Compact Binary Stars Research Articles

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.

The X-Ray Luminous Type Ibn SN 2022ablq: Estimates of Preexplosion Mass Loss and Constraints on Precursor Emission

Type Ibn supernovae (SNe Ibn) are rare stellar explosions powered primarily by interaction between the SN ejecta and H-poor, He-rich material lost by their progenitor stars. Multiwavelength observations, particularly in the X-rays, of SNe Ibn constrain their poorly understood progenitor channels and mass-loss mechanisms. Here we present Swift X-ray, ultraviolet, and ground-based optical observations of the Type Ibn SN 2022ablq, only the second SN Ibn with X-ray detections to date. While similar to the prototypical Type Ibn SN 2006jc in the optical, SN 2022ablq is roughly an order of magnitude more luminous in the X-rays, reaching unabsorbed luminosities L X ∼ 4 × 1040 erg s−1 between 0.2–10 keV. From these X-ray observations we infer time-varying mass-loss rates between 0.05 and 0.5 M ⊙ yr−1 peaking 0.5–2 yr before explosion. This complex mass-loss history and circumstellar environment disfavor steady-state winds as the primary progenitor mass-loss mechanism. We also search for precursor emission from alternative mass-loss mechanisms, such as eruptive outbursts, in forced photometry during the 2 yr before explosion. We find no statistically significant detections brighter than M ≈ −14—too shallow to rule out precursor events similar to those observed for other SNe Ibn. Finally, numerical models of the explosion of an ∼15 M ⊙ helium star that undergoes an eruptive outburst ≈1.8 yr before explosion are consistent with the observed bolometric light curve. We conclude that our observations disfavor a Wolf–Rayet star progenitor losing He-rich material via stellar winds and instead favor lower-mass progenitor models, including Roche-lobe overflow in helium stars with compact binary companions or stars that undergo eruptive outbursts during late-stage nucleosynthesis stages.

The DBL Survey I: discovery of 34 double-lined double white dwarf binaries

ABSTRACT We present the first discoveries of the double-lined double white dwarf (DBL) survey that targets overluminous sources with respect to the canonical white dwarf cooling sequence according to a set of well-defined criteria. The primary goal of the DBL survey is to identify compact double white dwarf binary star systems from a unique spectral detection of both stars, which then enables a precise quantification of the atmospheric parameters and radial velocity variability of a system. Our search of 117 candidates that were randomly selected from a magnitude-limited sample of 399 yielded a 29 per cent detection efficiency with 34 systems exhibiting a double-lined signature. A further 38 systems show strong evidence of being single-lined or potentially DBL binaries and seven single-lined sources from the full observed sample are radial velocity variable. The 45 remaining candidates appear as a single WD with no companion or a non-DA white dwarf, bringing the efficiency of detecting binaries to 62 per cent. Atmospheric fitting of all double-lined systems reveals a large fraction that have two similar mass components that combine to a total mass of 1.0–1.3 $\mathrm{M}_\odot$ – a class of double white dwarf binaries that may undergo a sub-Chandrasekhar mass type Ia detonation or merge to form a massive O/Ne WD, although orbital periods are required to infer on which time-scales. One double-lined system located 49 pc away, WDJ181058.67+311940.94, is super-Chandrasekhar mass, making it the second such double white dwarf binary to be discovered.

