Testing black hole metrics with binary black hole inspirals

Abstract In this article, we explore the Rényi law constraints on black hole merger in Gauß–Bonnet (GB) gravity. Specifically, we consider the case of static solutions in five-dimensional (5D) Anti-de-Sitter (AdS) spacetime and study the constraints on merger of two equal mass black holes. We calculate the general Rényi entropy expression and utilize it to study the bounds on the final black hole mass post-merger. We study its variation with the Rényi parameter. We also compare the results with those for black holes in General Relativity (GR). We find that the GB term has a significant impact on the bounds for black hole merger. The bounds for GB gravity become weaker for the zeroth order Rényi entropy and stronger for higher order Rényi entropies in comparison to GR.

Abstract GW231123 is a merger of two black holes (BHs) with estimated masses exceeding 100 M ⊙ , making them the most massive BHs discovered to date via gravitational-wave (GW) observations. We investigate whether GW231123-like events can originate from isolated Population (Pop) III binary stars using binary population synthesis calculations. Our findings indicate that isolated Pop III binaries can produce GW231123-like events at a rate sufficient to explain the discovery of GW231123, provided that three conditions are met: (i) Pop III stars evolve with inefficient convective overshooting, (ii) the 12 C( α , γ ) 16 O rate is 2 σ lower than the standard value, and (iii) Pop III binary stars share the same orbital parameters as Pop I/II binary stars at the initial time. In contrast, GW190521—the most massive BH merger in the Gravitational Wave Transient Catalog 3—can be formed from isolated Pop III binaries even with the standard 12 C( α , γ ) 16 O rate. We demonstrate that the discovery of GW231123 is increasingly constraining the parameter ranges of single-star evolution models, under the assumption that these GW events originate from isolated binary evolution.

We present a framework for relating gravitational wave (GW) sources to the astrophysical properties of spectroscopic galaxy samples. We show how this can enable using clustering measurements of GW sources to infer the relationship between the GW sources and the astrophysical properties of their host galaxies. We accomplish this by creating mock GW catalogs from the spectroscopic Sloan Digital Sky Survey (SDSS) DR7 galaxy survey. We populate the GWs using a joint host-galaxy probability function defined over stellar mass, star formation rate (SFR), and metallicity. This probability is modeled as the product of three broken power-law distributions, each with a turnover pointmotivated by astrophysical processes governing the relation between current-day galaxy properties and binary black hole (BBH) mergers, such as galaxy quenching and BBH delay time. Given that our analysis is anchored in the specific properties and selection characteristics of the adopted galaxy sample, as well as assumptions regarding the host-galaxy probability functions and BBH merger rate prescriptions, the resulting trends should be regarded as model-dependent.Within this framework, our results show that GW bias is most sensitive to host-galaxy probability dependence on stellar mass, with increases of up to ∼𝒪(10)% relative to galaxy bias as the stellar mass pivot scale rises. We also find a notable relationship between GW bias and SFR: when the host-galaxy probability favors low-SFR galaxies, the GW bias significantly increases. In contrast, we observe no strong correlation between GW bias and metallicity. These findings suggest that the spatial clustering of GW sources is primarily driven by the stellar mass and SFR of their host galaxies and shows how GW bias measurements can inform models of the host-galaxy probability function.

Targeted Search for Eccentric Supermassive Binary Black Holes in OJ 287 and nearby Galaxy Clusters with PPTA DR3

Abstract Gravitational-wave observations of binary black hole (BH) mergers provide a novel avenue for testing massive-star evolution and the resulting BH mass spectrum. Recent population analyses under the hierarchical-merger hypothesis have provided evidence for the BH mass gap and inferred its lower edge to ∼44–68 M ⊙ . Motivated by these findings, we compute low-metallicity ( Z = 10 −5 ) helium star models with MESA and systematically explore the effect of uncertainties in the 12 C( α , γ ) 16 O and 16 O+ 16 O reaction rates on the final fate of these massive stars. Varying the 12 C( α , γ ) 16 O reaction rate from −3 σ to +3 σ , we find that the predicted BH mass gap shifts from ∼104–184 M ⊙ to ∼45–135 M ⊙ . In contrast, scaling the 16 O+ 16 O reaction rate by global factors of 0.1, 1, and 10 has only a modest effect on the lower edge of the BH mass gap (less than 5 M ⊙ ), and shifts the upper edge by more than 10 M ⊙ . Using the predictions of our models together with the literature estimates for the lower edge of the BH mass gap, we constrain the astrophysical S factor of 12 C( α , γ ) 16 O reaction at 300 keV of S 300 ≃ 137.6–263.4 keV barn.

