Compact Objects Research Articles

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.

X-ray timing and spectral characteristics of compact symmetric objects

ABSTRACT Compact Symmetric Objects (CSOs) are a distinct category of jetted active galactic nuclei whose high-energy emission is not well understood. We examined the X-ray characteristics of 17 bona fide CSOs using observations from Chandra, XMM–Newton, and NuSTAR. Among the sources with XMM–Newton observations, we found two sources, J0713+4349 and J1326+3154 to show clear evidence of variations in the soft (0.3–2 keV), the hard (2–10 keV), and the total energy (0.3–10 keV) bands with the normalized excess variance (F$_{\mathrm{ var}}$) as large as 1.17$\pm$0.27. Also, the F$_{\mathrm{ var}}$ is found to be larger in the hard band relative to the soft band for J1326+3154. From the analysis of the hardness ratio (HR) with count rate, we found both sources to show a harder when brighter (HWB) trend. Similarly, in the Chandra observations, we found one source, J0131+5545, to show flux variations in the total energy band (0.5–7 keV). We discuss possible reasons for about 82 per cent of the CSOs being non-variable. From spectral analysis, carried out in a homogeneous manner, we found the existence of obscured as well as unobscured CSOs. Three CSOs, J0111+3906, J1407+2827, and J2022+6136, were found to have the intrinsic neutral hydrogen column density N$_{\rm H,z} \gt 10^{23}$ cm$^{-2}$, consistent with earlier analyses. For the majority of the CSOs, the observed hard X-ray emission is expected to be dominated by their mildly relativistic jet emission. For the sources, J0713+4349, J1347+1217, J1407+2827, J1511+0518, and J2022+6136, the confirmed detection of Fe K $\alpha$ emission line suggests a significant contribution from the disc/corona. Our results point to diverse X-ray characteristics of CSOs.

Detectability of Self-lensing Flares of White Dwarfs with Compact Companions

Binaries containing compact objects, viewed nearly edge on, can produce periodic brightening events under certain conditions on the masses, radii, and binary separation. Such flares are caused by one object gravitationally lensing another, in what is known as self-lensing flares. We present a simulation tool that efficiently reproduces the main features of self-lensing flares and facilitates a detection sensitivity analysis for various sky surveys. We estimate the detection prospects for a handful of representative surveys when searching for systems of either two white dwarfs or a white dwarf with other compact objects, i.e., neutron stars and black holes. We find only a marginal ability to detect such systems in existing surveys. However, we estimate many such systems could be detectable by surveys in the near future, including the Vera Rubin Legacy Survey of Space and Time (LSST). We provide a quantitative analysis of the detectability of double-compact object self-lensing flares across the landscape of system parameters, and a qualitative discussion of survey and follow-up approaches to distinguish such flares from confounding events, such as stellar flares, satellite glints, and cosmic rays. We estimate 0.3, 3 and 247 double white dwarf systems could be detected by Transiting Exoplanet Survey Satellite, Zwicky Transient Facility, and LSST, respectively. A similar number of systems with a neutron star or black hole companion could be detected, but we caution that the number densities of such binaries is model dependent and so are our detection estimates. Such binaries can be used to constrain models of the end states of binary evolution.

Comparison of the Origin of Short Gamma-Ray Bursts with or without Extended Emission

