Massive Compact Objects Research Articles

Prediction of mass and radii of anisotropic polytropic compact objects with Buchdahl-I metric

In this article, we discuss several compact objects (GW 190814, PSR J0952-0607, PSR J0030+0451, PSR J0740+6620, GW 170817, PSR J1614-2230, PSR J2215+5135, and 4U 1608-52) to predict their masses and radii. A generalized polytropic stellar model within the framework of general relativity is derived by employing the Buchdahl-I metric. All the physical quantities such as energy density, radial, and tangential pressure are well behaved, continuous and no singularity is observed. The obtained results are compatible with observational data for compact objects under consideration. The physical stability of this model is determined by using generalized hydrostatic equilibrium condition, energy conditions, causality conditions and Herrera’s cracking technique. We observe that our model fulfills all of the requirements for being a physically realistic model.

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.

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.

Compact object populations over cosmic time II. Compact object merger rates and masses over redshift from varying initial conditions

ABSTRACT We perform a first study of the impact of varying two components of the initial conditions in binary population synthesis of compact binary mergers – the initial mass function, which is made metallicity- and star formation rate-dependent, and the orbital parameter (orbital period, mass ratio, and eccentricity) distributions, which are assumed to be correlated – within a larger grid of initial condition models also including alternatives for the primary mass-dependent binary fraction and the metallicity-specific cosmic star formation history. We generate the initial populations with the sampling code bossa and evolve them with the rapid population synthesis code compas. We find strong suggestions that the main role of initial conditions models is to set the relative weights of key features defined by the evolution models. In the two models we compare, black hole–black hole (BHBH) mergers are the most strongly affected, which we connect to a shift from the common envelope to the stable Roche lobe overflow formation channels with decreasing redshift. We also characterize variations in the black hole–neutron star (BHNS) and neutron star–neutron star (NSNS) final parameter distributions. We obtain the merger rate evolution for BHBH, BHNS, and NSNS mergers up to $z=10$, and find a variation by a factor of $\sim 50\textnormal {--}60$ in the local BHBH and BHNS merger rates, suggesting a more important contribution from initial conditions than previously thought, and calling for a complete exploration of the initial conditions model permutations.

Slim-disk modeling reveals an accreting intermediate-mass black hole in the luminous fast blue optical transient AT2018cow

The origin of the most luminous subclass of the fast blue optical transients (LFBOTs) is still unknown. We present an X-ray spectral analysis of AT2018cow – the LFBOT archetype – using NuSTAR, Swift, and XMM-Newton data. The source spectrum can be explained by the presence of a slim accretion disk, and we find that the mass accretion rate decreases to sub–Eddington levels ≳200 days after the source’s discovery. Applying our slim-disk model to data obtained at multiple observational epochs, we constrain the mass of the central compact object in AT2018cow to be log(M•/M⊙) = 2.4−0.1+0.6 at the 68% confidence level. Our mass measurement is independent from, but consistent with, the results from previously employed methods. The mass constraint is consistent with both the tidal disruption and the black hole–star merger scenarios, if the latter model can be extrapolated to the measured black hole mass. Our work provides evidence for an accreting intermediate–mass black hole (102 − 106 M⊙) as the central engine in AT2018cow, and, by extension, in LFBOT sources similar to AT2018cow.

Bose–Einstein Condensation dark matter models generated by gravitational decoupling

