Binary Population Research Articles

The nature of very-faint X-ray binaries: Near-infrared spectroscopy of 1RXH J173523.7–354013 reveals a giant companion

Abstract Very-faint X-ray binaries (VFXBs) are a sub-class of black holes and neutron stars in binaries that appear to be accreting at a very low rate. In addition to providing interesting constraints on poorly understood forms of accretion, elucidating the nature of VFXBs is particularly interesting for binary evolution and population modeling. Through near-infrared (nIR) spectroscopy, we here investigate the nature of the bursting neutron star and VFXB 1RXH J173523.7–354013 (J1735), which persistently accretes at an X-ray luminosity of LX ∼ 1034 − 1035 erg s−1. Our analysis shows that the nIR emission is dominated by that of the companion star, which we find to be a late G or early K-type giant, making this the second neutron star identified as a VFXB found to have a giant companion. We discuss how several of the system properties are difficult to reconcile with a wind-fed symbiotic X-ray binary. We therefore also propose an alternative scenario wherein J1735 is a wide binary system (supported by the discovery of a 7.5 d modulation in the nIR light curves) with a quiescent luminosity of LX ∼ 1034 − 1035 erg s−1, in which the donor star is overflowing its Roche lobe. This raises the possibility that J1735 may, every century or more, exhibit very long and very bright outbursts during which it reaches accretion rates around the Eddington limit like the neutron star Z sources.

The influence of black holes on the binary population of the globular cluster Palomar 5

ABSTRACT The discovery of stellar-mass black holes (BHs) in globular clusters (GCs) raises the possibility of long-term retention of BHs within GCs. These BHs influence various astrophysical processes, including merger-driven gravitational waves and the formation of X-ray binaries. They also impact cluster dynamics by heating and creating low-density cores. Previous N-body models suggested that Palomar 5, a low-density GC with long tidal tails, may contain more than 100 BHs. To test this scenario, we conduct N-body simulations of Palomar 5 with primordial binaries to explore the influence of BHs on binary populations and the stellar mass function. Our results show that primordial binaries have minimal effect on the long-term evolution. In dense clusters with BHs, the fraction of wide binaries with periods >105 d decreases, and the disruption rate is independent of the initial period distribution. Multi-epoch spectroscopic observations of line-of-sight velocity changes can detect most bright binaries with periods below 104 d, significantly improving velocity dispersion measurements. Four BH-MS binaries in the model with BHs suggests their possible detection through the same observation method. Including primordial binaries leads to a flatter inferred mass function because of spatially unresolved binaries, leading to a better match of the observations than models without binaries, particularly in Palomar 5’s inner region. Future observations should focus on the cluster velocity dispersion and binaries with periods of 104–105 d in Palomar 5’s inner and tail regions to constrain BH existence.

Demographics of three-body binary black holes in star clusters: implications for gravitational waves

ABSTRACT To explain both the dynamics of a globular cluster and its production of gravitational waves from coalescing binary black holes, it is necessary to understand its population of dynamically formed (or, ‘three-body’) binaries. We provide a theoretical understanding of this population, benchmarked by direct N-body models. We find that N-body models of clusters on average have only one three-body binary at any given time. This is different from theoretical expectations and models of binary populations, which predict a larger number of binaries (∼5), especially for low-N clusters (∼100), or in the case of two-mass models, low number of black holes. We argue that the presence of multiple binaries is suppressed by a high rate of binary–binary interactions, which efficiently ionize one of the binaries involved. These also lead to triple formation and potentially gravitational wave captures, which may provide an explanation for the recently reported high efficiency of in-cluster mergers in models of low-mass clusters ($\lesssim 10^5\, {\rm M}_\odot)$.

