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1. Probing the dark Solar system: detecting binary asteroids with a space-based interferometric asteroid explorer

   https://doi.org/10.1093/mnras/stac669  

   Sullivan, Andrew G. ; Veske, Doğa ; Márka, Zsuzsa ; Bartos, Imre ; Márka, Szabolcs ( March 2022 , Monthly Notices of the Royal Astronomical Society)

   With the inception of gravitational wave astronomy, astrophysical studies using interferometric techniques have begun to probe previously unknown parts of the Universe. In this work, we investigate the potential of a new interferometric experiment to study a unique group of gravitationally interacting sources within our Solar system: binary asteroids. We present the first study into binary asteroid detection via gravitational signals. We identify the interferometer sensitivity necessary for detecting a population of binary asteroids in the asteroid belt. We find that the space-based gravitational wave detector LISA will have negligible ability to detect these sources as these signals will be well below the LISA noise curve. Consequently, we propose a 4.6 au and a 1 au arm-length interferometer specialized for binary asteroid detection, targeting frequencies between 10−6 and 10−4 Hz. Our results demonstrate that the detection of binary asteroids with space-based gravitational wave interferometers is possible though very difficult, requiring substantially improved interferometric technology over what is presently proposed for space-based missions. If that threshold can be met, an interferometer may be used to map the asteroid belt, allowing for new studies into the evolution of our Solar system.
2. Probing the Efficiency of Tidal Synchronization in Outspiralling Double White Dwarf Binaries with LISA

   https://doi.org/10.3847/1538-4357/acc585  

   Biscoveanu, Sylvia ; Kremer, Kyle ; Thrane, Eric ( June 2023 , The Astrophysical Journal)

   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, LISAmore »will be able to probe the behavior of electron-degenerate matter.

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3. Gravitational Wave Sources in Our Galactic Backyard: Predictions for BHBH, BHNS, and NSNS Binaries Detectable with LISA

   https://doi.org/10.3847/1538-4357/ac8675  

   Wagg, T. ; Broekgaarden, F. S. ; de Mink, S. E. ; Frankel, N. ; van Son, L. A. C. ; Justham, S. ( October 2022 , The Astrophysical Journal)

   Future searches for gravitational waves from space will be sensitive to double compact objects in our Milky Way. We present new simulations of the populations of double black holes (BHBHs), BH neutron stars (BHNSs), and double neutron stars (NSNSs) that will be detectable by the planned space-based gravitational-wave detector called Laser Interferometer Space Antenna (LISA). For our estimates, we use an empirically informed model of the metallicity-dependent star formation history of the Milky Way. We populate it using an extensive suite of binary population-synthesis predictions for varying assumptions relating to mass transfer, common-envelope, supernova kicks, remnant masses, and wind mass-loss physics. For a 4(10) yr LISA mission, we predict between 30–370(50–550) detections over these variations, out of which 6–154 (9–238) are BHBHs, 2–198 (3–289) are BHNSs, and 3–35 (4–57) are NSNSs. We expect that about 50% (60%) can be distinguished from double white dwarf sources based on their mass or eccentricity and localization. Specifically, for about 10% (15%), we expect to be able to determine chirp masses better than 10%. For 13% (13%), we expect sky-localizations better than 1°. We discuss how the variations in the physics assumptions alter the distribution of properties of the detectable systems, even whenmore »the detection rates are unchanged. We further discuss the possibility of multimessenger observations of pulsar populations with the Square Kilometre Array and assess the benefits of extending the LISA mission.

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4. On the detectability of massive black hole merger events by Laser Interferometry Space Antenna

   https://doi.org/10.1093/mnras/stac831  

   Banks, Samuel ; Lee, Katharine ; Azimi, Nazanin ; Scarborough, Kendall ; Stefanov, Nikolai ; Periwal, Indra ; Chen, Nianyi ; DeGraf, Colin ; Di Matteo, Tiziana ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   The launch of space-based gravitational-wave (GW) detectors (e.g. Laser Interferometry Space Antenna; LISA) and current and upcoming Pulsar Timing Arrays will extend the GW window to low frequencies, opening new investigations into dynamical processes involving massive black hole binaries (MBHBs) and their mergers across cosmic time. MBHBs are expected to be among the primary sources for the upcoming low-frequency (10−4–10−1 Hz) window probed by LISA. It is important to investigate the expected supermassive BH merger rates and associated signals, to determine how potential LISA events are affected by physics included in current models. To study this, we post-process the large population of MBHBs in the Illustris simulation to account for dynamical friction time delays associated with BH infall/inspiral. We show that merger delays associated with binary evolution have the potential to decrease the expected merger rates, with $M_{\rm {BH}}\ \gt\ 10^6\ \mathrm{M}_\odot$ MBHBs (the lowest mass in Illustris) decreasing from ∼3 to ∼0.1 yr−1, and shifting the merger peak from z ∼2 to ∼1.25. During this time, we estimate that accretion grows the total merging mass by as much as 7x the original mass. Importantly, however, dynamical friction-associated delays (which shift the mergers toward lower redshift and higher masses) leadmore »to a stronger signal/strain for the emitted GWs in the LISA band, increasing mean frequency from 10−3.1 to 10−3.4–10−4.0 Hz, and mean strain from 10−17.2 to 10−16.3–10−15.3. Finally, we show that after including a merger delay and associated MBH growth, mergers still tend to lie on the typical MBH–M* relation, but with an increased likelihood of an undermassive BH.

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5. Predicting the LISA white dwarf binary population in the Milky Way with cosmological simulations

   https://doi.org/10.1093/mnras/stz2834  

   Lamberts, Astrid ; Blunt, Sarah ; Littenberg, Tyson B. ; Garrison-Kimmel, Shea ; Kupfer, Thomas ; Sanderson, Robyn E. ( October 2019 , Monthly Notices of the Royal Astronomical Society)

   White dwarf binaries with orbital periods below 1 h will be the most numerous sources for the space-based gravitational wave detector Laser Interferometer Space Antenna (LISA). Based on thousands of individually resolved systems, we will be able to constrain binary evolution and provide a new map of the Milky Way and its close surroundings. In this paper we predict the main properties of populations of different types of detached white dwarf binaries detected by LISA over time. For the first time, we combine a high-resolution cosmological simulation of a Milky Way-mass galaxy (taken from the FIRE project) with a binary population synthesis model for low- and intermediate-mass stars. Our Galaxy model therefore provides a cosmologically realistic star formation and metallicity history for the Galaxy and naturally produces its different components such as the thin and thick disc, the bulge, the stellar halo, and satellite galaxies and streams. Thanks to the simulation, we show how different Galactic components contribute differently to the gravitational wave signal, mostly due to their typical age and distance distributions. We find that the dominant LISA sources will be He–He double white dwarfs (DWDs) and He–CO DWDs with important contributions from the thick disc and bulge. The resulting sky map ofmore »the sources is different from previous models, with important consequences for the searches for electromagnetic counterparts and data analysis. We also emphasize that much of the science-enabling information regarding white dwarf binaries, such as the chirp mass and the sky localization, becomes increasingly rich with long observations, including an extended mission up to 8 yr.

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