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1. Recovering Injected Astrophysics from the LISA Double White Dwarf Binaries

   https://doi.org/10.3847/1538-4357/ada9e2  

   Delfavero, Vera; Breivik, Katelyn; Thiele, Sarah; O’Shaughnessy, Richard; Baker, John G (February 2025, The Astrophysical Journal)

   Abstract We present the successful recovery of common-envelope ejection efficiency assumed in a simulated population of double white dwarf (DWD) binaries like those which may be observed by the future Laser Interferometer Space Antenna (LISA) mission. We simulate the formation of DWD binaries by using the COSMIC population synthesis code to sample binary formation conditions such as initial mass function, metallicity of star formation, initial orbital period, and initial eccentricity. These binaries are placed in the m12i synthetic Milky Way–like galaxy, and their signal-to-noise ratio (SNR) for the LISA instrument is estimated, considering a Galactic gravitational-wave foreground informed by the population. Through the use of Fisher estimates, we construct a likelihood function for the measurement error of the LISA-bright DWD binaries (≥20 SNR,f_GW≥ 5 mHz), in their gravitational-wave frequency (f_GW) and chirp mass. By repeating this process for different assumptions of the common-envelope ejection efficiency, we apply Bayesian hierarchical inference to find the best match to an injected astrophysical assumption for a fiducial population model. We conclude that the impact of common-envelope ejection efficiency on the mass-transfer processes involved in DWD formation may be statistically relevant in the future observed LISA population, and that constraints on binary formation may be found by comparing simulated populations to a future observed population. 

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2. Unraveling cosmological anisotropies within stochastic gravitational wave backgrounds

   https://doi.org/10.1088/1475-7516/2023/10/064  

   Cui, Yanou; Kumar, Soubhik; Sundrum, Raman; Tsai, Yuhsin (October 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract Identifying the anisotropies in a cosmologically sourced stochastic gravitational wave background (SGWB) would be of significance in shedding light on the nature of primordial inhomogeneities.For example, if SGWB carries isocurvature fluctuations, it would provide evidence for a multi-field inflationary origin of these inhomogeneities.However, this is challenging in practice due to finite detector sensitivity and also the presence of the astrophysical foregrounds that can compete with the cosmological signal.In this work, we explore the prospects for measuring cosmological SGWB anisotropies in the presence of an astrophysical counterpart and detector noise.To illustrate the main idea, we perform a Fisher analysis using a well-motivated cosmological SGWB template corresponding to a first order phase transition,and an astrophysical SGWB template corresponding to extra-galactic binary mergers, and compute the uncertainty with which various parameters characterizing the isotropic and anisotropic components can be extracted.We also discuss some subtleties and caveats involving shot noise in the astrophysical foreground.Overall, we show that upcoming experiments, e.g., LISA, Taiji, Einstein Telescope, Cosmic Explorer, and BBO, can all be effective in discovering plausible anisotropic cosmological SGWBs. 

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3. Probing anisotropies of the Stochastic Gravitational Wave Background with LISA

   https://doi.org/10.1088/1475-7516/2022/11/009  

   Bartolo, Nicola; Bertacca, Daniele; Caldwell, Robert; Contaldi, Carlo R.; Cusin, Giulia; De Luca, Valerio; Dimastrogiovanni, Emanuela; Fasiello, Matteo; Figueroa, Daniel G.; Franciolini, Gabriele; et al (November 2022, Journal of Cosmology and Astroparticle Physics)

   Abstract We investigate the sensitivity of the Laser Interferometer Space Antenna (LISA) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We first discussthe main astrophysical and cosmological sources of SGWB which are characterized by anisotropies in the GW energy density, and we build a Signal-to-Noise estimator to quantify the sensitivity of LISA to different multipoles. We then perform a Fisher matrix analysis of the prospects of detectability of anisotropic features with LISA for individual multipoles, focusing on a SGWB with a power-law frequency profile. We compute the noise angular spectrum taking into account the specific scan strategy of the LISA detector. We analyze the case of the kinematic dipole and quadrupole generated by Doppler boosting an isotropic SGWB. We find that β Ω GW ∼ 2 × 10 -11 is required to observe a dipolar signal with LISA. The detector response to the quadrupole has a factor ∼ 10 3 β relative to that of the dipole. The characterization of the anisotropies, both from a theoretical perspective and from a map-making point of view, allows us to extract information that can be used to understand the origin of the SGWB, and to discriminate among distinct superimposed SGWB sources. 

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4. Convection and spin-up during common envelope evolution: the formation of short-period double white dwarfs

   Wilson, E. C. Nordhaus (July 2020, Monthly notices of the Royal Astronomical Society)

   The formation channels and predicted populations of double white dwarfs (DWDs) are important because a subset will evolve to be gravitational-wave sources and/or progenitors of Type Ia supernovae. Given the observed population of short-period DWDs, we calculate the outcomes of common envelope (CE) evolution when convective effects are included. For each observed white dwarf (WD) in a DWD system, we identify all progenitor stars with an equivalent proto-WD core mass from a comprehensive suite of stellar evolution models. With the second observed WD as the companion, we calculate the conditions under which convection can accommodate the energy released as the orbit decays, including (if necessary) how much the envelope must spin-up during the CE phase. The predicted post-CE final separations closely track the observed DWD orbital parameter space, further strengthening the view that convection is a key ingredient in CE evolution. 

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5. The DBL Survey II: towards a mass – period distribution of double white dwarf binaries

   https://doi.org/10.1093/mnras/staf1198  

   Munday, James; Pelisoli, Ingrid; Tremblay, Pier-Emmanuel; Jones, David; Nelemans, Gijs; Kilic, Mukremin; Cunningham, Tim; Toonen, Silvia; Santos-García, Alejandro; Dawson, Harry; et al (July 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Double white dwarf (DWD) binaries are an important remnant of binary evolution as they are possible Type Ia supernova progenitors and strong sources of gravitational waves in the low-frequency regime. The double-lined double white dwarf (DBL) survey searches for compact DWDs where both stars can be spectrally disentangled. Candidates are identified by being overluminous compared to the cooling sequence of a typical-mass, single white dwarf. In this second instalment of the DBL survey, we present full orbital solutions of 15 DWD binaries from our ongoing campaign to accurately measure a magnitude-limited mass–period distribution. For the first time, 12 of these systems are fully solved. A long-standing bias in the full population has been evident, favouring systems with orbital periods up to a few hours, with little exploration of the majority of the compact DWD population, whose orbital period distribution centres at approximately 20 h. The 15 systems in this study span the orbital period range $$5< P_\textrm {orb}< 75$$ h, significantly augmenting the number of well-characterized systems over these periods, and in general have two similar mass stars combining to $$\approx$$1.0 M$$_\odot$$. We witness that the orbitally derived mass ratios generally show excellent agreement with those deduced from atmospheric fits to double-lined spectra in previous work, emphasizing the power of wide-scale spectroscopic surveys to efficiently locate the highest-mass, double-lined DWDs in the local Galaxy. 

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