A study on massive compact objects influenced by Bose–Einstein condensation (BEC) dark matter (DM) with a gravitational decoupling approach is presented within the context of f(Q) gravity, assuming a linear form of f(Q). In order to obtain physically valid solutions to the basic field equations of the decoupled systems, we take the Tolman IV potential ansatz for the seed system and the BEC-DM density profile for the new source into consideration. After that, we apply the boundary conditions on the decoupled system with Schwarzschild-de Sitter spacetime as the exterior metric. Eventually, we obtain a complete non-singular solution to the decoupled system, consisting of a strange stellar configuration with a BEC-DM density profile. All the physical properties, such as the effective energy density, pressure along the radial as well as the tangential direction, and anisotropy in the system, are rigorously investigated. Analyzing the stability conditions of the decoupled stellar configuration, we study mass–radius (M−R) relations with observational constraints related to the supermassive compact stars, such as PSR J0740+6620, PSR J2215+5135, GW190814, and GW200210, with observed masses larger than or equal to 2 M⊙. Notably, we examine the influence of DM on constraining the M−R measurements of the observed stars, which satisfy the MIT bag model equation of state as well as the condition of mimicking, i.e., ρθ=ρDM, in the background of f(Q) gravity. Interestingly, any possible conversion of massive compact stars to smaller black holes can be restrained, as the presence of a DM halo in the strange stellar system reduces the maximum mass effectively, as indicated by the M−R curves. This result regarding the maximum mass of the compact star can be associated with the astrophysical data concerning the mass gap of the events GW190814 and GW200210. Specifically, the present model predicts the radius for PSR J0740+6620 as 13.69−0.10+0.09 km, which nearly matches the NICER and XMM-Newton data.

The Evolution of Massive Binary Stars

Massive stars play a major role in the evolution of their host galaxies and serve as important probes of the distant Universe. It has been established that the majority of massive stars reside in close binaries and interact with their companion stars during their lifetimes. Such interactions drastically alter their life cycles and complicate our understanding of their evolution, but are also responsible for the production of interesting and exotic interaction products. ▪Extensive observation campaigns with well-understood detection sensitivities have enabled the conversion of observed properties into intrinsic characteristics, facilitating a direct comparison to theory.▪Studies of large samples of massive stars in our Galaxy and the Magellanic Clouds have unveiled new types of interaction products, providing critical constraints on the mass transfer phase and the formation of compact objects.▪The direct detection of gravitational waves has revolutionized the study of stellar mass compact objects, providing a new window to study massive star evolution. Their formation processes are, however, still unclear. The known sample of compact object mergers will increase by orders of magnitude in the coming decade, which is vastly outgrowing the number of stellar-mass compact objects detected through electromagnetic radiation.

Long-lived Equilibria in Kinetic Astrophysical Plasma Turbulence

Turbulence in classical fluids is characterized by persistent structures that emerge from the chaotic landscape. We investigate the analogous process in fully kinetic plasma turbulence by using high-resolution, direct numerical simulations in two spatial dimensions. We observe the formation of long-lived vortices with a profile typical of macroscopic, magnetically dominated force-free states. Inspired by the Harris pinch model for inhomogeneous equilibria, we describe these metastable solutions with a self-consistent kinetic model in a cylindrical coordinate system centered on a representative vortex, starting from an explicit form of the particle velocity distribution function. Such new equilibria can be simplified to a Gold–Hoyle solution of the modified force-free state. Turbulence is mediated by the long-lived structures, accompanied by transients in which such vortices merge and form self-similarly new metastable equilibria. This process can be relevant to the comprehension of various astrophysical phenomena, going from the formation of plasmoids in the vicinity of massive compact objects to the emergence of coherent structures in the heliosphere.

Two dynamically discovered compact object candidate binary systems from LAMOST low-resolution survey

ABSTRACT We report two binary systems, LAMOST J035540 + 381550 (hereafter J035540) and LAMOST J035916 + 400732 (hereafter J035916), identified through the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low-resolution survey (LRS). Each of these two systems contains an M-type star orbiting with a invisible compact object candidate. Follow-up spectroscopic observations of Palomar 200-in. telescope (P200) enhance radial velocity measurements. We use radial velocities from LAMOST and P200, as well as light curves from Zwicky Transient Facility (ZTF) to constrain orbital parameters. The masses of the visible M-type stars are estimated by fitting the MIST isochrones and spectral energy distributions. The mass functions for the unseen companions are: $0.22\pm 0.01\,{\rm M}_{\odot }$ for J035540 and $0.16\pm 0.01\, {\rm M}_{\odot }$ for J035916. With the orbital and stellar parameters derived above and assuming the orbital inclination is 90° (edge-on), we find that the minimum masses of the invisible companions exceeds that of the visible stars. The single-lined feature and the dynamical evidence suggest the presence of compact objects. J035540’s ZTF light curve, modelled with phoebe, yields a compact object mass of $0.70^{+0.12}_{-0.05}\, {\rm M}_{\odot }$. For J035916, ellipsoidal modulation analysis constrains the light-curve amplitude, yielding a compact object mass range of $0.57-0.90\, {\rm M}_{\odot }$. The mass estimates indicate that both are likely white dwarfs. These findings underscore the efficiency of optical time-domain surveys and dynamical methods in identifying faint, massive white dwarfs, along with other compact objects in binaries.