The high energy X-ray probe: resolved X-ray populations in extragalactic environments

We construct simulated galaxy data sets based on the High Energy X-ray Probe (HEX-P) mission concept to demonstrate the significant advances in galaxy science that will be yielded by the HEX-P observatory. The combination of high spatial resolution imaging (<20 arcsec FWHM), broad spectral coverage (0.2–80 keV), and sensitivity superior to current facilities (e.g., XMM-Newton and NuSTAR) will enable HEX-P to detect hard (4–25 keV) X-ray emission from resolved point-source populations within ∼800 galaxies and integrated emission from ∼6,000 galaxies out to 100 Mpc. These galaxies cover wide ranges of galaxy types (e.g., normal, starburst, and passive galaxies) and properties (e.g., metallicities and star-formation histories). In such galaxies, HEX-P will: 1) provide unique information about X-ray binary populations, including accretor demographics (black hole and neutron stars), distributions of accretion states and state transition cadences; 2) place order-of-magnitude more stringent constraints on inverse Compton emission associated with particle acceleration in starburst environments; and 3) put into clear context the contributions from X-ray emitting populations to both ionizing the surrounding interstellar medium in low-metallicity galaxies and heating the intergalactic medium in the z > 8 Universe.

Connecting core galaxy properties to the massive black hole binary population

ABSTRACT We investigate how the properties of massive black hole binaries influence the observed properties of core galaxies. We compare the observed trend in stellar mass deficit as a function of total stellar mass in the core galaxy with predicted trends in IllustrisTNG. We calculate mass deficits in simulated galaxies by applying subgrid, post-processing physics based on the results of literature N-body experiments. We find that the median value of the posterior distribution for the minimum binary mass ratio capable of creating a core is 0.7. For the gas mass fraction above which a core is erased, we find a median value of 0.6. Thus, low mass ratio binaries do not contribute to core formation and gas-rich mergers must lead to star formation within the nucleus, ultimately erasing the core. Such constraints have important implications for the overall massive black hole binary population, black hole–galaxy co-evolution, and the origin of the gravitational wave background.

The Impact of Angular Momentum Loss on the Outcomes of Binary Mass Transfer

We use the rapid binary population synthesis code COMPAS to investigate commonly used prescriptions for the determination of mass transfer stability in close binaries and the orbital separations after stable mass transfer. The degree of orbital tightening during nonconservative mass transfer episodes is governed by the poorly constrained angular momentum carried away by the ejected material. Increased orbital tightening drives systems toward unstable mass transfer leading to a common envelope. We find that the fraction of interacting binaries that will undergo only stable mass transfer throughout their lives fluctuates between a few and ∼20% due to uncertainty in the angular momentum loss alone. If mass transfer is significantly nonconservative, stability prescriptions that rely on the assumption of conservative mass transfer underpredict the number of systems which experience unstable mass transfer and stellar mergers. This may substantially impact predictions about the rates of various transients, including luminous red novae, stripped-envelope supernovae, X-ray binaries, and the progenitors of coalescing compact binaries.

Population synthesis of Be X-ray binaries: metallicity dependence of total X-ray outputs

ABSTRACT X-ray binaries (XRBs) are thought to regulate cosmic thermal and ionization histories during the Epoch of Reionization and Cosmic Dawn (z ∼ 5–30). Theoretical predictions of the X-ray emission from XRBs are important for modelling such early cosmic evolution. Nevertheless, the contribution from Be-XRBs, powered by accretion of compact objects from decretion discs around rapidly rotating O/B stars, has not been investigated systematically. Be-XRBs are the largest class of high-mass XRBs (HMXBs) identified in local observations and are expected to play even more important roles in metal-poor environments at high redshifts. In light of this, we build a physically motivated model for Be-XRBs based on recent hydrodynamic simulations and observations of decretion discs. Our model is able to reproduce the observed population of Be-XRBs in the Small Magellanic Cloud with appropriate initial conditions and binary stellar evolution parameters. We derive the X-ray output from Be-XRBs as a function of metallicity in the (absolute) metallicity range Z ∈ [10−4, 0.03] with a large suite of binary population synthesis (BPS) simulations. The simulated Be-XRBs can explain a non-negligible fraction ($\gtrsim 30{{\ \rm per\ cent}}$) of the total X-ray output from HMXBs observed in nearby galaxies for Z ∼ 0.0003–0.02. The X-ray luminosity per unit star formation rate from Be-XRBs in our fiducial model increases by a factor of ∼8 from Z = 0.02 to Z = 0.0003, which is similar to the trend seen in observations of all types of HMXBs. We conclude that Be-XRBs are potentially important X-ray sources that deserve greater attention in BPS of XRBs.

