstrate that for fixed coupling constants two different branches of charged scalar clouds exist around Schwarzschild black holes. This had not been noticed previously. We find that the new solutions possess a ‛hard wall’ at maximal possible gauge coupling. This wall separates the interior (containing the black hole horizon), in which the scalar field is trapped in the ‛false vacuum’, from the ‛true vacuum’ exterior. When taking back-reaction onto the space-time into account, we find that at maximal possible back-reaction, the black hole solutions corresponding to these two branches either become extremal black holes with diverging scalar field derivative on the horizon or inflating black holes with a second, ‛cosmological’ horizon which—outside this second horizon—correspond to extremal Reissner–Nordström black holes.

Determination of bending angle of light deflection subject to possible weak and strong quantum gravity effects

Explicit expressions for the bending angle of light deflection arising from phenomenologically deformed black hole metrics, subject to possible weak and strong quantum gravity effects, respectively, are obtained, by a highly effective method. The accuracy and effectiveness of these expressions are then illustrated by numerically solving the differential equation governing the deflection angle directly in the weak quantum-gravity effect situation.

Quantum Black Holes in the Sky

Black Holes are possibly the most enigmatic objects in our universe. From their detection in gravitational waves upon their mergers, to their snapshot eating at the centres of galaxies, black hole astrophysics has undergone an observational renaissance in the past four years. Nevertheless, they remain active playgrounds for strong gravity and quantum effects, where novel aspects of the elusive theory of quantum gravity may be hard at work. In this review article, we provide an overview of the strong motivations for why “Quantum Black Holes” may be radically different from their classical counterparts in Einstein’s General Relativity. We then discuss the observational signatures of quantum black holes, focusing on gravitational wave echoes as smoking guns for quantum horizons (or exotic compact objects), which have led to significant recent excitement and activity. We review the theoretical underpinning of gravitational wave echoes and critically examine the seemingly contradictory observational claims regarding their (non-)existence. Finally, we discuss the future theoretical and observational landscape for unraveling the “Quantum Black Holes in the Sky”.

Towards a complete description of spectra and polarization of black hole accretion discs: albedo profiles and returning radiation

ABSTRACT Accretion discs around stellarmass black holes (BHs) emit radiation peaking in the soft X-rays when the source is in the thermal state. The emerging photons are polarized and, for symmetry reasons, the polarization integrated over the source is expected to be either parallel or perpendicular to the (projected) disc symmetry axis, because of electron scattering in the disc. However, due to general relativity effects photon polarization vectors will rotate with respect to their original orientation, by an amount depending on both the BH spin and the observer’s inclination. Hence, X-ray polarization measurements may provide important information about strong gravity effects around these sources. Along with the spectral and polarization properties of radiation which reaches directly the observer once emitted from the disc, in this paper we also include the contribution of returning radiation, i.e. photons that are bent by the strong BH gravity to return again on the disc, where they scatter until eventually escaping to infinity. A comparison between our results and those obtained in previous works by different authors shows an overall good agreement, despite the use of different code architectures. We finally consider the effects of absorption in the disc material by including more realistic albedo profiles for the disc surface. Our findings in this respect show that considering also the ionization state of the disc may deeply modify the behaviour of polarization observables.

Accretion in strong field gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

Null points in the magnetosphere of a plunging neutron star

We explore the structure of a dipole-type vacuum field of a slowly rotating magnetic star near the horizon of a supermassive black hole, where the structure of field lines becomes highly distorted by effects of strong gravity. Such a situation may occur near a neutron star in the final stages of a plunging trajectory into a galactic center. We solve Maxwell's equations in the Rindler approximation for the rotating conducting source of dipolar magnetic field arbitrarily inclined with respect to the axis of rotation. For the fixed inclination angle we calculate the field including the radiative terms while in the general case we discuss the electromagnetic field considering the near-field terms only. In the latter case we investigate the emergence of magnetic null points within the vacuum magnetosphere. Null points become highly relevant in the presence of astrophysical plasma where they are connected with processes of magnetic reconnection and mass ejection.

