Dwarf Binaries Research Articles

Expected insights into Type Ia supernovae from LISA’s gravitational wave observations

The nature of progenitors of Type Ia supernovae has long been debated, primarily due to the elusiveness of the progenitor systems to traditional electromagnetic observation methods. We argue that gravitational wave observations with the upcoming Laser Interferometer Space Antenna (LISA) offer the most promising way to test one of the leading progenitor scenarios – the double-degenerate scenario, which involves a binary system of two white dwarf stars. In this study we review published results, supplementing them with additional calculations for the context of Type Ia supernovae. We discuss the fact that LISA will be able to provide a complete sample of double white dwarf Type Ia supernova progenitors with orbital periods shorter than 16–11 minutes (gravitational wave frequencies above 2–3 millihertz). Such a sample will enable a statistical validation of the double-degenerate scenario by simply counting whether LISA detects enough double white dwarf binaries to account for the measured Type Ia merger rate in Milky Way-like galaxies. Additionally, we illustrate how LISA’s capability to measure the chirp mass will set lower bounds on the primary mass, revealing whether detected double white dwarf binaries will eventually end up as a Type Ia supernova. We estimate that the expected LISA constraints on the Type Ia merger rate for the Milky Way will be 4–9%. We also discuss the potential gravitational wave signal from a Type Ia supernova assuming a double-detonation mechanism and explore how multi-messenger observations could significantly advance our understanding of these transient phenomena.

Triple trouble with PSR J1618−3921: Mass measurements and orbital dynamics of an eccentric millisecond pulsar

Context. PSR J1618−3921 is one of five known millisecond pulsars (MSPs) in eccentric orbits (eMPSs) located in the Galactic plane, whose formation is poorly understood. Earlier studies of these objects revealed significant discrepancies between observations and predictions from standard binary evolution scenarios of pulsar-helium white dwarf (HeWD) binaries, especially in the case of PSR J0955−6150, for which mass measurements ruled out most eMSP formation models. Aims. We aim to measure the masses of the pulsar and its companion, and constrain the orbital configuration of PSR J1618−3921. This facilitates understanding similarities among eMSPs and could offer hints as to their formation mechanism. Methods. We conducted observations with the L-band receiver of the MeerKAT radio telescope and the UWL receiver of the Parkes Murriyang radio telescope between 2019 and 2021. These data were added to archival Parkes and Nançay observations. We performed a full analysis on this joint data set with a timing baseline of 23 years. We also used the data from recent observations to give a brief account of the emission properties of J1618−3921, including a rotating vector model (RVM) fit of the linear polarisation position angle of the pulsar. Results. From the timing analysis, we measure a small but significant proper motion of the pulsar. The long timing baseline allowed for a highly significant measurement of the rate of advance of periastron of ω̇ = (0.00145±0.00010)°yr−1. Despite the tenfold improvement in timing precision from MeerKAT observations, we can only report a low-significance detection of the orthometric Shapiro delay parameters, h3 = 2.70−1.47+2.07 μs and ς = 0.68−0.09+0.13. Under the assumption of the validity of general relativity (GR), the self-consistent combination of these three parameters leads to mass estimates of the total and individual masses in the binary of Mtot = 1.42−0.19+0.20 M⊙, Mc = 0.20−0.03+0.11 M⊙, and Mp = 1.20−0.20+0.19 M⊙. We detect an unexpected change in the orbital period of Ṗb = −2.26−0.33+0.35 × 10−12, that is an order of magnitude larger and carries an opposite sign to what is expected from the Galactic acceleration and the Shklovskii effect, which are a priori the only non-negligible contributions expected for Ṗb. We also detect a significant second derivative of the spin frequency, f̈. The RVM fit reveals a viewing angle of ζ = (111 ± 1)°. Furthermore, we report an unexpected, abrupt change in the mean pulse profile in June 2021 of unknown origin. Conclusions. We propose that the anomalous Ṗb and f̈ we measure for J1618−3921 indicate an additional varying acceleration due to a nearby mass. The J1618−3921 binary system is likely part of a hierarchical triple, but with the third component much farther away than the outer component of the MSP in a triple star system, PSR J0337+1715. This finding suggests that at least some eMSPs might have formed in triple star systems. Although the uncertainties are large, the binary companion mass is consistent with the Pb − MWD relation, which has been verified for circular HeWD binaries and also for the two HeWDs in the PSR J0337+1715 system. Future regular observations with the MeerKAT telescope will, due to the further extension of the timing baseline, improve the measurement of Ṗb and f̈. This will help us further understand the nature of this system, and perhaps improve our understanding of eMSPs in general.