J0526+5934: A peculiar ultra-short-period double white dwarf

Context.Ultra-short-period compact binaries are important sources of gravitational waves. The class of short-period compact binaries includes, for example, the progenitors of type Ia supernovae and the progenitors of merger episodes that may lead to massive and magnetic single white dwarfs. J0526+5934 is one such example: it is an unresolved compact binary star with an orbital period of 20.5 min.Aims.The visible component of J0526+5934 was recently claimed to be a hot sub-dwarf star with a CO white dwarf companion. Our aim is to provide strong observational and theoretical evidence that the primary star is instead an extremely low-mass white dwarf, although the hot sub-dwarf nature cannot be completely ruled out.Methods.We analysed optical spectra together with time-series photometry of the visible component of J0526+5934 to constrain its orbital and stellar parameters. We also employed evolutionary sequences for low-mass white dwarfs to derive independent values of the primary mass.Results.From the analysis of our observational data, we find a stellar mass for the primary star in J0526+5934 of 0.26 ± 0.05M⊙, which perfectly matches the 0.237 ± 0.035M⊙independent measurement we derive from the theoretical evolutionary models. This value is considerably lower than the theoretically expected and generally observed mass range for hot sub-dwarf stars, but falls well within the mass limit values of extremely low-mass white dwarfs.Conclusions.We conclude J0526+5934 is the sixth ultra-short-period detached double white dwarf currently known.

Significance of Fabry-Perot Cavities for Space Gravitational Wave Antenna DECIGO

DECIGO is a future Japanese project for the detection of gravitational waves in space. To conduct various scientific missions, including the verification of cosmic inflation through the detection of primordial gravitational waves as the main objective, DECIGO is designed to have high sensitivity in the frequency band from 0.1 to 10 Hz, with arms of length 1000 km. Furthermore, the use of the Fabry-Perotcavity in these arms has been established for the DECIGO project. In this paper, we scrutinize the significance of the Fabry-Perot cavity for promoting this project, with a focus on the possibility of observing gravitational waves from cosmic inflation and binary compact star systems as indicators. The results show that using the Fabry-Perot cavity is extremely beneficial for detecting them, and it is anticipated to enable the opening of a new window in gravitational wave astronomy.

Strangelets at finite temperature: Nucleon emission rates, interface, and shell effects

We investigate the properties of strangelets at finite temperature T, where an equivparticle model is adopted with both the linear confinement and leading-order perturbative interactions accounted for using density-dependent quark masses. The shell effects are examined by solving the Dirac equations for quarks in the mean-field approximation, which diminish with temperature as the occupation probability of each single-particle levels fixed by the Fermi-Dirac statistics, i.e., shell dampening. Consequently, instead of decreasing with temperature, the surface tension extracted from a liquid-drop formula increases with T until reaching its peak at T≈20–40 MeV with vanishing shell corrections, where the formula roughly reproduces the free energy per baryon of all strangelets. The curvature term, nevertheless, decreases with T despite the presence of shell effects. The neutron and proton emission rates are fixed microscopically according to the external nucleon gas densities that are in equilibrium with strangelets, which generally increase with T (≲50 MeV) for stable strangelets but decrease for those that are unstable against nucleon emission at T=0. The energy, free energy, entropy, charge-to-mass ratio, strangeness per baryon, and root-mean-square radius of β-stable strangelets obtained with various parameter sets are presented as well. The results indicated in this work are useful for understanding the products of binary compact star mergers and heavy-ion collisions. Published by the American Physical Society 2024

The Progenitor and Central Engine of a Peculiar GRB 230307A

Recently, a lack of supernova-associated with long-duration gamma-ray burst (GRB 230307A) at such a low redshift z = 0.065, but associated with a possible kilonova emission, has attracted great attention. Its heavy element nucleosynthesis and the characteristic of soft X-ray emission suggest that the central engine of GRB 230307A is a magnetar that is originated from a binary compact star merger. The calculated lower value of ε ∼ 0.05 suggests that GRB 230307A seems to have an ambiguous progenitor. The lower value of f eff = 1.23 implies that GRB 230307A is not likely to be from the effect of “tip of iceberg.” We adopt the magnetar central engine model to fit the observed soft X-ray emission with varying efficiency and find that the parameter constraints of the magnetar falls into a reasonable range, i.e., B < 9.4 × 1015 G and P < 2.5 ms for Γsat = 103, and B < 3.6 × 1015 G and P < 1.05 ms for Γsat = 104. Whether the progenitor of GBR 230307A is from the mergers of neutron star–white dwarf (NS–WD) or neutron star–neutron star (NS–NS) remains unknown. The difference of GW radiation between NS–NS merger and NS–WD merger may be a probe to distinguish the progenitor of GRB 230307A-like events in the future.

