Compact Objects Research Articles

Hanging on the cliff: Extreme mass ratio inspiral formation with local two-body relaxation and post-Newtonian dynamics

Extreme mass ratio inspirals (EMRIs) are anticipated to be primary gravitational wave sources for the Laser Interferometer Space Antenna (LISA). They form in dense nuclear clusters when a compact object is captured by the central massive black holes (MBHs) as a consequence of the frequent two-body interactions occurring between orbiting objects. The physics of this process is complex and requires detailed statistical modelling of a multi-body relativistic system. We present a novel Monte Carlo approach to evolving the post-Newtonian (PN) equations of motion of a compact object orbiting an MBH. The approach accounts for the effects of two-body relaxation locally on the fly, without leveraging on the common approximation of orbit-averaging. We applied our method to study the function S(a_0), describing the fraction of EMRI to total captures (including EMRIs and direct plunges, DPs) as a function of the initial semi-major axis a_0 for compact objects orbiting central MBHs with M_ M M sun . The past two decades have consolidated a picture in which S(a_0)→ 0 at large initial semi-major axes, with a sharp transition from EMRIs to DPs occurring around a critical scale a_ c. A recent study challenges this notion for low-mass MBHs, finding EMRIs forming at a≫ c which were called `cliffhangers'. Our simulations confirm the existence of cliffhanger EMRIs, which we find to be more common then previously inferred. Cliffhangers start to appear for M_∙ M and can account for up to 55% of the overall EMRIs forming at those masses. We find S(a_0) ≫ 0 for a ≫ c M much higher than previously found. We test how these results are influenced by different assumptions on the dynamics used to evolve the system and treatment of two-body relaxation. We find that the PN description of the system greatly enhances the number of EMRIs by shifting a_ c to larger values at all MBH masses. Conversely, the local treatment of relaxation has a mass-dependent impact, significantly boosting the number of cliffhangers at low MBH masses compared to an orbit-averaged treatment. These findings highlight the shortcomings of standard approximations used in the EMRI literature and the importance of carefully modelling the (relativistic) dynamics of these systems. The emerging picture is more complex than previously thought, and should be considered in future estimates of rates and properties of EMRIs detectable by LISA.

Cyclical accretion regime change in the slow X-ray pulsar 4U 0114+65 observed with Chandra

4U 0114+65 is a high-mass X-ray binary system formed by the luminous supergiant B1Ia, known as V V662 Cas, and one of the slowest rotating neutron stars (NSs) with a spin period of about 2.6 hours. This provides a rare opportunity to study interesting details of the accretion within each individual pulse of the compact object. For this paper we analyzed 200 ks of Chandra grating data, divided into nine uninterrupted observations around the orbit. The changes in the circumstellar absorption column through the orbit suggest an orbital inclination of ∼ 40^∘ with respect to the observer and a companion mass-loss rate of ∼ 8.6 10^-7 M yr^-1. The peaks of the NS pulse exhibit a large pulse-to-pulse variability. Three of them show an evolution from a brighter regime to a weaker one. We propose that the efficiency of Compton cooling in this source fluctuates throughout an accumulation cycle. After significant depletion of matter within the magnetosphere, since the settling velocity is ∼ 2 lower than the free-fall velocity, the source gradually accumulates matter until the density exceeds a critical threshold. This increase in density triggers a transition to a more efficient Compton cooling regime, leading to a higher mass accretion rate and consequently to an increased brightness.

Uniform Modeling of Observed Kilonovae: Implications for Diversity and the Progenitors of Merger-driven Long Gamma-Ray Bursts

Abstract We present uniform modeling of eight kilonovae, five following short gamma-ray bursts (GRBs; including GRB 170817A) and three following long GRBs. We model their broadband afterglows to determine the relative contributions of afterglow and kilonova emission. We fit the kilonovae using a three-component model in MOSFiT, and report population median ejecta masses for the total, blue (κ B = 0.5 cm2 g−1), purple (κ P = 3 cm2 g−1), and red (κ R = 10 cm2 g−1) components. The kilonova of GW170817 is near the sample median in most derived properties. We investigate trends between the ejecta masses and the isotropic-equivalent and beaming-corrected γ-ray energies (E γ,iso, E γ ), as well as rest-frame durations (T 90,rest). We find long GRB kilonovae have higher median red ejecta masses (M ej,R ≳ 0.05 M ⊙) compared to on-axis short GRB kilonovae (M ej,R ≲ 0.02 M ⊙). We also observe a weak scaling between the total and red ejecta masses with E γ,iso and E γ , though a larger sample is needed to establish a significant correlation. These findings imply a connection between merger-driven long GRBs and larger tidal dynamical ejecta masses, which may indicate that their progenitors are asymmetric compact object binaries. We produce representative kilonova light curves, and find that the planned depths and cadences of the Rubin and Roman Observatory surveys will be sufficient for order-of-magnitude constraints on M ej,B (and, for Roman, M ej,P and M ej,R) of future kilonovae at z ≲ 0.1.