The evolutionary history of GD 1400AB, a white dwarf–brown dwarf binary

ABSTRACT GD1400AB was one of the first known white dwarf $+$ brown dwarf binaries and is the only one of these systems where the white dwarf is a ZZ Ceti pulsator. Here, we present both radial velocity measurements and time-series photometry, analysing both the white dwarf pulsations and the effects of irradiation on the brown dwarf. We find that the brightness temperatures of 1760 $\pm $ 10 K for the nightside and 1860 $\pm$ 10 K for the dayside indicate that the brown dwarf is hotter than spectra have previously suggested, although brightness temperatures calculated using a larger radius for the brown dwarf are consistent with previously determined spectral types. We also discuss the likely evolutionary pathway of this binary and put its common envelope phase into context with the other known systems.

LP349-25AB: Application of Magneto-convection Model to an M Dwarf Binary Where Radio and Astrometric Data have Yielded Masses with 0.7% Precision

Reliable (±(1–2)%) estimates of stellar masses have typically been obtained from eclipsing binaries. In such cases, derived values of radii and T eff are in many cases discrepant relative to standard models, especially in active stars. These discrepancies led to the development of magneto-convection theory. Recently, an independent technique has emerged for determining precise masses of radio-emitting binary components: interferometric observations at multiple points around the orbit. S. Curiel et al. have used this technique to determine the masses of both components of an M8+M9 binary (LP349-25AB) with remarkably high precision: ±0.7%. However, applying standard models suggests that the components are not co-eval, differing in age by ∼30%. But LP349-25 is such a close binary (with a separation of 1.2 au) that non-coevality seems unrealistic. Here, we report on magneto-convective modeling which resolves the apparent non-coevality while simultaneously explaining why the primary component is variable, but the secondary component is not.

Fundamental Tests of White Dwarf Cooling Physics with Wide Binaries

We present follow-up spectroscopy and a detailed model atmosphere analysis of 29 wide double white dwarfs, including eight systems with a crystallized C/O core member. We use the state-of-the-art evolutionary models to constrain the physical parameters of each star, including the total age. Assuming that the members of wide binaries are coeval, any age difference between the binary members can be used to test the cooling physics for white dwarf stars, including potential delays due to crystallization and 22Ne distillation. We use our control sample of 14 wide binaries with noncrystallized members to show that this method works well; the control sample shows an age difference of only ΔAge = −0.03 ± 0.15 Gyr between its members. For the eight crystallized C/O core systems we find a cooling anomaly of ΔAge = 1.13−1.07+1.20 Gyr. Even though our results are consistent with a small additional cooling delay (∼1 Gyr) from 22Ne distillation and other neutron-rich impurities, the large uncertainties make this result not statistically significant. Nevertheless, we rule out cooling delays longer than 3.6 Gyr at the 99.7% (3σ) confidence level for 0.6–0.9 M ⊙ white dwarfs. Further progress requires larger samples of wide binaries with crystallized massive white dwarf members. We provide a list of subgiant + white dwarf binaries that could be used for this purpose in the future.

Predicting gravitational wave signals from BPASS White Dwarf Binary and Black Hole Binary populations of a Milky Way-like galaxy model for LISA

Abstract Galactic white dwarf binaries (WDBs) and black hole binaries (BHBs) will be gravitational wave (GW) sources for LISA. Their detection will provide insights into binary evolution and the evolution of our Galaxy through cosmic history. Here, we make predictions of the expected WDB and BHB population within our Galaxy. We combine predictions of the compact remnant binary populations expected by stellar evolution from the detailed Binary Population and Spectral Synthesis (BPASS) code, with a Milky Way analogue galaxy model from the Feedback In Realistic Environment (FIRE) simulations. We use PhenomA and legwork to simulate LISA observations. Both packages make similar predictions that on average four Galactic BHBs and 673 Galactic WDBs are above the signal-to-noise ratio (SNR) threshold of 7 after a four-year mission. We compare these predictions to earlier results using the Binary Star Evolution (BSE) code with the same FIRE model galaxy. We find that BPASS predicts a few more LISA observable Galactic BHBs and a twentieth of the Galactic WDBs. The differences are due to the different physical assumptions that have gone into the binary evolution calculations. These results indicate that the expected population of compact binaries that LISA will detect depends very sensitively on the binary population synthesis models used and thus observations of the LISA population will provide tight constraints on our modelling of binary stars. Finally, from our synthetic populations we have created mock LISA signals that can be used to test and refine data processing methods of the eventual LISA observations.