The merger of compact binary stars produces short gamma-ray bursts (sGRBs), involving channels such as neutron star–neutron star (BNS) and neutron star–black hole (NS–BH). The association between sGRB 170817A and gravitational wave GW170817 provides reliable evidence for the BNS channel. Some speculations suggest that sGRBs with extended emission (EE) may represent another distinct population. The offset is the distance between the GRB sky localization and the host galaxy center. We compared the offset distributions of these two types of samples (46 sGRBs with EE and 9 without EE samples) and found that they follow the same distribution. Utilizing nonparametric methods, we examined the luminosity function and formation rate of sGRBs without any assumptions. The luminosity function can be described as ψ(L0)∝L0−0.12±0.01 for L0<L0b ( ψ(L0)∝L0−0.73±0.02 and for L0>L0b ) for sGRBs without EE and ψ(L0)∝L0−0.13±0.003 for L0<L0b ( ψ(L0)∝L0−0.61±0.01 and for L0>L0b ) for sGRBs with EE. The formation rate is characterized as ρ(z) ∝ (1 + z)−3.04±0.10 for z < 1 and as ρ(z) ∝ (1 + z)−0.29±0.38 for 1 < z < 3 for sGRBs without EE, while for sGRBs with EE, it is ρ(z) ∝ (1 + z)−3.85±0.15 for z < 1 and ρ(z) ∝ (1 + z)−0.40±1.11 for 1 < z < 3. Our findings suggest no significant difference in the progenitors of sGRBs with and without EE when considered in terms of spatial offsets, formation rates, and luminosity function.

Bremsstrahlung emission from nucleon-nucleus reactions in dense medium of compact stars

Abstract Bremsstrahlung photons emitted during nucleon-nucleus reactions in compact star are investigated.&amp;#xD;Influence of density of stellar medium on intensity of emission is studied at first time in quantum approach.&amp;#xD;Bremsstrahlung model is generalized, where new term describing influence of stellar medium is added to interactions between nucleons of nucleus (in frameworks of nuclear model of deformed oscillatoric shells).&amp;#xD;Polytropic EOS, Chandrasekar EOS and Harrison-Wheeler EOS are applied for calculations.&amp;#xD;Unified EOS of neutron-star matter of Haensel and Potekhin based on FPS an SLy EOSs is used for tests.&amp;#xD;Bremsstrahlung calculations are tested on existed measurements of bremsstrahlung in the scattering of protons off the \isotope[197]{Au} nuclei at energy of proton beam of $E_{\rm p}=190$~MeV.&amp;#xD;Many properties of bremsstrahlung emitted from nuclear processes in stellar medium of compact stars are studied at first time.&amp;#xD;In particular, the spectra of photons in the scattering of protons and neutrons off \isotope[4]{He}, \isotope[8]{Be}, \isotope[12]{C}, \isotope[16]{O}, \isotope[24]{Mg}, \isotope[40]{Ca}, \isotope[56]{Fe} are estimated in dependence on density of stellar medium.&amp;#xD;Medium of white dwarfs has small influence on the bremsstrahlung emission from nuclear processes, while&amp;#xD;bremsstrahlung emission is intensive in neutron stars and it is changed essentially in dependence on stellar density and structure.

UNCOVER: Candidate Red Active Galactic Nuclei at 3 < z < 7 with JWST and ALMA

The James Webb Space Telescope (JWST) is revolutionizing our knowledge of z > 5 galaxies and their actively accreting black holes. Using the JWST Cycle 1 Treasury program Ultradeep NIRSpec and NIRCam Observations before the Epoch of Reionization (UNCOVER) in the lensing field A2744, we report the identification of a sample of little red dots at 3 < z phot < 7 that likely contain highly reddened accreting supermassive black holes. Using a NIRCam-only selection to F444W < 27.7 mag, we find 26 sources over the ∼45 arcmin2 field that are blue in F115W − F200W ∼ 0 (or β UV ∼ –2.0 for f λ ∝ λ β ), red in F200W − F444W = 1−4 (β opt ∼ +2.0), and are dominated by a point-source-like central component. Of the 20 sources with deep Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm coverage, none are detected individually or in a stack. For the majority of the sample, spectral energy distribution fits to the JWST+ALMA observations prefer models with hot dust rather than obscured star formation to reproduce the red NIRCam colors and ALMA 1.2 mm nondetections. While compact dusty star formation cannot be ruled out, the combination of extremely small sizes (〈r e 〉 ≈ 50 pc after correction for magnification), red rest-frame optical slopes, and hot dust can be explained by reddened broad-line active galactic nuclei (AGNs). Our targets have faint M 1450 ≈ −14 to −18 mag but inferred bolometric luminosities of L bol = 1043–1046 erg s−1, reflecting their obscured nature. If the candidates are confirmed as AGNs with upcoming UNCOVER spectroscopy, then we have found an abundant population of reddened luminous AGNs that are at least ten times more numerous than UV-luminous AGNs at the same intrinsic bolometric luminosity.