Exploring Gamma-Ray Burst Diversity: Clustering Analysis of the Emission Characteristics of Fermi- and BATSE-detected Gamma-Ray Bursts

Gamma-ray bursts (GRBs), often attributed to massive star collapse or binary compact object mergers, exhibit diverse emission characteristics hinting at multiple GRB classes based on various factors like progenitors, radiation mechanisms, and central engines. This study employs unsupervised clustering using the nested Gaussian mixture model algorithm to analyze data from Fermi and BATSE, identifying four classes (A–D) based on duration, spectral peak, and spectral index of time-integrated spectra of GRBs. Class proportions are approximately 70%, 10%, 3%, and 17%, respectively, with A and B comprising mostly long GRBs, C mainly short GRBs, and D encompassing both types. The classes are further assessed based on spectral index α, indicating distinct radiation mechanisms: α > −0.67 for photospheric emission, α ≤ −1.5 for fast-cooling synchrotron, and −1.5 < α ≤ −0.67 for slow-cooling synchrotron. Classes B and C align with photospheric emission, while A and D predominantly exhibit synchrotron radiation. Short GRBs are predominantly photospheric emission, whereas long GRBs tend to favor synchrotron emission. Overall, 63% of the total bursts exhibit α profiles indicative of synchrotron emission, with the remaining 37% associated with photospheric emission. Considering the limited data of kilonova and supernova associated with GRBs, classes are examined for progenitor origins, suggesting a hybrid nature for A and D, and collapsar and merger origins for B and C, respectively. This clustering analysis results in four GRB classes, which, upon investigation, reveal the diverse and complex nature of GRBs in terms of their radiation, duration, and progenitor.

The High-resolution Fe K Spectrum of Cygnus X-3

We analyze features of the Fe K spectrum of the high-mass X-ray binary Cygnus X-3. The spectrum was obtained with the Chandra High Energy Transmission Grating Spectrometer in the third diffraction order. The increased energy resolution of the third order enables us to fully resolve the Fe xxv Heα complex and the Fe xxvi Lyα lines. The emission-line spectrum shows the expected features of photoionization equilibrium, excited in the dense stellar wind of the companion star. We detect discrete emission from inner-shell transitions, in addition to absorption likely due to multiple unresolved transitions in lower ionization states. The emission-line intensity ratios observed in the range of the spectrum occupied by the Fe xxv n = 1–2 forbidden and intercombination lines suggest that there is a substantial contribution from resonantly scattered inner-shell emission from the Li- and Be-like ionization states. The Fe xxv forbidden and intercombination lines arise in the ionization zone closest to the compact object, and since they are not subject to radiative-transfer effects, we can use them in principle to constrain the radial velocity amplitude of the compact object. We infer that the results indicate a compact object mass of the order of the mass of the Wolf–Rayet companion star, but we note that the presence of resonantly scattered radiation from Li-like ions may complicate the interpretation of the He-like emission spectrum, and specifically of the radial velocity curve of the Fe xxv forbidden line.

Shadow images of compact objects in beyond Horndeski theory

A beyond Horndeski theory is considered that admits wormholes, black holes and naked singularities. In this theory the shadow images of the black holes and the exotic compact objects (ECOs), illuminated by an optically and geometrically thin disk, are investigated. The results show that the three kinds of objects cast unlike shadow images, in particular, because the different objects possess a different number of light rings. The different boundaries of the accretion disk also affect the images. This may provide further insight into the nature of the shadow images of massive compact objects.

