White Dwarf Binaries Research Articles

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.

Four new eclipsing accreting ultracompact white dwarf binaries found with the Zwicky Transient Facility

Accreting ultracompact white dwarf binaries contain a white dwarf that is accreting from a degenerate object. They have orbital periods shorter than 65 minutes. We report the discovery and the orbital period of four new eclipsing accreting ultracompact white dwarf binaries found using the Zwicky Transient Facility (ZTF) and discuss their photometric properties. We searched through a list of 4171 dwarf novae compiled using the ZTF and used the box least square method to search for periodic signals in the data. We found four eclipsing accreting ultracompact binaries with orbital periods between 25.9 and 56 minutes.\ One had previously been published as an AM Canum Venaticorum (AM CVn), and the other three systems are new discoveries. The two shorter-period systems are likely also AM CVn systems, while the longest-period system, with a period of 56 minutes, showed multiple super-outbursts over two years, which is more consistent with it being a helium CV.

Ultra–short-period WD Binaries Are Not Undergoing Strong Tidal Heating

Double white dwarf (WD) binaries are increasingly being discovered at short orbital periods where strong tidal effects and significant tidal heating signatures may occur. We assume that the tidal potential of the companion excites outgoing gravity waves within the WD primary, the dissipation of which leads to an increase in the WD’s surface temperature. We compute the excitation and dissipation of the waves in cooling WD models in evolving MESA binary simulations. Tidal heating is self-consistently computed and added to the models at every time step. As a binary inspirals to orbital periods less than ∼20 minutes, the WD’s behavior changes from cooling to heating, with temperature enhancements that can exceed 10,000 K compared with nontidally heated models. We compare a grid of tidally heated WD models to observed short-period systems with hot WD primaries. While tidal heating affects their T eff, it is likely not the dominant luminosity. Instead, these WDs are probably intrinsically young and hot, implying that the binaries formed at short orbital periods. The binaries are consistent with undergoing common envelope evolution with a somewhat low efficiency α CE. We delineate the parameter space where the traveling wave assumption is most valid, noting that it breaks down for WDs that cool sufficiently, where standing waves may instead be formed.

Rotation plays a role in the generation of magnetic fields in single white dwarfs

ABSTRACT Recent surveys of close white dwarf binaries as well as single white dwarfs have provided evidence for the late appearance of magnetic fields in white dwarfs, and a possible generation mechanism, a crystallization and rotation-driven dynamo has been suggested. A key prediction of this dynamo is that magnetic white dwarfs rotate, at least on average, faster than their non-magnetic counterparts and/or that the magnetic field strength increases with rotation. Here we present rotation periods of ten white dwarfs within 40 pc measured using photometric variations. Eight of the light curves come from TESS observations and are thus not biased towards short periods, in contrast to most period estimates that have been reported previously in the literature. These TESS spin periods are indeed systematically shorter than those of non-magnetic white dwarfs. This means that the crystallization and rotation-driven dynamo could be responsible for a fraction of the magnetic fields in white dwarfs. However, the full sample of magnetic white dwarfs also contains slowly rotating strongly magnetic white dwarfs which indicates that another mechanism that leads to the late appearance of magnetic white dwarfs might be at work, either in addition to or instead of the dynamo. The fast-spinning and massive magnetic white dwarfs that appear in the literature form a small fraction of magnetic white dwarfs, and probably result from a channel related to white dwarf mergers.

The Common Envelope Evolution Outcome. II. Short-orbital-period Hot Subdwarf B Binaries Reveal a Clear Picture

Common envelope evolution (CEE) is vital for forming short-orbital-period compact binaries. It covers many objects, such as double compact merging binaries, Type Ia supernovae progenitors, binary pulsars, and X-ray binaries. Knowledge of the common envelope (CE) ejection efficiency still needs to be improved, though progress has been made recently. Short-orbital-period hot subdwarf B star (sdB) plus white dwarf (WD) binaries are the most straightforward samples with which to constrain CEE physics. We apply the known orbital period–WD mass relation to constrain the sdB progenitors of seven sdB+WD binaries with a known inclination angle. The average CE efficiency parameter is 0.32. This is consistent with previous studies. However, the CE efficiency need not be constant, but a function of the initial mass ratio, based on well-constrained sdB progenitor mass and evolutionary stage. Our results can be used as physical inputs for binary population synthesis simulations of related objects. A similar method can also be applied to study other short-orbital-period WD binaries.

How to distinguish white dwarf and neutron star X-ray binaries during their X-ray outbursts?