Stellar Physics and General Relativity

ABSTRACTThe general theory of relativity is currently established as the most precise theory of gravity supported by observations, and its application is diverse ranging from astronomy to cosmology, while its application to astrophysics has been restricted only to compact stars due to the assumption that the Newtonian approximation is sufficient for celestial bodies with medium density such as the sun. Surprisingly, the recent research of the author has implied that this long‐held assumption is not valid, and that nonperturbative effects significantly change relevant results obtained by Newtonian gravity. In particular, local physical quantities inside the sun are newly predicted to exhibit power law differently from the so‐called standard solar model. This surprising result is reviewed including brief discussion of physics behind the discrepancy and a little new application.

Analysis of Gamma-Ray Burst Closure Relationship in Multiple Wavelengths

Gamma-ray bursts (GRBs) are intense pulses of high-energy emission associated with the death of massive stars or compact objects’ coalescence. Their multiwavelength observations help verify the reliability of the standard fireball model. We analyze 14 GRBs observed contemporaneously in gamma rays by the Fermi Large Area Telescope, in X-rays by the Swift Telescope, and in the optical bands by Swift and many ground-based telescopes. We study the correlation between the spectral and temporal indices using closure relations according to the synchrotron forward-shock model in a stratified medium (n ∝ r −k ) with k ranging from 0 to 2.5. We find that the model without energy injection is preferred over the one with energy injection in all the investigated wavelengths. In gamma rays, we only explored the ν > max{ν c , ν m } (slow cooling, SC/fast cooling, FC) cooling condition (where ν c and ν m are the cooling and characteristic frequencies, namely the frequencies at the spectral break). In the X-ray and optical bands, we explored all the cooling conditions, including ν m < ν < ν c (SC), ν c < ν < ν m (FC), and SC/FC, and found a clear preference for SC for X-rays and SC/FC for optical. Within these cooling conditions, X-rays exhibit the highest rate of occurrence for the density profile with k = 0, while the optical band has the highest occurrence for k = 2.5 when considering no energy injection. Although we can pinpoint a definite environment for some GRBs, we find degeneracies in other GRBs.

Microlensing Discovery and Characterization Efficiency in the Vera C. Rubin Legacy Survey of Space and Time

Abstract The Vera C. Rubin Legacy Survey of Space and Time will discover thousands of microlensing events across the Milky Way, allowing for the study of populations of exoplanets, stars, and compact objects. We evaluate numerous survey strategies simulated in the Rubin Operation Simulations to assess the discovery and characterization efficiencies of microlensing events. We have implemented three metrics in the Rubin Metric Analysis Framework: a discovery metric and two characterization metrics, where one estimates how well the light curve is covered and the other quantifies how precisely event parameters can be determined. We also assess the characterizability of microlensing parallax, critical for detection of free-floating black hole lenses. We find that, given Rubin’s baseline cadence, the discovery and characterization efficiency will be higher for longer-duration and larger-parallax events. Microlensing discovery efficiency is dominated by the observing footprint, where more time spent looking at regions of high stellar density, including the Galactic bulge, Galactic plane, and Magellanic Clouds, leads to higher discovery and characterization rates. However, if the observations are stretched over too wide an area, including low-priority areas of the Galactic plane with fewer stars and higher extinction, event characterization suffers by &gt;10%. This could impact exoplanet, binary star, and compact object events alike. We find that some rolling strategies (where Rubin focuses on a fraction of the sky in alternating years) in the Galactic bulge can lead to a 15%–20% decrease in microlensing parallax characterization, so rolling strategies should be chosen carefully to minimize losses.