Superstructural ordering in self-sorting coacervate-based protocell networks.

Bottom-up assembly of higher-order cytomimetic systems capable of coordinated physical behaviours, collective chemical signalling and spatially integrated processing is a key challenge in the study of artificial multicellularity. Here we develop an interactive binary population of coacervate microdroplets that spontaneously self-sort into chain-like protocell networks with an alternating sequence of structurally and compositionally dissimilar microdomains with hemispherical contact points. The protocell superstructures exhibit macromolecular self-sorting, spatially localized enzyme/ribozyme biocatalysis and interdroplet molecular translocation. They are capable of topographical reconfiguration using chemical or light-mediated stimuli and can be used as a micro-extraction system for macroscale biomolecular sorting. Our methodology opens a pathway towards the self-assembly of multicomponent protocell networks based on selective processes of coacervate droplet-droplet adhesion and fusion, and provides a step towards the spontaneous orchestration of protocell models into artificial tissues and colonies with ordered architectures and collective functions.

Infrared spectroscopy of SWIFT J0850.8−4219: identification of the second red supergiant X-ray binary in the Milky Way

ABSTRACT High-mass X-ray binaries hosting red supergiant (RSG) donors are a rare but crucial phase in massive stellar evolution, with only one source previously known in the Milky Way. In this letter, we present the identification of the second Galactic RSG X-ray binary SWIFT J0850.8−4219. We identify the source 2MASS 08504008−4211514 as the likely infrared counterpart with a chance coincidence probability ≈5 × 10−6. We present a $1.0{\!-\!}2.5\, \mu$m spectrum of the counterpart, exhibiting features characteristic of late-type stars and an exceptionally strong He i emission line, corroborating the identification. Based on i) the strength of the 12CO(2,0) band, ii) strong CN bandheads and absent TiO bandheads at ≈1.1 µm and iii) equivalent width of the Mg i$1.71\, \mu$m line, we classify the counterpart to be a K3–K5 type RSG with an effective temperature of 3820 ± 100 K, located at a distance of ≈12 kpc. We estimate the source X-ray luminosity to be (4 ± 1) × 1035 erg s−1, with a hard photon index (Γ < 1), arguing against a white dwarf accretor but consistent with a magnetized neutron star in the propeller phase. Our results highlight the potential of systematic near-infrared spectroscopy of Galactic hard X-ray sources in completing our census of the local X-ray binary population.

Detecting New Visual Binaries in Gaia DR3 with Gaia and Two Micron All Sky Survey (2MASS) Photometry. I. New Candidate Binaries within 200 pc of the Sun

We present a method to identify likely visual binaries in Gaia eDR3 that does not rely on parallax or proper motion. This method utilizes the various point-spread function sizes of Two Micron All Sky Survey (2MASS)/Gaia, where at <2.″5 two stars may be unresolved in 2MASS but resolved by Gaia. Due to this, if close neighbors listed in Gaia are a resolved pair, the associated 2MASS source will have a predictable excess in the J band that depends on the ΔG of the pair. We demonstrate that the expected relationship between 2MASS excess and ΔG differs for chance alignments, as compared to true binary systems, when parameters like magnitude and location on the sky are also considered. Using these multidimensional distributions, we compute the likelihood of a close pair of stars to be a chance alignment, resulting in a total(clean) catalog of 68,725(50,230) likely binaries within 200 pc with a completeness rate of ∼75%(∼64%) and contamination rate of ∼14%(∼0.4%). Within this, we find 590 previously unidentified binaries from Gaia eDR3 with projected physical separations <30 au, where 138 systems were previously identified, and for s < 10 au we find that 4 out of 15 new likely binaries have not yet been observed with high-resolution imaging. We also demonstrate the potential of our catalog to determine physical separation distributions and binary fraction estimates, from this increase in low-separation binaries. Overall, this catalog provides a good complement for the study of local binary populations by probing smaller physical separations and mass ratios, and provides prime targets for speckle monitoring.