We present spectral signatures of neutron stars (NSs) and white dwarfs (WDs) hosted in accreting X-ray binaries that can be easily identified in X-ray observations. We perform spectral and timing analysis of 4U 1636–53 and SS Cygni, as typical representatives of such NS and WD binaries, based on their X-ray observations by RXTE, ASCA, Suzaku and BeppoSAX. As a result, we formulate a criterion that makes it easy to distinguish NS from WD in such binaries: NS X-rays exhibits quasi-stable behavior with the index Γ → 2 and is characterized by quasi periodic oscillations (QPOs) at 𝜈𝑄𝑃𝑂 > 0.5 Hz, although WD X-rays is stable with Γ → 1.85 and is accompanied by QPOs at 𝜈𝑄𝑃𝑂 < 0.05 Hz during source outbursts. In addition, we revealed that in 4U 1636–53 the mHz QPOs anti-correlate with the temperature, 𝑇𝑒 of the corona around a NS. This allowed us to associate mHz-QPOs with the corona dynamics during outburst cycle. The above index effect, now well established for 4U 1636–53 and SS Cygni using extensive observations, has previously been found in other low-mass X-ray NS and WD binaries and agrees well with the criterion for distinguishing NSs and WDs presented here.

Spectral Modeling of the Supersoft X-Ray Source CAL87 Based on Radiative Transfer Codes

Supersoft X-ray sources (SSSs) are white dwarf (WD) binaries that radiate almost entirely below ∼1 keV. Their X-ray spectra are often complex when viewed with the X-ray grating spectrometers, where numerous emission and absorption features are intermingled and hard to separate. The absorption features are mostly from the WD atmosphere, for which radiative transfer models have been constructed. The emission features are from the corona surrounding the WD atmosphere, in which incident emission from the WD surface is reprocessed. Modeling the corona requires different solvers and assumptions for the radiative transfer, which has yet to be achieved. We chose CAL87, an SSS in the Large Magellanic Cloud, which exhibits emission-dominated spectra from the corona, as the WD atmosphere emission is assumed to be completely blocked by the accretion disk. We constructed a radiative transfer model for the corona using two radiative transfer codes: xstar for a one-dimensional two-stream solver and MONACO for a three-dimensional Monte Carlo solver. We identified their differences and limitations in comparison to the spectra taken with the Reflection Grating Spectrometer on board the XMM-Newton satellite. We finally obtained a sufficiently good spectral model of CAL87 based on the radiative transfer of the corona plus an additional collisionally ionized plasma. In the coming X-ray microcalorimeter era, it will be required to interpret spectra based on radiative transfer in a wider range of sources than what is presented here.

Measuring the initial-final mass relation using wide double white dwarf binaries from Gaia DR3

ABSTRACT The initial-final mass relation (IFMR) maps the masses of main-sequence stars to their white dwarf descendants. The most common approach to measure the IFMR has been to use white dwarfs in clusters. However, it has been shown that wide double white dwarfs can also be used to measure the IFMR using a Bayesian approach. We have observed a large sample of 90 Gaia double white dwarfs using FORS2 on the VLT. Considering 52 DA + DA, DA + DC, and DC + DC pairs, we applied our extended Bayesian framework to probe the IFMR in exquisite detail. Our monotonic IFMR is well constrained by our observations for initial masses of 1–5 M⊙, with the range of 1–4 M⊙ mostly constrained to a precision of 0.03 M⊙ or better. We add an important extension to the framework, using a Bayesian mixture-model to determine the IFMR robustly in the presence of systems departing from single star evolution. We find a large but uncertain outlier fraction of 59 ± 21 per cent, with outlier systems requiring an additional $0.70_{-0.22}^{+0.40}$ Gyr uncertainty in their cooling age differences. However, we find that this fraction is dominated by a few systems with massive components near 0.9 M⊙, where we are most sensitive to outliers, but are also able to establish four systems as merger candidates.

Abundances and Transients from Neutron Star–White Dwarf Mergers

We systematically investigate the mergers of neutron star–white dwarf binaries from beginning to end, with a focus on the properties of the inflows and outflows in accretion disks and their electromagnetic emissions. Using population synthesis models, we determine a subset of these binaries in which the white dwarf companion undergoes unstable mass transfer and complete tidal disruption, forming a large accretion disk around the neutron star. The material evolves according to a one-dimensional advection-dominated accretion-disk model with nuclear burning, neutrino emissions, and disk-surface wind ejection. The extreme dynamics of the entire process have proven difficult to analyze, and thus currently, the properties are poorly understood. The outflows from the mergers are iron- and nickel-rich, giving rise to optical and infrared emissions powered by the decay of the radioactive iron-type isotopes, calculated via the SuperNu light-curve code. We find these systems capable of powering bright, yet short-lived, optical transients with the potential to power gamma-ray bursts.