A Pilot Search for Gravitational Self-Lensing Binaries with the Zwicky Transient Facility

Binary systems containing a compact object may exhibit periodic brightening episodes due to gravitational lensing as the compact object transits the companion star. Such “self-lensing” signatures have been detected before for white dwarf binaries. We attempt to use these signatures to identify detached stellar-mass neutron star and black hole binaries using data from the Zwicky Transient Facility (ZTF). We present a systematic search for self-lensing signals in Galactic binaries from a subset of high-cadence ZTF data taken in 2018. We identify 12 plausible candidates from the search, although because each candidate is observed to only brighten once, other origins such as stellar flares are more likely. We discuss prospects for more comprehensive future searches of the ZTF data.

A new code for low-resolution spectral identification of white dwarf binary candidates

Close white dwarf binaries (CWDBs) are considered to be progenitors of several exotic astronomical phenomena (e.g., type Ia supernovae, cataclysmic variables). These violent events are broadly used in studies of general relativity and cosmology. However, obtaining precise stellar parameter measurements for both components of CWDBs is a challenging task given their low luminosities, swift time variation, and complex orbits. High-resolution spectra (R$> 20 000$) are preferred but expensive, resulting in a sample size that is insufficient for robust population study. Recently, studies have shown that the more accessible low-resolution (R$ 2000$) spectra (LRS) may also provide enough information for spectral decomposition. To release the full potential of the less expensive low-resolution spectroscopic surveys, and thus greatly expand the CWDB sample size, it is necessary to develop a robust pipeline for spectra decomposition and analysis. We aim to develop a spectroscopic fitting program for white dwarf binary systems based on photometry, LRS, and stellar evolutionary models. The outputs include stellar parameters of both companions in the binary including effective temperature, surface gravity, mass, radius, and metallicity in the case of MS stars. We used an artificial neural network (ANN) to build spectrum generators for DA/DB white dwarfs and main-sequence stars. Characteristic spectral lines were used to decompose the spectrum of each component. The best-fit stellar parameters were obtained by finding the least $ solution to these feature lines and the continuum simultaneously. Compared to previous studies, our code is innovative in the following aspects: (1) implementing a sophisticated binary decomposition technique in LRS for the first time; (2) using flux-calibrated spectra instead of photometry plus spectral lines, in which the latter requires multi-epoch observations; (3) applying an ANN in binary decomposition, which significantly improves the efficiency and accuracy of generated spectra. We demonstrate the reliability of our code with two well-studied CWDBs, WD 1534+503 and PG 1224+309. We also estimate the stellar parameters of 14 newly identified CWDB candidates, most of which are fitted with double component models for the first time. Our estimates agree with previous results for the common stars and follow the statistical distribution in the literature. We provide a robust program for fitting binary spectra of various resolutions. Its application to a large volume of white dwarf binary candidates will offer important statistic samples to stellar evolution studies and future gravitational wave monitoring.

Constraining gravitational wave velocities using gravitational and electromagnetic wave observations of white dwarf binaries

ABSTRACT Although the general theory of relativity (GR) predicts that gravitational waves (GWs) have exactly the same propagation velocity as electromagnetic (EM) waves, many theories of gravity beyond GR expect otherwise. Accurate measurement of the difference in their propagation speed, or a tight constraint on it, could be crucial to validate or put limits on theories beyond GR. The proposed future space-borne GW detectors are poised to detect a substantial number of Galactic white dwarf binaries (GWDBs), which emit the GW as semimonochromatic signals. Concurrently, these GWDBs can also be identified as optical variable sources. Here we proposed that allocating a GWDB’s optical light curve and contemporaneous GW signal can be used to trace the difference between the velocity of GW and EM waves. Simulating GW and EM wave data from 14 verification binaries (VBs), our method constrains propagation-originated phase differences, limiting the discrepancy between the speed of light (c) and GW ($c_{GW}$). Through the utilization of LISA’s design sensitivity and the current precision in optical observation on GWDB, our study reveals that a four-year observation of the 14 recognized VBs results in a joint constraint that confines $\Delta c/c$ ($\Delta c = c_{\mathrm{GW}} - c$) to the range of $-2.1\times 10^{-12}$ and $4.8\times 10^{-12}$. Additionally, by incorporating this constraint on $c_{\mathrm{GW}}$, we are able to establish boundaries for the mass of the graviton, limiting it to $m_{\mathrm{g}}\le 3\times 10^{-23}\, e\mathrm{V}\,c^{-2}$, and for the parameter associated with local Lorentz violation, $\bar{s}_{00}$, constrained within the range of $-3.4\times 10^{-11}\le \bar{s}_{00}\le 1.5\times 10^{-11}$.

