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1. Euclid detectability of pair instability supernovae in binary population synthesis models consistent with merging binary black holes

   https://doi.org/10.1093/mnrasl/slac149  

   Tanikawa, Ataru ; Moriya, Takashi J. ; Tominaga, Nozomu ; Yoshida, Naoki ( November 2022 , Monthly Notices of the Royal Astronomical Society: Letters)

   We infer the expected detection number of pair instability supernovae (PISNe) during the operation of the Euclid space telescope based on binary population models. Our models reproduce the global maximum at the primary BH mass of ∼9–10 M⊙ and the overall gradient of the primary BH mass distribution in the binary BH merger rate consistent with recent observations. We consider different PISN conditions depending on the 12C(α, γ)16O reaction rate. The fiducial and 3σ models adopt the standard and 3σ smaller reaction rates, respectively. Our fiducial model predicts that Euclid detects several hydrogen-poor PISNe. For the 3σ model, detection of ∼1 hydrogen-poor PISN by Euclid is expected if the stellar mass distribution extends to Mmax  = 600 M⊙, but the expected number becomes significantly smaller if Mmax  = 300 M⊙. We may be able to distinguish the fiducial and 3σ models by the observed PISN rate. This will help us to constrain the origin of binary BHs and the reaction rate, although there remains a degeneracy between Mmax  and the reaction rate. PISN ejecta mass estimates from light curves and spectra obtained by follow-up observations would be important to disentangle the degeneracy.
2. Is GW190521 the merger of black holes from the first stellar generations?

   Farrell, E.J. ; Groh, J.H. ; Hirschi, R. ; Murphy, L. ; Kaiser, E. ; Ekström, S. ; Georgy, C. ; Meynet, G. ( September 2020 , ArXivorg)

   GW190521 challenges our understanding of the late-stage evolution of massive stars and the effects of the pair-instability in particular. We discuss the possibility that stars at low or zero metallicity could retain most of their hydrogen envelope until the pre-supernova stage, avoid the pulsational pair-instability regime and produce a black hole with a mass in the mass gap by fallback. We present a series of new stellar evolution models at zero and low metallicity computed with the Geneva and MESA stellar evolution codes and compare to existing grids of models. Models with a metallicity in the range 0-0.0004 have three properties which favour higher BH masses as compared to higher metallicity models. These are (i) lower mass-loss rates during the post-MS phase, (ii) a more compact star disfavouring binary interaction and (iii) possible H-He shell interactions which lower the CO core mass. We conclude that it is possible that GW190521 may be the merger of black holes produced directly by massive stars from the first stellar generations. Our models indicate BH masses up to 70-75 Msun. Uncertainties related to convective mixing, mass loss, H-He shell interactions and pair-instability pulsations may increase this limit to ~85 Msun.
3. Resolving the Peak of the Black Hole Mass Spectrum

   https://doi.org/10.3847/1538-4357/ac8b83  

   Farag, Ebraheem ; Renzo, Mathieu ; Farmer, Robert ; Chidester, Morgan T. ; Timmes, F. X. ( October 2022 , The Astrophysical Journal)

   Gravitational-wave (GW) detections of binary black hole (BH) mergers have begun to sample the cosmic BH mass distribution. The evolution of single stellar cores predicts a gap in the BH mass distribution due to pair-instability supernovae (PISNe). Determining the upper and lower edges of the BH mass gap can be useful for interpreting GW detections of merging BHs. We useMESAto evolve single, nonrotating, massive helium cores with a metallicity ofZ= 10⁻⁵, until they either collapse to form a BH or explode as a PISN, without leaving a compact remnant. We calculate the boundaries of the lower BH mass gap for S-factors in the range S(300 keV) = (77,203) keV b, corresponding to the ±3σuncertainty in our high-resolution tabulated¹²C(α,γ)¹⁶O reaction rate probability distribution function. We extensively test temporal and spatial resolutions for resolving the theoretical peak of the BH mass spectrum across the BH mass gap. We explore the convergence with respect to convective mixing and nuclear burning, finding that significant time resolution is needed to achieve convergence. We also test adopting a minimum diffusion coefficient to help lower-resolution models reach convergence. We establish a new lower edge of the upper mass gap asM_lower≃${60}_{-14}^{+32}$M_⊙from the ±3σuncertainty inmore »the¹²C(α,γ)¹⁶O rate. We explore the effect of a larger 3αrate on the lower edge of the upper mass gap, findingM_lower≃${69}_{-18}^{+34}$M_⊙. We compare our results with BHs reported in the Gravitational-Wave Transient Catalog.

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4. Light Curves and Event Rates of Axion Instability Supernovae

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

   Mori, Kanji ; Moriya, Takashi J. ; Takiwaki, Tomoya ; Kotake, Kei ; Horiuchi, Shunsaku ; Blinnikov, Sergei I. ( January 2023 , The Astrophysical Journal)

   It was recently proposed that exotic particles can trigger a new stellar instability that is analogous to thee⁻e⁺pair instability if they are produced and reach equilibrium in the stellar plasma. In this study, we construct axion instability supernova (AISN) models caused by the new instability to predict their observational signatures. We focus on heavy axion-like particles (ALPs) with masses of ∼400 keV–2 MeV and coupling with photons ofg_aγ∼ 10⁻⁵GeV⁻¹. It is found that the⁵⁶Ni mass and the explosion energy are significantly increased by ALPs for a fixed stellar mass. As a result, the peak times of the light curves of AISNe occur earlier than those of standard pair-instability supernovae by 10–20 days when the ALP mass is equal to the electron mass. Also, the event rate of AISNe is 1.7–2.6 times higher than that of pair-instability supernovae, depending on the high mass cutoff of the initial mass function.
5. Ain’t No Mountain High Enough: Semiparametric Modeling of LIGO–Virgo’s Binary Black Hole Mass Distribution

   https://doi.org/10.3847/1538-4357/ac3667  

   Edelman, Bruce ; Doctor, Zoheyr ; Godfrey, Jaxen ; Farr, Ben ( January 2022 , The Astrophysical Journal)

   We introduce a semiparametric model for the primary mass distribution of binary black holes (BBHs) observed with gravitational waves (GWs) that applies a cubic-spline perturbation to a power law. We apply this model to the 46 BBHs included in the second gravitational-wave transient catalog (GWTC-2). The spline perturbation model recovers a consistent primary mass distribution with previous results, corroborating the existence of a peak at 35M_⊙(>97% credibility) found with the Powerlaw+Peakmodel. The peak could be the result of pulsational pair-instability supernovae. The spline perturbation model finds potential signs of additional features in the primary mass distribution at lower masses similar to those previously reported by Tiwari and Fairhurst. However, with fluctuations due to small-number statistics, the simpler Powerlaw+Peakand BrokenPowerlawmodels are both still perfectly consistent with observations. Our semiparametric approach serves as a way to bridge the gap between parametric and nonparametric models to more accurately measure the BBH mass distribution. With larger catalogs we will be able to use this model to resolve possible additional features that could be used to perform cosmological measurements and will build on our understanding of BBH formation, stellar evolution, and nuclear astrophysics.