GRB 220831A: a hostless, intermediate Gamma-ray burst with an unusual optical afterglow

Abstract GRB 220831A is a gamma-ray burst (GRB) with a duration and spectral peak energy that places it at the interface between the distribution of long-soft and short-hard GRBs. In this paper, we present the multi-wavelength follow-up campaign to GRB 220831A and its optical, near-infrared, X-ray and radio counterparts. Our deep optical and near-infrared observations do not reveal an underlying host galaxy, and establish that GRB 220831A is observationally hostless to depth, mi ≳ 26.6 AB mag. Based on the Amati relation and the non-detection of an accompanying supernova, we find that this GRB is most likely to have originated from a collapsar at z > 2, but it could also possibly be a compact object merger at z < 0.4 with a large separation distance from its host galaxy. Regardless of its origin, we show that its optical and near-infrared counterpart departs from the evolution expected from a dominated synchrotron afterglow, exhibiting a steep post-break temporal powerlaw index of $-3.83^{+0.62}_{-0.79}$, too steep to be the jet-break. By analysing a range of models, we find that the observed steep departure from forward shock closure relations is likely due to an internal process producing either a flare or a plateau.

Constructing stellar solutions with spherical symmetry through quadratic anisotropy in f(Q) gravity

This article examines anisotropic models to characterize compact stars (CSs) in the context of modified f(Q) gravity theory. To achieve this, we employ the linear functional form f(Q)=αQ+β. A physically meaningful metric potential grr is considered, and a quadratic form of anisotropy is utilized to solve the Einstein field equations in closed form. This class of solutions is then applied to characterize observed pulsars from various perspectives. In the scope of f(Q) gravity, we address the Darmois–Israel junction requirements to guarantee a smooth matching of the inner metric with the external metric (Schwarzschild (Anti-) de Sitter solution) at the boundary hypersurface. By applying these junction conditions, we determine the model parameters involved in the solutions. Additionally, this study evaluates the physical viability and dynamical stability of the solution for different values of the f(Q)-parameter α within the compact star (CS). The mass–radius relationships associated with observational constraints are analyzed for several compact stars, including Vela X-1, PSR J1614-2230, and PSR J0952-0607. The investigation indicates that the estimated radius of the compact object PSR J0952-0607, with mass 2.35±0.17M⊙, is around 15.79-0.09+0.05 km for a particular parameter value of α=2.0, and the moment of inertia for the de Sitter space is determined as 4.31×1045gcm2. The I-M curve shows greater sensitivity to the stiffness of the equation of state than the M-R curve, reinforcing our conclusion about the I-M framework’s responsiveness. Finally, we predicted the corresponding radii and moments of inertia for various values of α based on the M-R and M-I curves.

Quasistationary hair for binary black hole initial data in scalar Gauss-Bonnet gravity

Recent efforts to numerically simulate compact objects in alternative theories of gravity have largely focused on the time-evolution equations. Another critical aspect is the construction of constraint-satisfying initial data with precise control over the properties of the systems under consideration. Here, we augment the extended conformal thin sandwich framework to construct quasistationary initial data for black hole systems in scalar Gauss-Bonnet theory and numerically implement it in the open-source p code. Despite the resulting elliptic system being singular at black hole horizons, we demonstrate how to construct numerical solutions that extend smoothly across the horizon. We obtain quasistationary scalar hair configurations in the test-field limit for black holes with linear/angular momentum as well as for black hole binaries. For isolated black holes, we explicitly show that the scalar profile obtained is stationary by evolving the system in time and compare against previous formulations of scalar Gauss-Bonnet initial data. In the case of the binary, we find that the scalar hair near the black holes can be markedly altered by the presence of the other black hole. The initial data constructed here enable targeted simulations in scalar Gauss-Bonnet simulations with reduced initial transients. Published by the American Physical Society 2025

Critical parameters and equilibrium profiles for gravitational collapse with shear viscosity

Abstract In this letter, we investigate the instability around equilibrium of a poor thermal conductor with shear viscosity. The analytical formulation departs from a Boltzmann relation satisfied by the non-homogeneous density. The equilibrium condition together with the equation of state provide rescaled frequency, time, and viscosity. A viscosity parameter is then suitably identified. We find the radial profiles for the gravitational field and potential, and density at equilibrium. As a result, we obtain the critical radius and mass of the collapsing self-gravitating gas cloud as functions of the viscosity parameter. We find that shear viscosity cannot drive gravitational collapse of compact objects. Applications of our theory as a benchmark test for astrohydrocodes are addressed.