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.

On Its Way to the Neutron Star–White Dwarf Binary Graveyard, IGR J16194−2810, A First Ascent M Giant X-Ray Binary

A single-lined spectroscopic orbit for the M giant in the X-ray binary IGR J16194−2810 is determined from a time series of optical spectra. The spectroscopic orbital period of 192.5 days is twice that of the photometric period, confirming that the M giant in the system is an ellipsoidal variable. The giant is identified as a first ascent giant approaching the red giant tip. The primary is a neutron star (NS) with its M giant companion filling its Roche lobe, verifying the system classification as a low-mass X-ray binary (LMXB). Stellar C, N, O, and Fe abundances are derived for the M giant with the C, N, and O values typical for a field giant with [Fe/H] = −0.14. The system does not have a large kick velocity. Models for the evolution of the system into a binary NS–white dwarf are presented. The X-ray properties are discussed in the context of this model. This binary is a rare example of a luminous, long orbital period LMXB early in the transient ellipsoidal phase.

Compact white dwarf binaries in the combined SRG/eROSITA/SDSS eFEDS survey

Context. X-ray surveys combined with optical follow-up observations are used to generate complete flux-limited samples of the main X-ray emitting source classes. eROSITA on the Spectrum-Roentgen-Gamma mission provides sufficient sensitivity to build significantly enhanced samples of rare X-ray emitting sources. Aims. We strive to identify and classify compact white dwarf binaries, cataclysmic variables (CVs), and related objects, which were detected in the sky area of eFEDS, the eROSITA Final Equatorial Depths Survey, and they were observed in the plate program of SDSS-V. Methods. Compact white dwarf binaries were selected from spectra obtained in the early SDSS-V plate program. A dedicated set of SDSS plate observations were carried out in the eFEDS field, providing spectroscopic classifications for a significant fraction of the optically bright end (r < 22.5) of the X-ray sample. The identification and subclassification rests on visual inspections of the SDSS spectra, spectral variability, color-magnitude and color-color diagrams involving optical and X-ray fluxes, optical variability, and literature work. Results. Upon visual inspection of SDSS spectra and various auxiliary data products, we have identified 26 accreting compact white dwarf binaries (aCWDBs) in eFEDS, of which 24 are proven X-ray emitters. Among those 26 objects, there are 12 dwarf novae, three WZ Sge-like disk-accreting nonmagnetic CVs with low accretion rates, five likely nonmagnetic high accretion rate nova-like CVs, two magnetic CVs of the polar subcategory, and three double degenerates (AM CVn objects). Period bouncing candidates and magnetic systems are rarer than expected in this sample, but it is too small for a thorough statistical analysis. Fourteen of the systems are new discoveries, of which five are fainter than the Gaia magnitude limit. Thirteen aCWDBs have measured or estimated orbital periods, of which five were presented here. Through a Zeeman analysis, we revise the magnetic field estimate of the polar system J0926+0105, which is likely a low-field polar at B = 16 MG. We quantified the success of X-ray versus optical/UV selection of compact white dwarf binaries which will be relevant for the full SDSS-V survey. We also identified six white dwarf main sequence (WDMS) systems, among them there is one confirmed pre-CV at an orbital period of 17.6 h and another pre-CV candidate. Conclusions. This work presents successful initial work in building large samples of all kinds of accreting and X-ray emitting compact white dwarf binaries that will be continued over the full hemisphere in the years to come.