Predicting the X-ray polarization of type 2 Seyfert galaxies

Infrared, optical and ultraviolet spectropolarimetric observations have proven to be ideal tools for the study of the hidden nuclei of type-2 active galactic nuclei (AGN) and for constraining the composition and morphology of the sub-parsec scale emission components. In this paper, we extend the analysis to the polarization of the X-rays from type-2 AGN. Combining two radiative transfer codes, we performed the first simulations of photons originating in the gravity dominated vicinity of the black hole and scattering in structures all the way out to the parsec-scale torus and polar winds. We demonstrate that, when strong gravity effects are accounted for, the X-ray polarimetric signal of Seyfert-2s carries as much information about the central AGN components as spectropolarimetric observations of Seyfert-1s. The spectropolarimetric measurements can constrain the spin of the central supermassive black hole even in edge-on AGN, the hydrogen column density along the observer's line-of-sight, and the composition of the polar outflows. However, the polarization state of the continuum source is washed out by multiple scattering, and should not be measurable unless the initial polarization is exceptionally strong. Finally, we estimate that modern X-ray polarimeters, either based on the photo-electric effect or on Compton scattering, will require long observational times on the order of a couple of mega-seconds to be able to properly measure the polarization of type-2 AGN.

Spinfoams near a classical curvature singularity

We apply the technique of spinfoam to study the space-time which, classically, contains a curvature singularity. We derive from the full covariant Loop Quantum Gravity (LQG) that the region near curvature singularity has to be of strong quantum gravity effect. We show that the spinfoam configuration describing the near-singularity region has to be of small spins $j$, in order that its contribution to the full spinfoam amplitude is nontrivial. The spinfoams in low and high curvature regions of the space-time may be viewed as in two different {phases} of covariant LQG. There should be a phase transition as the space-time described by spinfoam becomes more and more curved. A candidate of order parameter is proposed for understanding the phase transition. Moreover, we also analyze the spin-spin correlation function of spinfoam, and show the correlation is of long-range in the low curvature phase. This work is a first step toward understanding the physics of black hole and early universe from the full covariant LQG theory.

CONTROVERSY OF THE GRO J1655-40 BLACK HOLE MASS AND SPIN ESTIMATES AND ITS POSSIBLE SOLUTIONS

ABSTRACT Estimates of the black hole mass M and the dimensionless spin a in the microquasar GRO J1655-40 implied by strong gravity effects related to the timing and spectral measurements are controversial, if the mass restriction determined by the dynamics related to independent optical measurements, M opt = (5.4 ± 0.3) M ⊙, is applied. The timing measurements of twin high-frequency (HF) quasiperiodic oscillations (QPOs) with the frequency ratio 3:2 and the simultaneously observed low-frequency (LF) QPO imply spin in the range if models based on the frequencies of geodesic epicyclic motion are used to fit the timing measurements, and the correlated creation of the twin HF QPOs and the LF QPO at a common radius is assumed. On the other hand, the spectral continuum method implies , and the Fe-line-profile method implies . This controversy can be cured if we abandon the assumption of the occurrence of the twin HF QPOs and the simultaneously observed LF QPO at a common radius. We demonstrate that the epicyclic resonance model of the twin HF QPOs is able to predict the spin in agreement with the Fe-profile method, but no model based on the geodesic epicyclic frequencies can be in agreement with the spectral continuum method. We also show that the non-geodesic string loop oscillation model of twin HF QPOs predicts spin a > 0.3 under the optical measurement limit on the black hole mass, which is in agreement with both the spectral continuum and Fe-profile methods.

Minimum X‐ray source size of the on‐axis corona in AGN

AbstractThe “lamppost” model is often used to describe the X‐ray source geometry in AGN, where an infinitesimal point source is located on the black hole spin axis. This is especially invoked for narrow line Seyfert 1 (NLS1) galaxies, where an extremely broad iron line seen in episodes of low X‐ray flux can both be explained by extremely strong relativistic effects as the source approaches the black hole horizon. The most extreme spectrum seen from the NLS1 1H0707‐495 requires that the source is less than 1 GM /c2 above the event horizon in this geometry. However, the source must also be large enough to intercept sufficient seed photons from the disc to make the hard X‐ray Compton continuum which produces the observed iron line/reflected spectrum. We use a fully relativistic ray tracing code to show that this implies that the source must be substantially larger than 1GM /c2 in 1H0707‐495 if the disc is the source of seed photons. Hence the source cannot fit as close as 1 GM /c2 to the horizon, so the observed spectrum and variability are not formed purely by effects of strong gravity but probably also by changes in corona and inner accretion flow geometry. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)