The effect of multiple cooling channels on the formation of dark compact objects

A dissipative dark sector can result in the formation of compact objects with masses comparable to stars and planets. In this work, we investigate the formation of such compact objects from a subdominant inelastic dark matter model, and study the resulting distributions of these objects. In particular, we consider cooling from dark Bremsstrahlung and a rapid decay process that occurs after inelastic upscattering. Inelastic transitions introduce an additional radiative processes which can impact the formation of compact objects via multiple cooling channels. We find that having multiple cooling processes changes the mass and abundance of compact objects formed, as compared to a scenario with only one cooling channel. The resulting distribution of these astrophysical compact objects and their properties can be used to further constrain and differentiate between dark sectors.

Testing the nature of gravitational wave propagation using dark sirens and galaxy catalogues

The dark sirens method enables us to use gravitational wave events without electromagnetic counterparts as tools for cosmology and tests of gravity. Furthermore, the dark sirens analysis code gwcosmo can now robustly account for information coming from both galaxy catalogues and the compact object mass distribution. We present here an extension of the gwcosmo code and methodology to constrain parameterized deviations from General Relativity that affect the propagation of gravitational waves under the ΛCDM expansion history of the universe. We show results of our analysis using data from the GWTC-3 gravitational wave catalogues, in preparation for application to the O4 observing run. After testing our pipelines using the First Two Years mock data set, we reanalyse 46 events from GWTC-3, and combine the posterior for BBH and NSBH sampling results for the first time. We obtain joint constraints on H 0 and parameterized deviations from General Relativity in the Power Law + Peak BBH population model. With increased galaxy catalogue support in the future, our work sets the stage for dark sirens to become a powerful tool for testing gravity.

The Hidden Companion in J1527: A 0.69 Solar-mass White Dwarf?

Finding nearby neutron stars can probe the supernova and metal-enrichment histories near our solar system. Recently, Lin et al. reported an exciting neutron star candidate, Two Micron All Sky Survey J15274848+3536572 (hereafter J1527), with a small Gaia distance of 118 pc. They claim that J1527 harbors an unseen neutron star candidate with an unusually low mass of 0.98 ± 0.03 M ⊙. In this work, we use the Canada–France–Hawaii Telescope high-resolution spectrum to measure J1527's orbital inclination independently. Our spectral fitting suggests an orbital inclination of 63° ± 2°. Instead, by fitting a complex hybrid variability model consisting of the ellipsoidal-variation component and the starspot modulation to the observed light curve, Lin et al. obtain an orbital inclination of 45.2−0.20+0.13 degrees. We speculate that the orbital inclination obtained by the light-curve fitting is underestimated since J1527's light curves are obviously not pure ellipsoidal variations. According to our new inclination (i ∼ 63°), the mass of the unseen compact object is reduced to 0.69 ± 0.02 M ⊙, which is as massive as a typical white dwarf.

Kinetic gases in static spherically symmetric modified dispersion relations

We study the dynamics of a collisionless kinetic gas in the most general static, spherically symmetric dispersion relation. For a static, spherically symmetric kinetic gas, we derive the most general solution to these dynamics, and find that any solution is given by a one-particle distribution function which depends on three variables. For two particular solutions, describing a shell of monoenergetic orbiting particles and a purely radial inflow, we calculate the particle density as a function of the radial coordinate. As a particular example, we study a κ-Poincaré modification of the Schwarzschild metric dispersion relation and derive its influence on the particle density. Our results provide a possible route towards quantum gravity phenomenology via the observation of matter dynamics in the vicinity of massive compact objects.