Charged anisotropic strange stars in generalized rastall gravity

Abstract In this work, we have studied about the configuration of a charged anisotropic strange star in the context of generalized Rastall Gravity. We have discussed the equation of motion for a static type symmetric and spherically space-time using the MIT Bag model. Various physical aspects, like energy density, radial pressure and tangential pressure etc., have been constructed with the help of Krori-Barua metric potentials. We have matched interior and exterior Reissner-Nordstr¨om geometry to evaluate the unknown constants of the model. Several necessary physical aspects have been examined like energy density, both radial and tangential pressure components, energy conditions, compactness, surface redshift and stability of strange stars with respect to our proposed model analytically and graphically. All graphical analyses are governed for a particular compact star 4U1820 − 30 and by two coupling parameters α and β. We have concluded that the effects of extra coupling term RT are minimum than the term R as in the original Rastall Gravity on the physical characteristics of a star. Moreover, all physical aspects satisfied their specific limits, and hence our proposed model is viable and stable.

The Puzzling Long GRB 191019A: Evidence for Kilonova Light

Abstract GRB 191019A was a long gamma-ray burst (GRB) lasting ~65 s and, as such, originally thought to be linked to a core-collapse supernova. However, even though follow-up observations identified the optical counterpart close to the bright nucleus of a nearby ancient galaxy (z = 0.248), no associated supernova was found. This led to the suggestion that the burst was caused by the merger of two compact stellar objects, likely in a dense circumnuclear environment. By using a recently developed diagnostic tool based on prompt emission temporal properties, we noticed that GRB 191019A falls among those long GRBs which are associated with compact mergers and with evidence of kilonova light. We thus reanalyzed unpublished GROND multicolor ( g ′ r ′ i ′ z ′ J H K s ) data obtained between 0.4 and 15 days posttrigger. Image subtraction confirmed the optical counterpart in all four optical bands, with GROND tracking its fading until 1.5 days postburst. Incorporating publicly available Swift-XRT data, a joint fit of an afterglow plus a kilonova model revealed a better match than an afterglow-only scenario. The resulting kilonova properties resemble those of AT2017gfo associated with the binary neutron star merger GW170817, with a total ejected mass of ~0.06 M ⊙. Contrary to previous findings inferring a high-density circumburst environment (n 0 ~ 107−8 cm−3), our analysis finds standard conditions (n 0 ~ 1 cm−3), suggesting the long duration of GRB 191019A was intrinsic rather than due to jet interaction with a dense external medium.

Stellar population and metal production in AGN disks

Abstract As gravitational wave detections increase the number of observed compact binaries (consisting of neutron stars or blacks), we begin to probe the different conditions producing these binaries. Most studies of compact remnant formation focus either on stellar collapse from the evolution of field binary stars in gas-free environments or the formation of stars in clusters where dynamical interactions capture the compact objects, forming binaries. But a third scenario exists. In this paper, we study the fate of massive stars formed, accrete gas, and evolve in the dense disks surrounding supermassive black holes. We calculate the explosions produced and compact objects formed by the collapse of these massive stars. Nucleosynthetic yields may provide an ideal, directly observable, diagnostic of the formation and fate of these stars in active galactic nuclei. We present a first study of the explosive yields from these stars, comparing these yields with the observed nucleosynthetic signatures in the disks around supermassive stars with quasars. We show that, even though these stars tend to form black holes, their rapid rotation leads to disks that can eject a considerable amount of iron during the collapse of the star. The nucleosynthetic yields from these stars can produce constraints on the number of systems formed in this manner, but further work is needed to exploit variations from the initial models presented in this paper.