Dual active galactic nuclei (AGNs) are expected in hierarchical galaxy evolution models, in which low-mass galaxies merge to build more massive ones. While observational evidence for dual AGNs is growing in massive galaxies, no clear detection has yet been found in the low-mass regime. We used photometry and spectroscopy from the first Euclid Quick Data Release, combined with a collection of multi-wavelength data from the Dark Energy Spectroscopic Instrument (DESI), the LOw-Frequency ARray (LOFAR) high band antenna, and counterparts in X-ray and mid-infrared catalogues to identify dual AGNs at redshift z łesssim 1. Focusing on low-mass galaxies with stellar masses below 10 10 $,M_⊙, we find nine dual AGN candidates with projected separations ranging from $∼20 to 51,kpc. We also find 49 dual AGN candidates in more massive galaxies. We derive a dual AGN fraction of 0.1% for the low-mass galaxies and estimate that these systems likely trace a population of progenitor black hole pairs that may evolve into bound binaries and eventually coalesce, emitting gravitational waves in the LISA band. These results constitute the first sample of spectroscopically confirmed dual AGN candidates in low-mass galaxies and have important implications for models in which supermassive black holes grow from lower-mass black holes located in low-mass galaxies, as well as for predictions of gravitational waves from low-mass binary black holes.

Gravitational wave spectral sirens can provide cosmological constraints by using the shape of the binary black hole (BBH) mass distribution (MD). However, the precision and accuracy of these constraints depend critically on the capturing all the MD features. In this work, we analyzed 137 BBH events from the latest GWTC-4.0 with a novel data-driven semiparametric approach based on that adaptively places knots around the most informative structures in the MD, while keeping the dimensionality of the parameter space moderate. Our flexible models resolved three distinct peaks at sim10, 18, and 33, _⊙ and are statistically preferred over standard parametric models, with Bayes factors up to 226. Because these features are correlated with H_0, the semiparametric model yielded, under different prior assumptions, 12%-21% improvement in the precision of H_0 relative to parametric models, providing H_0 = 57.8^ Bspline M +21.9 _ -20.6 , in the best case. Our results demonstrate that capturing the full complexity of the BBH mass distribution is essential for realizing the cosmological potential of spectral sirens as gravitational wave catalogs continue to grow. km/s/Mpc

Gravitational wave observations have recently revealed with high significance, and high precision, the existence of O ( 100 ) M ⊙ rapidly rotating black holes, allowing gravitational wave events to be used for the first time to probe unexplored axion parameter space using the phenomenon known as black hole superradiance. Here, we present new limits on axions using the binary black hole merger event GW231123, whose constituent black holes are among the fastest spinning observed with gravitational waves to date. We demonstrate that the most viable binary formation channels lead to conservative constraints on axion masses μ ∼ [ 0.6 − 5 ] × 10 − 13 eV and decay constants f Φ ≳ 10 14 GeV , extending existing superradiance constraints derived using x-ray observations to yet lower axion masses.

Abstract Little red dots (LRDs) are a newly identified class of broad-line active galactic nuclei (AGNs) with a distinctive V-shaped spectrum characterized by red optical and blue UV continuum emission. Their high abundance at redshifts of z ∼ 6–8 and decline at lower redshifts suggest a transient origin. We propose that the spectral shape of LRDs originates from compact binary black hole systems, in which each black hole is surrounded by a mini-disk and embedded within a larger circumbinary disk. With a binary separation of ≲10 3 Schwarzschild radii, the Wien tail of a T ≃ 5000 K blackbody spectrum at the inner edge of the circumbinary disk produces the red optical emission, while the mini-disks power the UV continuum. Binary torques carve out a gap between the circumbinary disk and the mini-disks, setting the turnover wavelength of the V-shaped spectrum around the Balmer limit. This scenario naturally reproduces LRD spectra requiring only modest dust attenuation ( A V ≲ 1 mag), resolving overestimated luminosities for LRDs in previous studies and alleviating a tension with the so-called Sołtan argument. This model predicts distinct spectral evolution as the binary orbit decays through binary disk interactions and gravitational-wave (GW) emission, linking early-stage “proto-LRD” binaries to the broader AGN population and late-stage “LRD descendants” to coalescing binaries detectable in GW experiments.

Search for precessing binary black holes in advanced LIGO’s third observing run using harmonic decomposition

Parameter estimation of eccentric massive black hole binaries with LISA and its cosmological implications

Abstract Recently discovered quasar pairs at high redshifts ( z ≳ 5) are likely precursors to supermassive black hole mergers, providing a promising window to high-redshift quasar growth mechanisms. However, the large uncertainties on their relative distances along the line of sight ( d LOS ) limit our ability to characterize quasar pairs. In this study, we explore synthetic quasar proximity zone spectra as an alternative method to constrain the line-of-sight distance of quasar pairs. We find that for small sky-plane separations ( d sky ≈ 10–100 pkpc), a simple peak-finding algorithm can easily distinguish between scenarios of d LOS ≲ 1 pMpc and ≳1 pMpc. For cases where the true d LOS ≥ 3 pMpc, the accuracy of d LOS estimation is ≈0.2 pMpc. Large sky-plane separations of d sky = 1 pMpc have larger absolute uncertainties in d LOS estimates, but the method can still easily distinguish between scenarios where d LOS ≲ 4 pMpc and ≳4 pMpc. The d LOS estimates have an uncertainty of ≈0.5 pMpc when true d LOS ≳ 4 pMpc. Our proof-of-concept study illustrates the potential use of quasar proximity zones to constrain the three-dimensional quasar pair configuration, providing an avenue to characterize quasar pairs.