Dynamical Evolution of White Dwarfs in Triples in the Era of Gaia

The Gaia mission has detected many white dwarfs (WDs) in binary and triple configurations, and while observations suggest that triple-stellar systems are common in our Galaxy, not much attention was devoted to WDs in triples. For stability reasons, these triples must have hierarchical configurations, i.e., two stars are on a tight orbit (the inner binary), with the third companion on a wider orbit about the inner binary. In such a system, the two orbits torque each other via the eccentric Kozai–Lidov mechanism, which can alter the orbital configuration of the inner binary. We simulate thousands of triple-stellar systems for over 10 Gyr, tracking gravitational interactions, tides, general relativity, and stellar evolution up to their WD fate. As demonstrated here, three-body dynamics coupled with stellar evolution is a critical channel to form tight WD binaries or merge a WD binary. Among these triples, we explore their manifestations as cataclysmic variables, Type Ia supernovae, and gravitational-wave events. The simulated systems are then compared to a sample of WD triples selected from the Gaia catalog. We find that including the effect of mass-loss-induced kicks is crucial for producing a distribution of the inner binary–tertiary separations that is consistent with Gaia observations. Lastly, we leverage this consistency to estimate that, at minimum, 30% of solar-type stars in the local 200 pc were born in triples.

Multimessenger parameter inference of gravitational-wave and electromagnetic observations of white dwarf binaries

ABSTRACT The upcoming Laser Interferometer Space Antenna (LISA) will detect a large gravitational-wave foreground of Galactic white dwarf binaries. These sources are exceptional for their probable detection at electromagnetic wavelengths, some long before LISA flies. Studies in both gravitational and electromagnetic waves will yield strong constraints on system parameters not achievable through measurements of one messenger alone. In this work, we present a Bayesian inference pipeline and simulation suite in which we study potential constraints on binaries in a variety of configurations. We show how using LISA detections and parameter estimation can significantly improve constraints on system parameters when used as a prior for the electromagnetic analyses. We also provide rules of thumb for how current measurements will benefit from LISA measurements in the future.

Convective dynamos of black widow companions

ABSTRACT Black widows and redbacks are binary millisecond pulsars with close low-mass companions that are irradiated and gradually ablated by the pulsar’s high-energy luminosity Lirr. These binaries evolve primarily through magnetic braking, which extracts orbital angular momentum and pushes the companion to overflow its Roche lobe. Here, we use the stellar evolution code mesa to examine how the irradiation modifies the companion’s structure. Strong Lirr inhibits convection to the extent that otherwise fully convective stars become almost fully radiative. By computing the convective velocities and assuming a dynamo mechanism, we find that the thin convective envelopes of such strongly irradiated companions ($L_{\rm irr}\gtrsim 3\, {\rm L}_\odot$) generate much weaker magnetic fields than previously thought – halting binary evolution. With our improved magnetic braking model, we explain most observed black widow and redback companions as remnants of main-sequence stars. We also apply our model (with Lirr) to evolved companions that overflow their Roche lobe close to the end of their main-sequence phase. The evolutionary tracks of such companions bifurcate, explaining the shortest period systems (which are potential gravitational wave sources) as well as the longest period ones (which are the progenitors of common pulsar–white dwarf binaries). The variety of black widow structures and evolutionary trajectories may be utilized to calibrate the dependence of magnetic braking on the size of the convective layer and on the existence of a radiative–convective boundary, with implications for single stars as well as other binaries, such as cataclysmic variables and AM Canum Venaticorum stars.

LISA Galactic binaries in the Roman Galactic Bulge Time-Domain Survey

ABSTRACT Short-period Galactic white dwarf binaries detectable by Laser Interferometer Space Antenna are the only guaranteed persistent sources for multimessenger gravitational-wave astronomy. Large-scale surveys in the 2020s present an opportunity to conduct preparatory science campaigns to maximize the science yield from future multimessenger targets. The Nancy Grace Roman Space Telescope Galactic Bulge Time-Domain Survey will (in its Reference Survey design) image seven fields in the Galactic Bulge approximately 40 000 times each. Although the Reference Survey cadence is optimized for detecting exoplanets via microlensing, it is also capable of detecting short-period white dwarf binaries. In this paper, we present forecasts for the number of detached short-period binaries the Roman Galactic Bulge Time-Domain Survey will discover and the implications for the design of electromagnetic surveys. Although population models are highly uncertain, we find a high probability that the baseline survey will detect of the order of ∼5 detached white dwarf binaries. The Reference Survey would also have a ${\gtrsim} 20\,{\rm per\,cent}$ chance of detecting several known benchmark white dwarf binaries at the distance of the Galactic Bulge.