A stochastic gravitational wave background in LISA from unresolved white dwarf binaries in the Large Magellanic Cloud

ABSTRACT The Laser Interferometer Space Antenna (LISA) is expected to detect a wide variety of gravitational wave sources in the mHz band. Some of these signals will elude individual detection, instead contributing as confusion noise to one of several stochastic gravitational-wave backgrounds (SGWBs) – notably including the ‘Galactic foreground’, a loud signal resulting from the superposition of millions of unresolved double white dwarf binaries (DWDs) in the Milky Way. It is possible that similar, weaker SGWBs will be detectable from other DWD populations in the local Universe, including the Large Magellanic Cloud (LMC). We use the Bayesian LISA Inference Package (blip) to investigate the possibility of an anisotropic SGWB generated by unresolved DWDs in the LMC. To do so, we compute the LMC SGWB from a realistic DWD population generated via binary population synthesis, simulate 4 years of time-domain data with blip comprised of stochastic contributions from the LMC SGWB and the LISA detector noise, and analyse this data with blip’s spherical harmonic anisotropic SGWB search. We also consider the case of spectral separation from the Galactic foreground. We present the results of these analyses and show, for the first time, that the unresolved DWDs in the LMC will comprise a significant SGWB for LISA.

Exploring the orbital alignments of Galactic close white dwarf binaries with LISA

Exploring the orbital alignments of Galactic close white dwarf binaries with LISA

The search for DA double white dwarf binary candidates from SDSS DR14

Aims. Double white dwarf (DWD) binaries are one of the channels through which type Ia supernovae explosions occur. With the release of more and more sky survey data, the search for additional DWDs has become a possibility. We utilized the spectroscopic data from SDSS DR14 to search for DWD binaries based on variations in radial velocities (RVs). Methods. We obtained a sample of 4089 DA white dwarfs (WDs) with two or more spectra from SDSS DR14, and their RVs were derived using the cross-correlation function. Using the chi-squared (χ2) distribution of RVs as a base, we calculated the corresponding logarithmic probabilities (log p) for different degrees of freedom. Results. We selected the targets with log p < −3.0 and obtained 65 highly credible DWD candidates, of which 56 were newly discovered. We compared the distributions of the Teff, log g, and mass of the DWD candidates and found that the mass distribution of DWDs has two peaks. The primary peak, located at 0.45 M⊙, is lower than the peak of the total WD sample, while the secondary peak, located at 0.60 M⊙, is similar to the peak of the total sample. Finally, we crossmatched our sample with Zwicky Transient Facility (ZTF) photometry data and identified two targets with clear periodic variability. Based on the shape of their light curve, we think both could be white dwarf main-sequence binary stars

Latitude-dependent Atmospheric Waves and Long-period Modulations in Luhman 16 B from the Longest Light Curve of an Extrasolar World

In this work, we present the longest photometric monitoring of up to 1200 hr of the strongly variable brown dwarf binaries Luhman 16 AB and provide evidence of ±5% variability on a timescale of several to hundreds of hours for this object. We show that short-period rotational modulation around 5 hr (k = 1 wavenumber) and 2.5 hr (k = 2 wavenumber) dominate the variability under 10 hr, where the planetary-scale wave model composed of k = 1 and k = 2 waves provides good fits to both the periodograms and light curve. In particular, models consisting of three to four sine waves could explain the variability of the light-curve durations up to 100 hr. We show that the relative range of the k = 2 periods is narrower compared to the k = 1 periods. Using simple models of zonal banding in solar system giants, we suggest that the difference in period range arises from the difference in wind-speed distribution at low and mid-to-high latitudes in the atmosphere. Last, we show that Luhman 16 AB also exhibits long-period ±5% variability, with periods ranging from 15 hr up to 100 hr over the longest monitoring of this object. Our results for the k = 1 and k = 2 waves and long-period evolution are consistent with previous 3D atmosphere simulations, demonstrating that both latitude-dependent waves and slow-varying atmospheric features are potentially present in Luhman 16 AB atmospheres and are a significant contribution to the light-curve modulation over hundreds of rotations.

Sky location of Galactic white dwarf binaries in space-based gravitational wave detection