The Precursor of GRB211211A: A Tide-induced Giant Quake?

The equilibrium configuration of a solid strange star in the final inspiral phase with another compact object is generally discussed, and the starquake-related issue is revisited, for a special purpose to understand the precursor emission of binary compact star merger events (e.g., that of GRB211211A). As the binary system inspirals inward due to gravitational wave radiation, the ellipticity of the solid strangeon star increases due to the growing tidal field of its compact companion. Elastic energy is hence accumulated during the inspiral stage which might trigger a starquake before the merger when the energy exceeds a critical value. The energy released during such starquakes is calculated and compared to the precursor observation of GRB211211A. The result shows that the energy might be insufficient for binary strangeon-star case unless the entire solid strangeon star shatters, and hence favors a black hole-strangeon star scenario for GRB211211A. The timescale of the precursor as well as the frequency of the observed quasi-periodic-oscillation have also been discussed in the starquake model.

Halted-pendulum Relaxation: Application to White Dwarf Binary Initial Data

Studying compact-star binaries and their mergers is integral to determining progenitors for observable transients. Today, compact-star mergers are typically studied via state-of-the-art computational fluid dynamics codes. One such numerical technique, smoothed particle hydrodynamics (SPH), is frequently chosen for its excellent mass, energy, and momentum conservation. The natural treatment of vacuum and the ability to represent highly irregular morphologies make SPH an excellent tool for the study of compact-star binaries and mergers. For many scenarios, including binary systems, the outcome of simulations is only as accurate as the initial conditions. For SPH, it is essential to ensure that the particles are distributed regularly, representing the initial density profile but without long-range correlations. Particle noise in the form of high-frequency local motion and low-frequency global dynamics must be damped out. Damping the latter can be as computationally intensive as the actual simulation. We discuss a new and straightforward relaxation method, halted-pendulum relaxation (HPR), to remove global oscillation modes of SPH particle configurations. In combination with effective external potentials representing gravitational and orbital forces, we show that HPR has an excellent performance in efficiently relaxing SPH particles to the desired density distribution and removing global oscillation modes. We compare the method to frequently used relaxation approaches and test it on a white dwarf binary model at its Roche-lobe overflow limit. We highlight the importance of our method in achieving accurate initial conditions and its effect on achieving circular orbits and realistic accretion rates when compared with other general relaxation methods.

Overlap reduction functions for a polarized stochastic gravitational-wave background in the Einstein Telescope-Cosmic Explorer and the LISA-Taiji networks

The detection of gravitational waves from the coalescences of binary compact stars by current interferometry experiments has opened up a new era of gravitational-wave astrophysics and cosmology. The search for a stochastic gravitational-wave background is underway by correlating signals from a pair of detectors in the detector network formed by the LIGO, Virgo, and KAGRA. In a previous work, we have developed a method based on spherical harmonic expansion to calculate the overlap reduction functions of the LIGO-Virgo-KAGRA network for a polarized stochastic gravitational-wave background. In this work, we will apply the method to calculate the overlap reduction functions of third-generation detectors such as a ground-based network linking the Einstein Telescope, the Cosmic Explorer, and the LISA-Taiji joint space mission.

DMPP-3: confirmation of short-period S-type planet(s) in a compact eccentric binary star system, and warnings about long-period RV planet detections