Dynamical He Flashes in Double White Dwarf Binaries

The detonation of an overlying helium layer on a 0.8–1.1 M ⊙ carbon–oxygen (CO) white dwarf (WD) can detonate the CO WD and create a thermonuclear supernova (SN). Many authors have recently shown that when the mass of the He layer is low (≲0.03 M ⊙), the ashes from its detonation minimally impact the spectra and light curve from the CO detonation, allowing the explosion to appear remarkably similar to Type Ia SNe. These new insights motivate our investigation of dynamical He shell burning and our search for a binary scenario that stably accumulates thermally unstable He shells in the 0.01–0.08 M ⊙ range, thick enough to detonate, but also often thin enough for minimal impact on the observables. We first show that our improved nonadiabatic evolution of convective He shell burning in this range of shell mass leads to conditions ripe for a He detonation. We also find that a stable mass transfer scenario with a high-entropy He WD donor of mass 0.15–0.25 M ⊙ yields the He shell masses needed to achieve the double detonations. This scenario also predicts that the surviving He donor leaves with a spatial velocity consistent with the unusual runaway object, D6-2. We find that hot He WD donors originate in common-envelope events when a 1.3–2.0 M ⊙ star fills its Roche lobe at the base of the red giant branch at orbital periods of 1–10 days with the CO WD.

Investigating galactic double white dwarfs for the sub-mHz gravitational wave mission ASTROD-GW

A large number of galactic binary systems emit gravitational waves (GW) continuously with frequencies below $\ensuremath{\sim}10\text{ }\text{ }\mathrm{mHz}$. The LISA mission could identify tens of thousands of binaries over years of observation and will be subject to the confusion noise around 1 mHz yielded by the unresolved sources. Beyond LISA, there are several missions have been proposed to observe GWs in the sub-mHz range where the galactic foreground is expected to be overwhelming the instrumental noises. In this study, we investigate the detectability of sub-mHz GW missions to detect the galactic double white dwarf (DWD) binaries and evaluate the confusion noise produced by the undistinguished DWDs. This confusion noise could also be viewed as a stochastic GW foreground and be effectively observed in the sub-mHz band. The parameter determinations for the modeled foreground are examined by employing different detector sensitivities and population models. By assuming the determined foregrounds could be subtracted from the data, we evaluate the residuals which are expected to have power spectral densities two orders of magnitude lower than the originals data.

A GALEX view of the DA white dwarf population

ABSTRACT We present a detailed model atmosphere analysis of 14001 DA white dwarfs from the Montreal White Dwarf Database with ultraviolet photometry from the GALEX mission. We use the 100 pc sample, where the extinction is negligible, to demonstrate that there are no major systematic differences between the best-fitting parameters derived from optical only data and the optical + UV photometry. GALEX FUV and NUV data improve the statistical errors in the model fits, especially for the hotter white dwarfs with spectral energy distributions that peak in the UV. Fitting the UV to optical spectral energy distributions also reveals UV-excess or UV-deficit objects. We use two different methods to identify outliers in our model fits. Known outliers include objects with unusual atmospheric compositions, strongly magnetic white dwarfs, and binary white dwarfs, including double degenerates and white dwarf + main-sequence systems. We present a list of 89 newly identified outliers based on GALEX UV data; follow-up observations of these objects will be required to constrain their nature. Several current and upcoming large-scale spectroscopic surveys are targeting >105 white dwarfs. In addition, the ULTRASAT mission is planning an all-sky survey in the NUV band. A combination of the UV data from GALEX and ULTRASAT and optical data on these large samples of spectroscopically confirmed DA white dwarfs will provide an excellent opportunity to identify unusual white dwarfs in the solar neighbourhood.