Quickly localizing the identified white dwarf (WD) binaries is the basic requirement for the space-based gravitational wave (GW) detection. In fact, the amplitude of GW signals are modulated by the periodic motion of GW detectors on the solar orbit. The intensity of the observed signals is enhanced according to the observation time beyond a year to enhance a high signal to noise ratio (SNR). As data gap exists, the completeness of the data observed for a long time depends on filling gaps in the data. Actually, in a year period, the GW sources have a best observation orbit position of GW detectors, where the intensity of GW is maximum. Thus, the best positions can be searched for the verified GW sources of the sky map to enhance SNR too, which avoids filling data gaps. For the three arms response intensity of the GW signals changing more clearly with the location of the GW sources relative to the detector, the noises and the suppression of noise by time delay interferometer are ignored. As a verification case, the four WD binaries are chosen, whose best observation orbit positions of the GW detectors are related to the direction of WD binaries perpendicular to the detection arms. The two verification binaries: J0806 and V407 Vul are observed at the best orbit positions by TAIJI for the short time of 2 and 3 days respectively. The corresponding intensities of GWs are above the values of the TAIJI sensitivity curve, significantly. Location parameter estimation of the verification WD binaries are performed using the Metropolis–Hastings MCMC method. The confidence level of the parameters obtained in the best position is significantly several times higher than that in the worst position where the direction of WD binaries are almost parallel to the detection arms. Compared with a single detector, the network of two detectors improve slightly the accuracy of location of the verification binaries. These results imply that the searching of GW signals and parameter estimation of GW sources from the experimental data of the space-based mission do not ignore the orbit positions relevant to GW sources.

Neutron star – white dwarf binaries: probing formation pathways and natal kicks with LISA

ABSTRACT Neutron star–white dwarf (NS + WD) binaries offer a unique opportunity for studying NS-specific phenomena with gravitational waves. In this paper, we employ the binary population synthesis technique to study the Galactic population of NS + WD binaries with the future Laser Interferometer Space Antenna (LISA). We anticipate approximately $\mathcal {O}(10^2)$ detectable NS + WD binaries by LISA, encompassing both circular and eccentric ones formed via different pathways. Despite the challenge of distinguishing these binaries from more prevalent double white dwarfs (especially at frequencies below 2 mHz), we show that their eccentricity and chirp mass distributions may provide avenues to explore the NS natal kicks and common envelope evolution. Additionally, we investigate the spatial distribution of detectable NS + WD binaries relative to the Galactic plane and discuss prospects for identifying electromagnetic counterparts at radio wavelengths. Our results emphasise LISA’s capability to detect and characterize NS + WD binaries and to offer insights into the properties of the underlying population. Our conclusions carry significant implications for shaping LISA data analysis strategies and future data interpretation.

Constraining mass transfer models with galactic neutron star–white dwarf binaries as gravitational wave sources

ABSTRACT Neutron star–white dwarf (NSWD) binaries are one of the most abundant sources of gravitational waves (GWs) in the Milky Way. These GW sources are the evolutionary products of primordial binaries that experienced many processes of binary interaction. We employ a binary population synthesis method to investigate the properties of Galactic NSWD binaries detectable by the Laser Interferometer Space Antenna (LISA). In this paper, only the NSWD systems with a COWD or ONeWD component are included. We consider various models related to mass-transfer efficiencies during primordial binary evolution, supernova explosion mechanisms at NS formation, common envelope ejection efficiencies, and critical WD masses that determine the stability of mass transfer between WDs and NSs. Based on our calculations, we estimate that tens to hundreds of LISA NSWD binaries exist in the Milky Way. We find that the detection of LISA NSWD binaries is able to provide profound insights into mass-transfer efficiencies during the evolution of primordial binaries and critical WD masses during mass transfer from a WD to an NS.

Likelihood of white dwarf binaries to dominate the astrophysical gravitational wave background in the mHz band

Context. The astrophysical gravitational wave background (AGWB) is a collective signal of astrophysical gravitational wave sources dominated by compact binaries. One key science goal of current and future gravitational wave detectors is to obtain its measurement. Aims. We aim to determine the population of compact binaries dominating the AGWB in the mHz band. We revisit and update an earlier work by Farmer & Phinney (2003, MNRAS, 346, 1197) to model the astrophysical gravitational wave background sourced by extragalactic white dwarf binaries in the mHz frequency band. Methods. We calculated the signal using a single-metallicity model for the white dwarf population in the Universe using the global star formation history. Results. We estimate the white dwarf AGWB amplitude to be ∼60% higher than the earlier estimate. We also find that the overall shape of the white dwarf AGWB shows a good fit with a broken power law combined with an exponential cut-off. Conclusions. We compare our results to present-day best estimates for the background due to black hole and neutron star binaries, finding that the white dwarf component is likely to dominate in the mHz band. We provide an order-of-magnitude estimate that explains this hierarchy and we comment on the implications for future missions that aim to detect the AGWB. We also note that the black hole AGWB may only be detectable at high frequency. We outline several improvements that can be made to our estimate, however, these points are unlikely to change our main conclusion, which posits that the white dwarf AGWB dominates the mHz band.