Pair-instability and pulsational pair-instability supernovae (PPISN) have not been unambiguously observed so far. They are, however, promising candidates for the progenitors of the heaviest binary black hole (BBH) mergers detected. If these BBHs are the product of binary evolution, then PPISNe could occur in very close binaries. Motivated by this, we discuss the implications of a PPISN happening with a close binary companion, and what impact these explosions have on the formation of merging BBHs through binary evolution. For this, we have computed a set of models of metal-poor (Z /10) helium stars using the MESA software instrument. For PPISN progenitors with pre-explosion masses > 50M we find that, after a pulse, heat deposited throughout the layers of the star that remain bound cause it to expand to more than 100R for periods of 10 2 − 10 4 yrs depending on the mass of the progenitor. This results in long-lived phases of Roche-lobe overflow or even common-envelope events if there is a close binary companion, leading to additional electromagnetic transients associated to PPISN eruptions. If we ignore the effect of these interactions, we find that mass loss from PPISNe reduces the final black hole spin by ∼ 30%, induces eccentricities that can be detected by the LISA observatory, and can produce a double-peaked distribution of measured chirp masses in BBH mergers observed by ground-based detectors.

As the number of merging binary black holes observed with ground-based gravitational-wave detectors grows, increasingly accurate theoretical models are required to compare them to the observed sample and disentangle contributions from multiple channels. In formation models involving isolated binary stars, important uncertainties remain regarding the stability of mass transfer and common-envelope evolution. To study some of these uncertainties, we have computed binary simulations using the MESA code consisting of a 30 M⊙ star in a low metallicity (Z⊙/10) environment with a black-hole companion. We have developed an updated prescription to compute mass transfer rates including the possibility of outflows from outer Lagrangian points, as well as a method to self-consistently determine the core-envelope boundary in cases where there is common-envelope evolution. We find that binaries survive common-envelope evolution only if unstable mass transfer happens after the formation of a deep convective envelope, resulting in a narrow range (0.2 dex) in period for successful envelope ejection. All cases where binary interaction is initiated with a radiative envelope have large binding energies (∼1050 erg), and they result in mergers during the common-envelope phase even under the assumption that all the internal and recombination energy of the envelope, as well as the energy from an inspiral, is used to eject the envelope. This is independent of whether or not helium is ignited in the core of the donor, conditions under which various rapid-population synthesis calculations assume a successful envelope ejection is possible. Moreover, we find that the critical mass ratio for instability is such that across a large range in initial orbital periods (∼1−1000 days), merging binary black holes can be formed via stable mass transfer. A large fraction of these systems undergo overflow of their L2 equipotential, in which case we find that stable mass transfer produces merging binary black holes even under extreme assumptions of mass and angular momentum outflows. Our conclusions are limited to the study of one donor mass at a single metallicity, but they suggest that population synthesis calculations overestimate the formation rate of merging binary black holes produced by common-envelope evolution and that stable mass transfer could dominate the formation rate from isolated binaries. This is in agreement with a few other recent studies. Further work is required to extend these results to different masses and metallicities as well as to understand how they can be incorporated into rapid population synthesis calculations.

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The last decade has seen unprecedented effort in dark matter model building at all mass scales coupled with the design of numerous new detection strategies. Transformative advances in quantum technologies have led to a plethora of new high-precision quantum sensors and dark matter detection strategies for ultralight (< 10 eV) bosonic dark matter that can be described by an oscillating classical, largely coherent field. This white paper focuses on searches for wavelike scalar and vector dark matter candidates.

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