Triple Evolution: An Important Channel in the Formation of Type Ia Supernovae

Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear explosions in white dwarfs (WDs). Commonly considered formation pathways include two merging WDs (the double-degenerate channel) and a single WD accreting material from a H or He donor (the single-degenerate channel). Since the predicted SN Ia rates from WDs in binaries are thought to be insufficient to explain the observed SN Ia rate, it is important to study similar interactions in higher-order multiple-star systems such as triple systems. We use the evolutionary population synthesis code Multiple Stellar Evolution (MSE) to study the stellar evolution, binary interactions, and gravitational dynamics of the triple-star systems. Also, unlike previous studies, prescriptions are included to simultaneously take into account the single- and double-degenerate channels, and we consider triples across the entire parameter space (including those with tight inner binaries). We explore the impact of typically ignored or uncertain physics such as flybys and common envelope prescription parameters on our results. The majority of systems undergo circular mergers to explode as SNe Ia, while eccentric collisions contribute to 0.4%−4% of SN Ia events. The time-integrated SN Ia rate from the triple channel is found to be , which is, surprisingly, similar to that of the isolated binary channel, where the SN Ia rate is . This implies that triples, when considering their entire parameter space, yield an important contribution to the overall SN Ia rate.

Post-Keplerian waveform model for binary compact object as sources of space-based gravitational wave detector and its implications

Binary compact objects will be among the important sources for the future space-based gravitational wave detectors. Such binary compact objects include stellar massive binary black hole, binary neutron star, binary white dwarf and mixture of these compact objects. Regarding to the relatively low frequency, the gravitational interaction between the two objects of the binary is weak. Post-Newtonian approximation of general relativity is valid. Previous works about the waveform model for such binaries in the literature consider the dynamics for specific situations which involve detailed complicated matter dynamics between the two objects. We here take a different idea. We adopt the trick used in pulsar timing detection. For any gravity theories and any detailed complicated matter dynamics, the motion of the binary can always be described as a post-Keplerian expansion. And a post-Keplerian gravitational waveform model will be reduced. Instead of object masses, spins, matter’s equation of state parameters and dynamical parameters beyond general relativity, the involved parameters in our post-Keplerian waveform model are the Keplerian orbit elements and their adiabatic variations. Respect to current planning space-based gravitational wave detectors including LISA, Taiji and Tianqin, we find that the involved waveform model parameters can be well determined. And consequently the detail matter dynamics of the binary can be studied then. For binary with purely gravitational interactions, gravity theory can be constrained well.

Dynamically Forming Extremely Low-mass White Dwarf Binaries in Wide Orbits

The detection of a 0.2 M ⊙ extremely low-mass white dwarf (EW) in a wide orbit (P orb ≈ 450 days) with a 1.1 M ⊙ main-sequence companion, KIC 8145411, challenges our current understanding of how EWs form. The traditional channel for EW formation, via mass transfer from the EW’s progenitor, is expected to form EW binaries in tight orbits. Indeed, the majority of known EWs are found in tight binaries with a median P orb ≈ 5.4 hr. Using numerical scattering experiments, we find that binary–binary strong encounters in star clusters can sufficiently widen the orbit of a typical EW binary, to explain the observed wide orbit of the KIC 8145411 system. The P orb distribution for EW binaries produced through binary–binary encounters is bimodal: one mode corresponds to the initial orbital period of the EW binary, while the other is near P orb ∼ few 102 days, similar to the orbital period of the KIC 8145411 system. We find that the production of wide EW binaries that are also ejected from the cluster peaks at a star cluster mass of ∼105 M ⊙ with a rate of ∼10−3 Gyr−1. Assuming that 50% of all stars form in star clusters and an initial cluster mass function ∝m −2, we estimate a galactic formation rate of ∼4.16 × 103 Gyr−1 for wide EW binaries.

Probing the Efficiency of Tidal Synchronization in Outspiralling Double White Dwarf Binaries with LISA

Compact-object binaries including a white dwarf component are unique among gravitational-wave sources because their evolution is governed not just by general relativity and tides, but also by mass transfer. While the black hole and neutron star binaries observed with ground-based gravitational-wave detectors are driven to inspiral due to the emission of gravitational radiation—manifesting as a “chirp-like” gravitational-wave signal—the astrophysical processes at work in double white dwarf (DWD) systems can cause the inspiral to stall and even reverse into an outspiral. The dynamics of the DWD outspiral thus encode information about tides, which tell us about the behavior of electron-degenerate matter. We carry out a population study to determine the effect of the strength of tides on the distributions of the DWD binary parameters that the Laser Interferometer Space Antenna (LISA) will be able to constrain. We find that the strength of tidal coupling parameterized via the tidal synchronization timescale at the onset of mass transfer affects the distribution of gravitational-wave frequencies and frequency derivatives for detectably mass-transferring DWD systems. Using a hierarchical Bayesian framework informed by binary population synthesis simulations, we demonstrate how this parameter can be inferred using LISA observations. By measuring the population properties of DWDs, LISA will be able to probe the behavior of electron-degenerate matter.