Evidence for a Correlation between Binary Black Hole Mass Ratio and Black Hole Spins

The astrophysical origins of the binary black hole systems seen with gravitational waves are still not well understood. However, features in the distribution of black hole masses, spins, redshifts, and eccentricities provide clues into how these systems form. Much has been learned by investigating these distributions one parameter at a time. However, we can extract additional information by studying the covariance between pairs of parameters. Previous work has shown preliminary support for an anticorrelation between mass ratio q ≡ m 2/m 1 and effective inspiral spin χ eff in the binary black hole population. In this study, we test for the existence of this anticorrelation using updated data from the third gravitational-wave transient catalog and improve our copula-based framework to employ a more robust model for black hole spins. We find evidence for an anticorrelation in (q, χ eff) with 99.7% credibility. This may imply high common-envelope efficiencies, stages of super-Eddington accretion, or a tendency for binary black hole systems to undergo mass-ratio reversal during isolated evolution. Covariance in (q, χ eff) may also be used to investigate the physics of tidal spinup as well as the properties of binary black hole–forming active galactic nuclei.

Limits on scalar-induced gravitational waves from the stochastic background by pulsar timing array observations

Recently, the NANOGrav, PPTA, EPTA, and CPTA Collaborations independently reported their evidence of the Stochastic Gravitational Waves Background (SGWB). While the inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from the population of supermassive black-hole binaries (SMBHBs), the search for new physics remains plausible in this observational window. In this work, we explore the possibility of explaining such a signal by the scalar-induced gravitational waves (IGWs) in the very early universe. We use a parameterized broken power-law function as a general description of the energy spectrum of the SGWB and fit it to the new results of NANOGrav, PPTA and EPTA. We find that this approach can put constraints on the parameters of IGW energy spectrum and further yield restrictions on various inflation models that may produce primordial black holes (PBHs) in the early universe, which is also expected to be examined by the forthcoming space-based GW experiments.

Nonparametric Inference of the Population of Compact Binaries from Gravitational-wave Observations Using Binned Gaussian Processes

The observation of gravitational waves from multiple compact binary coalescences by the LIGO–Virgo–KAGRA detector networks has enabled us to infer the underlying distribution of compact binaries across a wide range of masses, spins, and redshifts. In light of the new features found in the mass spectrum of binary black holes and the uncertainty regarding binary formation models, nonparametric population inference has become increasingly popular. In this work, we develop a data-driven clustering framework that can identify features in the component mass distribution of compact binaries simultaneously with those in the corresponding redshift distribution, from gravitational-wave data in the presence of significant measurement uncertainties, while making very few assumptions about the functional form of these distributions. Our generalized model is capable of inferring correlations among various population properties, such as the redshift evolution of the shape of the mass distribution itself, in contrast to most existing nonparametric inference schemes. We test our model on simulated data and demonstrate the accuracy with which it can reconstruct the underlying distributions of component masses and redshifts. We also reanalyze public LIGO–Virgo–KAGRA data from events in GWTC-3 using our model and compare our results with those from some alternative parametric and nonparametric population inference approaches. Finally, we investigate the potential presence of correlations between mass and redshift in the population of binary black holes in GWTC-3 (those observed by the LIGO–Virgo–KAGRA detector network in their first three observing runs), without making any assumptions about the specific nature of these correlations.