Evidence for a black hole in the historical X-ray transient A 1524-61 (= KY TrA)

ABSTRACT We present Very Large Telescope spectroscopy, high-resolution imaging, and time-resolved photometry of KY TrA, the optical counterpart to the X-ray binary A 1524-61. We perform a refined astrometry of the field, yielding improved coordinates for KY TrA and the field star interloper of similar optical brightness that we locate 0.64 ± 0.04 arcsec SE. From the spectroscopy, we refine the radial velocity semi-amplitude of the donor star to K2 = 501 ± 52 km s−1 by employing the correlation between this parameter and the full width at half-maximum of the H α emission line. The r-band light curve shows an ellipsoidal-like modulation with a likely orbital period of 0.26 ± 0.01 d (6.24 ± 0.24 h). These numbers imply a mass function f(M1) = 3.2 ± 1.0 M⊙. The KY TrA de-reddened quiescent colour (r − i)0 = 0.27 ± 0.08 is consistent with a donor star of spectral type K2 or later, in case of significant accretion disc light contribution to the optical continuum. The colour allows us to place a very conservative upper limit on the companion star mass, M2 ≤ 0.94 M⊙, and, in turn, on the binary mass ratio, q = M2/M1 ≤ 0.31. By exploiting the correlation between the binary inclination and the depth of the H α line trough, we establish i = 57 ± 13 deg. All these values lead to a compact object and donor mass of $M_1 = 5.8^{+3.0}_{-2.4}$$\, M_\odot$ and M2 = 0.5 ± 0.3 M⊙, thus confirming the black hole nature of the accreting object. In addition, we estimate a distance towards the system of 8.0 ± 0.9 kpc.

Too small to fail: characterizing sub-solar mass black hole mergers with gravitational waves

The detection of a sub-solar mass black hole could yield dramatic new insights into the nature of dark matter and early-Universe physics, as such objects lack a traditional astrophysical formation mechanism. Gravitational waves allow for the direct measurement of compact object masses during binary mergers, and we expect the gravitational-wave signal from a low-mass coalescence to remain within the LIGO frequency band for thousands of seconds. However, it is unclear whether one can confidently measure the properties of a sub-solar mass compact object and distinguish between a sub-solar mass black hole or other exotic objects. To this end, we perform Bayesian parameter estimation on simulated gravitational-wave signals from sub-solar mass black hole mergers to explore the measurability of their source properties. We find that the LIGO/Virgo detectors during the O4 observing run would be able to confidently identify sub-solar component masses at the threshold of detectability; these events would also be well-localized on the sky and may reveal some information on their binary spin geometry. Further, next-generation detectors such as Cosmic Explorer and the Einstein Telescope will allow for precision measurement of the properties of sub-solar mass mergers and tighter constraints on their compact-object nature.

The orbital period, black hole mass, and distance to the X-ray transient GRS 1716-249 ( =N Oph 93)

ABSTRACT We present evidence for a 0.278(8) d ( =6.7 h) orbital period in the X-ray transient GRS 1716-249 (=N Oph 93), based on a superhump modulation detected during the 1995 mini-outburst plus ellipsoidal variability in quiescence. With a quiescent magnitude of r = 23.19 ± 0.15 N Oph 93 is too faint to warrant a full dynamical study through dedicated time-resolved spectroscopy. Instead, we apply the FWHM–K2 correlation to the disc H α emission line detected in Gran Telescopio Canarias spectra and obtain K2 = 521 ± 52 km s−1. This leads to a mass function f(M) = 4.1 ± 1.2 M⊙, thus indicating the presence of a black hole in this historic X-ray transient. Furthermore, from the depth of the H α trough and the quiescent light curve we constrain the binary inclination to i = 61 ± 15°, while the detection of superhumps sets an upper limit to the donor to compact star mass ratio q = M2/M1 ≲ 0.25. Our de-reddened (r − i) colour is consistent with a ≈ K6 main sequence star that fills its Roche lobe in a 0.278 d orbit. Using all this information we derive a compact object mass $M_{1}=6.4^{+3.2}_{-2.0}$ M⊙ at 68 per cent confidence. We also constrain the distance to GRS 1716-249 to 6.9 ± 1.1 kpc, placing the binary ∼0.8 kpc above the Galactic Plane, in support of a large natal kick.