ABSTRACT We present additional HARPS radial velocity observations of the highly eccentric (e ∼ 0.6) binary system DMPP-3AB, which comprises a K0V primary and a low-mass companion at the hydrogen burning limit. The binary has a 507 d orbital period and a 1.2 au semimajor axis. The primary component harbours a known 2.2 M⊕ planet, DMPP-3A b, with a 6.67-d orbit. New HARPS measurements constrain periastron passage for the binary orbit and add further integrity to previously derived solutions for both companion and planet orbits. Gaia astrometry independently confirms the binary orbit and establishes the inclination of the binary is 63.89 ± 0.78°. We performed dynamical simulations that establish that the previously identified ∼800 d RV signal cannot be attributed to an orbiting body. The additional observations, a deviation from strict periodicity, and our new analyses of activity indicators suggest the ∼800 d signal is caused by stellar activity. We conclude that there may be long-period planet ‘detections’ in other systems, which are similar misinterpreted stellar activity artefacts. Without the unusual eccentric binary companion to the planet-hosting star, we could have accepted the ∼800 d signal as a probable planet. Further monitoring of DMPP-3 will reveal which signatures can be used to most efficiently identify these imposters. We also report a threshold detection (0.2 per cent FAP) of a ∼2.26 d periodicity in the RVs, potentially attributed to an Earth-mass S-type planet interior to DMPP-3A b.

New constraints on the Bray conservation-of-momentum natal kick model from multiple distinct observations

ABSTRACT Natal supernova kicks, the linear momentum compact remnants receive during their formation, are an essential part of binary population synthesis (BPS) models. Although these kicks are well supported by evidence, their underlying distributions and incorporation into BPS models are uncertain. In this work, we investigate the nature of natal kicks using a previously proposed analytical prescription where the strength of the kick is given by $v_\text{k}=\alpha \frac{m_\text{ejecta}}{m_\text{remnant}}+\beta \, \mathrm{km\, s}^{-1}$ , for free parameters α and β. We vary the free parameters over large ranges of possible values, comparing these synthetic populations simultaneously against four constraints: the merger rate of compact binary neutron star (BNS) systems, the period–eccentricity distribution of Galactic BNSs, the velocity distribution of single-star pulsars, and the likelihood for low ejecta mass supernovae to produce low-velocity kicks. We find that different samples of the parameter space satisfy each test, and only 1 per cent of the models satisfy all four constraints simultaneously. Although we cannot identify a single best kick model, we report $\alpha =115^{+40}_{-55}\, \mathrm{km\, s}^{-1}, \beta =15^{+10}_{-15}\, \mathrm{km\, s}^{-1}$ as the centre of the region of the parameter space that fulfils all of our constraints, and expect $\beta \ge 0\, \mathrm{km\, s}^{-1}$ as a further constraint. We also suggest further observations that will enable future refinement of the kick model. A sensitive test for the kick model will be the redshift evolution of the BNS merger rate since this is effectively a direct measure of the delay-time distribution for mergers. For our best-fitting values, we find that the peak of the BNS merger rate is the present day.

Thermal and Nonthermal Emission from a Peculiar Long-duration GRB 211211A

Long-duration GRB 211211A that lacks a supernova emission even down to very stringent limits at such a low redshift z = 0.076 and is associated with kilonova emission, suggests that its physical origin is from a binary compact star merger. By reanalyzing its data observed with the Gamma-Ray Burst Monitor on board the Fermi mission, we find that both time-integrated and time-resolved spectra can be fitted well by using a 2SBPL plus blackbody (2SBPL+BB) model in the prompt emission. The bulk Lorentz factors (Γph) of the outflow can be inferred by invoking the observed thermal emission at the photosphere radius within a pure fireball model, and we find out that the temporal evolution of Γph seems to be tracking with the light curve. The derived values of Γph are also consistent with the Γph–L γ,iso/E γ,iso correlations that had been found in other bursts. Moreover, we also calculate the magnetization factor σ 0 in the central engine and σ ph at the photosphere radius within the framework of a hybrid jet model, and find that the values of both 1 + σ 0 and 1 + σ ph are larger than 1 for different time slices. It suggests that at least the Poynting-flux component is indeed existent in the outflow. If this is the case, one possible physical interpretation of thermal and nonthermal emissions in GRB 211211A is from the contributions of both annihilation and the Blandford–Znajek mechanisms in the relativistic jet when a stellar mass black hole resides in the central engine.