On the formation of eccentric millisecond pulsars by accretion-induced collapse of massive white dwarfs

ABSTRACT Millisecond pulsars (MSPs) are believed to be old neutron stars that have undergone spin-up by the accreting material from the donor. However, the discovery of eccentric MSPs (eMSPs) in the Galactic field challenges such a scenario of producing MSP–white dwarf systems only in a circular orbit. As the orbital periods and companion masses of these eMSPs are located in a narrow range, it is reasonable to postulate that they have the same origin. Although many models have been proposed to interpret the origin of eMSPs, the origin of the narrow range of the orbital period is still an open question. The accretion-induced collapse (AIC) of the oxygen–neon white dwarf is considered to be an important pathway to form MSPs, as it is expected to result in the formation of MSPs in a circular orbit due to tidal circularization. Here we revisited this scenario using binary population synthesis including the specific circularization calculation. Our results indicate that binaries with insufficient circularization in this scenario can evolve into eMSPs. The narrow initial binary parameters required by insufficient circularization can naturally account for the narrow range of the orbital period. Although the evolution of the white dwarf AIC process is not yet well understood, the characteristic of a narrow range in the orbital period of eMSPs can still set constraints on the physics of their evolution.

Aluminium-26 production in low- and intermediate-mass binary systems

ABSTRACT Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring 26Al production within AGB stars typically focus on single-stars; however, observations show that low- and intermediate-mass stars commonly exist in binaries. We use the binary population synthesis code binary_c to explore the impact of binary evolution on 26Al yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most 26Al overproduction is for stars to enter the thermally pulsing AGB (TP-AGB) phase with small cores relative to their total masses, allowing those stars to spend abnormally long times on the TP-AGB compared to single-stars of identical mass. Our population with a binary fraction of 0.75 has an 26Al weighted population yield increase of 25 per cent compared to our population of only single-stars. Stellar-models calculated from the Mt Stromlo/Monash Stellar Structure Program, which we use to test our results from binary_c and closely examine the interior structure of the overproducing stars, support our binary_c results only when the stellar envelope gains mass after core-He depletion. Stars which gain mass before core-He depletion still overproduce 26Al, but to a lesser extent. This introduces some physical uncertainty into our conclusions as 55 per cent of our 26Al overproducing stars gain envelope mass through stellar wind accretion onto pre-AGB objects. Our work highlights the need to consider binary influence on the production of 26Al.

The evolutionary route to form planetary nebulae with central neutron star–white dwarf binary systems

ABSTRACT We present a possible evolutionary pathway to form planetary nebulae (PNe) with close neutron star (NS)–white dwarf (WD) binary central stars. By employing the binary population synthesis technique, we find that the evolution involves two common envelope evolution (CEE) phases and a core collapse supernova explosion between them that forms the NS. Later the lower mass star engulfs the NS as it becomes a red giant, a process that leads to the second CEE phase and to the ejection of the envelope. This leaves a hot horizontal branch star that evolves to become a helium WD and an expanding nebula. Both the WD and the NS power the nebula. The NS in addition might power a pulsar wind nebula inside the expanding PN. From our simulations we find that the Galactic formation rate of NS–WD PNe is $1.8 \times 10^{-5}\, {\rm yr}^{-1}$ while the Galactic formation rate of all PNe is $0.42 \, {\rm yr}^{-1}$. There is a possibility that one of the observed Galactic PNe might be a NS–WD PN, and a few NS–WD PNe might exist in the Galaxy. The central binary systems might be sources for future gravitational wave detectors like LISA, and possibly of electromagnetic telescopes.

Identify main-sequence binaries from the Chinese Space Station Telescope Survey with machine learning

ABSTRACT The statistical properties of double main sequence (MS) binaries are very important for binary evolution and binary population synthesis. To obtain these properties, we need to identify these MS binaries. In this paper, we have developed a method to differentiate single MS stars from double MS binaries from the Chinese Space Station Telescope (CSST) Survey with machine learning. This method is reliable and efficient to identify binaries with mass ratios between 0.20 and 0.80, which is independent of the mass ratio distribution. But the number of binaries identified with this method is not a good approximation to the number of binaries in the original sample due to the low detection efficiency of binaries with mass ratios smaller than 0.20 or larger than 0.80. Therefore, we have improved this point by using the detection efficiencies of our method and an empirical mass ratio distribution and then can infer the binary fraction in the sample. Once the CSST data are available, we can identify MS binaries with our trained multi-layer perceptron model and derive the binary fraction of the sample.