Abstract While the Einstein telescope and cosmic explorer proposals for next-generation (XG), ground-based detectors promise vastly improved sensitivities to gravitational-wave signals, only joint observations are expected to enable the full scientific potential of these facilities, making timing and coordination between the efforts crucial to avoid missed opportunities. This study investigates the impact of long-term delays on the scientific capabilities of XG detector networks. We use the Fisher information formalism to simulate the performance of a set of detector networks for large, fiducial populations of binary black holes, binary neutron stars, and primordial black-hole binaries. Bootstrapping the simulated populations, we map the expected observation times required to reach a number of observations fulfilling scientific targets for key sensitivity and localization metrics across various network configurations. We also investigate the sensitivity to stochastic backgrounds. We find that purely sensitivity-driven metrics such as the signal-to-noise ratio are not strongly affected by delays between facilities. This is contrasted by the localization metrics, which are very sensitive to the number of detectors in the network and, by extension, to delayed observation campaigns for a detector. Effectively, delays in one detector behave like network-wide interruptions for the localization metrics for networks consisting of two XG facilities. We examine the impact of a supporting, current-generation detector such as LIGO India operating concurrently with XG facilities and find such an addition will greatly mitigate the negative effects of delays for localization metrics, with important consequences on multi-messenger science and stochastic searches.

Accurate models for recoil velocity distribution in black hole mergers with comparable to extreme mass-ratios and their astrophysical implications

Tidal heating effects in binary black hole mergers

Abstract The individual component spins of binary black holes (BBHs) are difficult to resolve using gravitational-wave observations but carry key signatures of the processes shaping their formation and evolution. Recent analyses have found conflicting evidence for a sub-population of black holes with negligible spin, but the Default spin magnitude population model used in LIGO-Virgo-KAGRA (LVK) analyses cannot formally accommodate an excess of systems with zero spin. In this work, we analyze several different simulated BBH populations to demonstrate that even in the face of this mismodeling, spinning and nonspinning populations can be reliably distinguished using the Default spin magnitude population model coupled with spin sorting. While typical analyses sort the binary components by their masses, sorting the components by their spin magnitudes instead offers a complementary view of the properties of individual systems consistent with equal mass and of population-level properties, given binary evolution processes like tidal-spin up that predict asymmetric spin magnitudes among the binary components. We conclude that current observations of the BBH population are inconsistent with a fully nonspinning population, but could be explained by a population with only one spinning black hole per binary or a population with up to ~80% nonspinning sources.

The Steep Redshift Evolution of the Hierarchical Binary Black Hole Merger Rate May Cause the z-χeff Correlation

We present eight direct N-body simulations with of extremely massive, initially rotating Population III star clusters with 1.01 Nbody6++GPU 10^5 stars. Our models include primordial binaries, a continuous initial mass function, differential rotation, tidal mass loss, updated fitting formulae for extremely massive metal-poor Population III stars, and general-relativistic merger recoil kicks. We assess their impact on cluster dynamics. All runs form black holes below, within, and above the pair-instability gap, with multi-generation growth. Faster-rotating clusters core-collapse earlier; post-collapse clusters host a rotating, axisymmetric subsystem of intermediate-mass black holes (IMBHs) at the centre and an expanding halo of lower-mass objects. Pair-instability supernovae and compact-object formation at ∼2-3 Myr sharply reduce total mass and a large fraction of the cluster’s angular momentum. All Population III clusters in our simulations have the gravothermal-gravogyro catastrophe phase. We confirm two of the hypothesized formation channels of galactic nucleus seed black holes: gravitational runaway mergers of black holes and of Population III stars, which core-collapse into IMBHs thereafter. A higher initial star cluster bulk rotation correlates with earlier core collapse and, in the event counts reported here, with more coalescences and collisions, as well as lower retained (compact) binary abundances. Initial bulk rotation is a primary control parameter of cluster evolution: faster rotation accelerates early angular-momentum transport, gravothermal collapse, mass segregation, and amplifies post-collapse expansion, which also favours the formation of a compact central IMBH subsystem.

The processes of splitting and merging of black holes obey the composition law generated by the Tsallis–Cirto δ = 2 statistics. The same composition law expresses the full entropy of the Reissner–Nordström black hole via the entropies of its outer and inner horizons. Here we apply this composition law to the thermodynamics of the Kerr black hole. As distinct from Reissner–Nordström black hole, where the full entropy depends only on mass M and does not depend on its charge Q , the entropy of Kerr black hole is the sum of contributions from its mass M and angular momentum J , i.e. S ( M , J ) = S ( M , 0) + 4π $$\sqrt {J(J + 1)} $$ . Here S ( M , 0) is the entropy of the Schwarzschild black hole. This demonstrates that when the Kerr black hole with J $$ \gg $$ 1 absorbs or emits a massless particle with spin s z = ±1/2, its entropy changes by |Δ S | = 2π. We also considered the quantization of entropy suggested by the toy model, in which the black hole thermodynamics is represented by the ensemble of the Planck-scale black holes – Planckons. The Tsallis–Cirto composition law is also extended to the thermodynamics of Kerr-Newman black hole and Schwarzschild–de Sitter black hole.