Comprehensive Study of Mass Ejection and Nucleosynthesis in Binary Neutron Star Mergers Leaving Short-lived Massive Neutron Stars

By performing general relativistic hydrodynamics simulations with an approximate neutrino radiation transfer, the properties of ejecta in the dynamical and post-merger phases are investigated in the cases in which the remnant massive neutron star collapses into a black hole in ≲20 ms after the onset of the merger. The dynamical mass ejection is investigated in three-dimensional simulations. The post-merger mass ejection is investigated in two-dimensional axisymmetric simulations with viscosity using the three-dimensional post-merger systems as the initial conditions. We show that the typical neutron richness of the dynamical ejecta is higher for the merger of more asymmetric binaries; hence, heavier r-process nuclei are dominantly synthesized. The post-merger ejecta are shown to have only mild neutron richness, which results in the production of lighter r-process nuclei, irrespective of the binary mass ratios. Because of the larger disk mass, the post-merger ejecta mass is larger for more asymmetric binary mergers. Thus, the post-merger ejecta can compensate for the underproduced lighter r-process nuclei for asymmetric merger cases. As a result, by summing up both ejecta components, the solar residual r-process pattern is reproduced within the average deviation of a factor of three, irrespective of the binary mass ratio. Our result also indicates that the (about a factor of a few) light-to-heavy abundance scatter observed in r-process-enhanced stars can be attributed to variation in the binary mass ratio and total mass. Implications of our results associated with the mass distribution of compact neutron star binaries and the magnetar scenario of short gamma-ray bursts are discussed.

A Short Gamma-Ray Burst from a Protomagnetar Remnant

The contemporaneous detection of gravitational waves and gamma rays from GW170817/GRB 170817A, followed by kilonova emission a day after, confirmed compact binary neutron star mergers as progenitors of short-duration gamma-ray bursts (GRBs) and cosmic sources of heavy r-process nuclei. However, the nature (and life span) of the merger remnant and the energy reservoir powering these bright gamma-ray flashes remains debated, while the first minutes after the merger are unexplored at optical wavelengths. Here, we report the earliest discovery of bright thermal optical emission associated with short GRB 180618A with extended gamma-ray emission—with ultraviolet and optical multicolor observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission 15 minutes post-burst, which fades abruptly and chromatically (flux density F ν ∝ t −α , α = 4.6 ± 0.3) just 35 minutes after the GRB. Our observations from gamma rays to optical wavelengths are consistent with a hot nebula expanding at relativistic speeds, powered by the plasma winds from a newborn, rapidly spinning and highly magnetized neutron star (i.e., a millisecond magnetar), whose rotational energy is released at a rate L th ∝ t −(2.22±0.14) to reheat the unbound merger-remnant material. These results suggest that such neutron stars can survive the collapse to a black hole on timescales much larger than a few hundred milliseconds after the merger and power the GRB itself through accretion. Bright thermal optical counterparts to binary merger gravitational wave sources may be common in future wide-field fast-cadence sky surveys.

Cosmology with Gravitational Waves: A Review

AbstractStandard sirens have been the central paradigm in gravitational‐wave cosmology so far. From the gravitational wave signature of compact star binaries, it is possible to measure the luminosity distance of the source directly, and if additional information on the source redshift is provided, a measurement of the cosmological expansion can be performed. This review article discusses several methodologies that have been proposed to use gravitational waves for cosmological studies. Methods that use only gravitational‐wave signals and methods that use gravitational waves in conjunction with additional observations such as electromagnetic counterparts and galaxy catalogs will be discussed. The review also discusses the most recent results on gravitational‐wave cosmology, starting from the binary neutron star merger GW170817 and its electromagnetic counterpart and finishing with the population of binary black holes, observed with the third Gravitational‐wave Transient Catalog GWTC–3.

GRB 201104A: A “Repetitive” Short Gamma-Ray Burst?