Long Plateau Doth So: How Internal Heating Sources Affect Hydrogen-rich Supernova Light Curves

Abstract Some hydrogen-rich core-collapse supernovae (SNeIIP) exhibit evidence of a sustained energy source powering their light curves, resulting in a brighter and/or longer-lasting hydrogen recombination plateau phase. We present a semi-analytic SNIIP light-curve model that accounts for the effects of an arbitrary internal heating source, considering as special cases 56Ni/56Co decay, a central engine (magnetar or accreting compact object), and shock interaction with a dense circumstellar disk. While a sustained internal power source can boost the plateau luminosity commensurate with the magnitude of the power, the duration of the recombination plateau can typically be increased by at most a factor of ∼2–3 compared to the zero-heating case. For a given ejecta mass and initial kinetic energy, the longest plateau duration is achieved for a constant heating rate at the highest magnitude that does not appreciably accelerate the ejecta. This finding has implications for the minimum ejecta mass required to explain particularly long-lasting SNe, such as iPTF14hls, and for confidently identifying rare explosions of the most massive hydrogen-rich (e.g., Population III) stars. We present a number of analytic estimates that elucidate the key features of the detailed model.

Modified Finch And Skea Stellar Model In Higher Dimensions

Within the framework of higher dimensions, we enhance the model of Pandya and Thomas and assume that the system is anisotropic in the Finch and Skea ansatz. Our model explores various physical parameters in higher dimensions, including mass, energy density, radial and transverse pressures, and the anisotropy factor. We have used graphical technique to analyse the energy conditions, equilibrium conditions, and stability across different dimensions. Furthermore, the mass of a particular compact object have shown to increase with radial parameter as space-time dimensions increase. Additionally, by generating a mass-radius (M-R) plot, we demonstrate the influence of dimensional factor on the maximum mass and radius allowed by our toy model.

Stability of compact stars in a uniform density background cloud

We are discussing a scenario where a compact star (neutron star, NS) is embedded in a thin, uniform density background cloud (a remnant cloud after a supernova or a cloud generated from the late stages of a star e.g., a planetary nebula or asymptotic red giant phases) and its effect on the stability of the compact star. Due to the thin background cloud, the spacetime geometry is minimally deformed allowing us to employ the technique of minimal geometric decoupling (MGD). Assuming a uniform background cloud density simplifies the problem, and through the MGD method, one can take Θtt=Θ>0 0$$\\end{document}]]>, where Θ is the density of the cloud. The background cloud interacts with the compact star through a coupling strength α. By varying α, one can tune the cloud density to analyze the stability of the embedded compact star. We found that for α<3×10-5, all the thermodynamic quantities are well-behaved, indicating a stable configuration. Once the coupling parameter exceeds 3×10-5, the adiabatic index drops below Γmax′, triggering a gravitational collapse. Beyond this limit of α, the pressure and speed of sound also become non-physical. At the end, we have used the M-R curve generated from the solution to determine the radii of a few compact stars, namely PSR J1614-2230, PSR J0952-0607, GW190814, and GW200210. Furthermore, we have discussed the possibility of the secondary component of GW200210 i.e. the less massive compact object with an upper mass of 3.3M⊙, which may be a stellar black hole with a Schwarzschild radius RBH=9.746 km. However, if the mass is 2.83M⊙ as observed, then its predicted minimum radius is 10.74 km, corresponding to α=0. This radius is far beyond RBH=8.357 km and therefore is most probably a massive NS in the mass gap.

LAMOST J171013+532646: A detached short-period noneclipsing hot subdwarf + white dwarf binary

pc \), with a Gaia G-band magnitude of 12.59. This renders it conducive for continuous observations. The spectral temperature is around 25164 K, in agreement with fitting results K \)) based on a spectral energy distribution. The TESS light curve displays ellipsoidal variation and Doppler beaming without eclipsing features. Through fitting the TESS light curve using the Wilson-Devinney code, we determined the masses for the sdB (\(M_1 = odot \)) and the compact object (\(M_2 = odot \)); the compact object likely is a WD. Furthermore, MESA models suggest that the sdB, with a helium core mass of 0.431 odot \) and a hydrogen envelope mass of \(1.3 odot \), is in the early helium main-sequence phase. The MESA binary evolution shows that the J1710 system is expected to evolve into a double WD system. This means that it is an important source of low-frequency gravitational waves.