A Compact And Aberration Effects-Shielded Objective Intraocular Scatter Measurement System

The measurement of the double-pass (DP) point spread function (PSF) provides an objective, non-invasive method for estimating intraocular scatter in the human eye. In this paper, we propose a compact double-pass objective intraocular scatter measurement system that eliminates the influence of aberrations. The system includes a far-field DP PSF detection channel and a Shack-Hartmann wavefront aberration detection channel, which are used to obtain the far-field DP PSF image and 7 orders Zernike aberration coefficients, respectively. The far-field DP PSF image is used to calculate the initial objective scatter index of the human eye. The aberration coefficients are used to reconstruct the DP PSF image caused by aberrations and calculate the influence coefficient of aberrations on intraocular scatter. By subtracting this influence coefficient from the initial objective scatter index (OSI0), the effect of aberrations on scatter measurement can be eliminated, resulting in an accurate objective scatter coefficient. Experimental verification showed that when the exit pupil aperture of this system was set to 4 mm and 6 mm, the measurement accuracy increased by at least 11.9% and 28.9%, respectively, compared to before eliminating the influence of aberrations. While improving the measurement accuracy, the system also keeps the device size and manufacturing costs at a low level, making it more suitable for clinical applications.

Hess J1731-347 is likely a quark star based on the density-dependent vMIT bag model

In this study, we extend the MIT bag model by incorporating the vector interaction among quarks and introducing a density-dependent bag pressure. Then we proceed to investigate the thermodynamic properties of strange quark matter (SQM) and pure up-down quark matter (udQM) in quark stars (QSs). Our findings demonstrate that the density dependence of bag pressure B(nb) and the vector interaction GV among quarks can significantly stiffen the equation of state (EOS) for both SQM and udQM which allows for the description of massive compact stars such as those observed in GW190814 and PSR J0740+6620 as plausible candidates for QSs. Ultimately, we derived a series of mass–radius relations of QS based on several combinations of (Bas, β). Our results support the hypothesis that HESS J1731-347 is a quark star.

Impact of Charge on Strange Compact Stars in Rastall Theory

Within the framework of Rastall theory, we investigate the impact of charge on the structural development of different types of spherically symmetric anisotropic stars. To do so, we present modified field equations based upon the Finch–Skea metric potentials expressed in terms of three parameters (A,B,C). These constants are determined using suitable matching conditions and observational data for compact objects which include Her X-1, SAX J 1808.4-3658, PSR J038-0842, LMC X-4 and SMC X-1. The equation of state offered by the MIT bag model for quark–gluon plasma is used to investigate the inner structure and other characteristics of these compact objects. For a fixed bag constant, B=60MeV/fm3, and two sets of the Rastall and charge parameters, ζ=0.255,0.259 and Q˜=0.2,0.7, respectively, we analyze the consistency of the matter variables in the model and other physical parameters such as energy conditions, stellar mass, compactness, and surface redshift. In addition, we assess the stability of the constructed model through two different approaches. It is found that the obtained model is physically viable and stable.

Revisiting gravitational angular momentum and mass dipole losses in the eikonal framework

Abstract We review the description of classical gravitational scatterings of two compact objects by means of the eikonal framework. This encodes via scattering amplitudes both the motion of the bodies and the gravitational-wave signals that such systems produce. As an application, we combine the next-to-leading post-Minkowskian (PM) waveform derived in the post-Newtonian (PN) limit with the 4PM static loss due to the linear memory effect to reproduce known results for the 
total angular mometum loss in the center-of-mass frame up to $\mathcal O(G^4)$ and 2.5PN order. We also provide similar expressions for the change in the system's mass dipole, discussing the subtleties related to its sensitivity to the Coulombic components of the field and to the nonlinear memory effect.

An Interior Solution for the Kerr Metric: A Novel Approach

We present a novel approach for the construction of interior solutions for the Kerr metric, extending J. Ovalle’s foundational work through ellipsoidal coordinate transformations. By deriving a physically plausible interior solution that smoothly matches the Kerr exterior metric, we analyze the energy conditions across various rotation parameters. Our findings reveal anisotropic fluid properties and energy condition behaviors in specific space-time regions, providing insights into the strong-field regime of rotating black holes. The proposed solution offers a more realistic description of rotating black hole interiors, with implications for understanding compact astrophysical objects.