Gamma-ray bursts (GRBs) are divided into short gamma-ray bursts (SGRBs) and long gamma-ray bursts (LGRBs) based on the bimodal distribution of their durations. LGRBs and SGRBs are typically characterized by different statistical characteristics. Nevertheless, there are some samples that challenge such a framework, such as GRB 060614, a long-duration burst with short-burst characteristics. Furthermore, GRBs are generally considered to be an event with no periodic or repetitive behavior, since the progenitors usually undergo destructive events, such as massive explosions or binary compact star mergers. In this work, we investigated Fermi data for possible quasiperiodic oscillations and repetitive behaviors of GRBs using timing analysis methods and report a special event GRB 201104A, which is a long-duration burst with the characteristics of an SGRB, and it exhibits a “repetitive” behavior. We propose that such a situation may arise from lensed SGRBs and attempt to verify it by Bayesian inference. In addition, we extend the spectral analysis to Bayesian inference. In spite of the existence of at least two distinct time periods with a nearly identical spectrum, there is no strong evidence that they result from a lensing GRB. Taking the gravitational-lensing scenario out of consideration, a long burst would resemble a short burst in its repetitive behavior, which presents a challenge for the current classification scheme.

GRB 160821B late afterglow rebrightening: A new candidate for magnetar-powered Merger-novae

For the first time on August 17, 2017, the LIGO-Virgo scientific collaboration detected a gravitational wave (GW) signal from a binary neutron star (BNS) merger event (i.e., GW170817), along with its multiwavelength electromagnetic counterparts. The detection of GW170817 confirmed that the BNS merger can produce both Gamma-ray Bursts (GRBs) and Merger-novae. Therefore, searching for the Merger-novae signature could help to justify whether those short GRBs without the detected GW parts are indeed resulting from the merger process of double compact stars. In this study, we thoroughly investigate the special characteristics of GRB 160821B's afterglow (including its internal X-ray plateau, late optical, and simultaneous X-ray rebrightening). We found that its multiband afterglow data could be well-interpreted by invoking magnetar dipole radiation, external shock afterglow emission and magnetar-powered Merger-novae radiation, inferring that the rebrightening signature of GRB 160821B is a novel candidate for magnetar-powered Merger-novae. The discovery of Merger-novae candidates, or GRBs, is important as they can provide an important basis for studying the merger rate of compact binary stars at high redshift, which is essential for studying the origin of heavy elements in the universe.

Dark photon bursts from compact binary systems and constraints

In this work, we consider the burst signal of the dark photon, the hypothetical vector boson of the $U(1)_B$ or $U(1)_{B-L}$ gauge group, generated by a compact binary star system. The absence of the signal in the laser interferometer puts bounds on the coupling constant $\epsilon$ to the ordinary matter. It turns out that if the dark photon is massless, $\epsilon^2$ is on the order of $10^{-37}-10^{-33}$ at most; in the massive case, the upper bound of $\epsilon^2$ is about $10^{-38}-10^{-31}$ in the mass range from $10^{-19}$ eV to $10^{-11}$ eV. These are the first bounds derived from the interferometer observations independent of the assumption of dark photons being dark matter.

Status of Advanced Virgo and upgrades before next observing runs

The upgrade to the second generation of ground-based gravitational waves detectors, finally allowed in 2015 the first direct detection of a Gravitational Wave, thus opening a new window of observation on the Universe. This outstanding achievement was the result of many years of joint research and development (R&D) efforts by the LIGO and Virgo collaborations. Since then, during the observing runs O1, O2 and O3 , many other events have been detected by both Virgo and LIGO (Laser Interferometer Gravitational-Wave Observatory). Since April 2020, right after the end of the O3 run, Advanced Virgo has undergone a period of instrument upgrade, still in progress, before the next observing run at present scheduled to start in mid-2022. This paper, after a brief introduction on Gravitational Wave detection, will provide an overview about Advanced Virgo design, its status and the planned instrument upgrades to the Advanced Virgo+ design, which will push the detector sensitivity towards the maximum achievable limit that is a detection range for compact binary neutron stars of 100 Mpc during O4 and 260 Mpc during O5 (2025-2026) [].