A multimessenger model for neutron star–black hole mergers

ABSTRACT We present a semi-analytic model for predicting kilonova light curves from the mergers of neutron stars with black holes (NSBH). The model is integrated into the mosfit platform, and can generate light curves from input binary properties and nuclear equation-of-state considerations, or incorporate measurements from gravitational wave (GW) detectors to perform multimessenger parameter estimation. The rapid framework enables the generation of NSBH kilonova distributions from binary populations, light curve predictions from GW data, and statistically meaningful comparisons with an equivalent binary neutron star (BNS) model in mosfit. We investigate a sample of kilonova candidates associated with cosmological short gamma-ray bursts, and demonstrate that they are broadly consistent with being driven by NSBH systems, though most have limited data. We also perform fits to the very well sampled GW170817, and show that the inability of an NSBH merger to produce lanthanide-poor ejecta results in a significant underestimate of the early (≲2 d) optical emission. Our model indicates that NSBH-driven kilonovae may peak up to a week after merger at optical wavelengths for some observer angles. This demonstrates the need for early coverage of emergent kilonovae in cases where the GW signal is either ambiguous or absent; they likely cannot be distinguished from BNS mergers by the light curves alone from ∼2 d after the merger. We also discuss the detectability of our model kilonovae with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST).

Can Cosmologically Coupled Mass Growth of Black Holes Solve the Mass Gap Problem?

Observations of elliptical galaxies suggest that black holes (BHs) might serve as dark energy candidates, coupled to the expansion of the Universe. According to this hypothesis, the mass of a BH could increase as the Universe expands. BH low-mass X-ray binaries (LMXBs) in the Galactic disk were born several gigayears ago, making the coupling effect potentially significant. In this work, we calculate the evolution of BH binaries with a binary population synthesis method to examine the possible influence of cosmologically coupled growth of BHs, if it really exists. The measured masses of the compact objects in LMXBs show a gap around ∼2.5–5 M ⊙, separating the most-massive neutron stars from the least-massive BHs. Our calculated results indicate that considering the mass growth seems to (partially) account for the mass gap and the formation of compact BH LMXBs, alleviating the challenges in modeling the formation and evolution of BH LMXBs with traditional theory. However, critical observational evidence like the detection of intermediate-mass BH binaries is required to test this hypothesis.

Astrophysical uncertainties in the gravitational-wave background from stellar-mass compact binary mergers

ABSTRACT We investigate the stochastic gravitational-wave background (SGWB) produced by merging binary black holes (BBHs) and binary neutron stars (BNSs) in the frequency ranges of Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo/Kagra and Laser Interferometer Space Antenna (LISA). We develop three analytical models, which are calibrated to the measured local merger rates, and complement them with three population synthesis models based on the cosmic code. We discuss the uncertainties, focusing on the impact of the BBH mass distribution, the effect of the metallicity of the progenitor stars, and the time delay distribution between star formation and compact binary merger. We also explore the effect of uncertainties in binary stellar evolution on the background. For BBHs, our analytical models predict ΩGW in the range [4 × 10−10 to 1 × 10−9] (25 Hz) and [1 × 10−12 to 4 × 10−12] (3 mHz), and between [2 × 10−10 to 2 × 10−9] (25 Hz) and [7 × 10−13 to 7 × 10−12] (3 mHz) for our population synthesis models. This background is unlikely to be detected during the LIGO/Virgo/Kagra O4 run, but could be detectable with LISA. We predict about 10 BBH and no BNS mergers that could be individually detectable by LISA for a period of observation of 4 yr. Our study provides new insights into the population of compact binaries and the main sources of uncertainty in the astrophysical SGWB.