Stability of the spacetime of a magnetized compact object

Stability of the spacetime of a magnetized compact object

Manifestation of antiquark nuggets in collisions with the Earth

Antiquark nuggets are hypothetical compact composite objects conjectured to account for a significant fraction of dark matter in the Universe. In contrast to quark nuggets, these objects consist of antimatter. They may remain undetected if they possess a sufficiently small cross section relative to their mass. In this paper, we investigate the allowed region in the parameter space of this model that is consistent with the observed neutrino flux from the Sun and the Earth, and the nonobservation of seismic events with specific signatures of dark matter particles. We found the allowed values of the antibaryon charge number in this model to be in the interval 2×1024<A<8×1025, while the probability of nucleon annihilation upon collisions with the antiquark core is constrained by 0.1≲κ<0.25. These values of A and κ are, however, constrained by the IceCube experiment and nonobservation of impacts of antiquark nuggets on humans. Although very large values of the antibaryon charge, A>1033, are not fully excluded by the present study, we show that they conflict with the nonobservation of rare catastrophic explosionlike events on the Earth. Published by the American Physical Society 2025

Reality of inverse cascading in neutron star crusts

The braking torque that dictates the timing properties of magnetars is closely tied to the large-scale dipolar magnetic field on their surface. The formation of this field has been a topic of ongoing debate. One proposed mechanism, based on macroscopic principles, involves an inverse cascade within the neutron star's crust. However, this phenomenon has not been observed in realistic simulations. In this study, we provide compelling evidence supporting the feasibility of the inverse cascading process in the presence of an initial helical magnetic field within realistic neutron star crusts and discuss its contribution to the amplification of the large-scale magnetic field. Our findings, derived from a systematic investigation that considers various coordinate systems, peak wavenumber positions, crustal thicknesses, magnetic boundary conditions, and magnetic Lundquist numbers, reveal that the specific geometry of the crustal domain—with its extreme aspect ratio—requires an initial peak wavenumber from small-scale structures for the inverse cascade to occur. However, this same aspect ratio confines the cascade to structures on the scale of the crust, making the formation of a large-scale dipolar surface field unlikely. Despite these limitations, the inverse cascade remains a significant factor in the magnetic field evolution within the crust and may help explain highly magnetized objects with weak surface dipolar fields, such as low-field magnetars and central compact objects.

Distinguishing Compact Objects in Extreme-Mass-Ratio Inspirals by Gravitational Waves

Extreme-mass-ratio inspirals (EMRIs) are promising gravitational-wave (GW) sources for space-based GW detectors. EMRI signals typically have long durations, ranging from several months to several years, necessitating highly accurate GW signal templates for detection. In most waveform models, compact objects in EMRIs are treated as test particles without accounting for their spin, mass quadrupole, or tidal deformation. In this study, we simulate GW signals from EMRIs by incorporating the spin and mass quadrupole moments of the compact objects. We evaluate the accuracy of parameter estimation for these simulated waveforms using the Fisher Information Matrix (FIM) and find that the spin, tidal-induced quadruple, and spin-induced quadruple can all be measured with precision ranging from 10−2 to 10−1, particularly for a mass ratio of ∼10−4. Assuming the “true” GW signals originate from an extended body inspiraling into a supermassive black hole, we compute the signal-to-noise ratio (SNR) and Bayes factors between a test-particle waveform template and our model, which includes the spin and quadrupole of the compact object. Our results show that the spin of compact objects can produce detectable deviations in the waveforms across all object types, while tidal-induced quadrupoles are only significant for white dwarfs, especially in cases approaching an intermediate-mass ratio. Spin-induced quadrupoles, however, have negligible effects on the waveforms. Therefore, our findings suggest that it is possible to distinguish primordial black holes from white dwarfs, and, under certain conditions, neutron stars can also be differentiated from primordial black holes.

Non-singular anisotropic solutions for strange star model in f(R,T,RζγTζγ) gravity theory

This article focuses on different anisotropic models within the framework of a specific modified f(R,T,RζγTζγ) gravity theory. The study adopts a static spherically symmetric spacetime to determine the field equations for two different modified models: (i) f(R,T,RζγTζγ)=R+ηRζγTζγ, and (ii) f(R,T,RζγTζγ)=R(1+ηRζγTζγ), where η is a constant parameter. To address the additional degrees of freedom in the field equations and obtain their corresponding unique solution, the Durgapal-Fuloria spacetime geometry and MIT bag model are utilized. Matching conditions are applied to determine unknown constants within the chosen spacetime geometry. We adopt a certain range of model parameters to analyze the physical characteristics of the developed models in the interior distribution of a particular compact star candidate 4U 1820-30. Energy conditions and some other tests are also implemented to ensure their viability and stability. Additionally, the disappearing radial pressure constraint is employed to find the values of the model parameter, aligning with the observed information of an array of stars. The study concludes that both of our models are well-behaved and satisfy all necessary conditions, and thus we observe them suitable for the modeling of astrophysical objects.