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1. On the Single-event-based Identification of Primordial Black Hole Mergers at Cosmological Distances

   https://doi.org/10.3847/2041-8213/ac6bea  

   Ng, Ken K. Y. ; Chen, Shiqi ; Goncharov, Boris ; Dupletsa, Ulyana ; Borhanian, Ssohrab ; Branchesi, Marica ; Harms, Jan ; Maggiore, Michele ; Sathyaprakash, B. S. ; Vitale, Salvatore ( May 2022 , The Astrophysical Journal Letters)

   The existence of primordial black holes (PBHs), which may form from the collapse of matter overdensities shortly after the Big Bang, is still under debate. Among the potential signatures of PBHs are gravitational waves (GWs) emitted from binary black hole (BBH) mergers at redshiftsz≳ 30, where the formation of astrophysical black holes is unlikely. Future ground-based GW detectors, the Cosmic Explorer and Einstein Telescope, will be able to observe equal-mass BBH mergers with total mass of$\mathit{}(10–100)M_{\odot }$at such distances. In this work, we investigate whether the redshift measurement of a single BBH source can be precise enough to establish its primordial origin. We simulate BBHs of different masses, mass ratios and orbital orientations. We show that for BBHs with total masses between 20M_⊙and 40M_⊙merging atz≥ 40, one can inferz> 30 at up to 97% credibility, with a network of one Einstein Telescope, one 40 km Cosmic Explorer in the US, and one 20 km Cosmic Explorer in Australia. This number reduces to 94% with a smaller network made of one Einstein Telescope and one 40 km Cosmic Explorer in the US. We also analyze how the measurement depends on the Bayesian priors usedmore »in the analysis and verify that priors that strongly favor the wrong model yield smaller Bayesian evidences.

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2. Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for Their Neutron Star Merger Origins

   https://doi.org/10.3847/1538-4357/ac91d1  

   Nugent, Anya E. ; Fong, Wen-Fai ; Dong, Yuxin ; Leja, Joel ; Berger, Edo ; Zevin, Michael ; Chornock, Ryan ; Cobb, Bethany E. ; Kelley, Luke Zoltan ; Kilpatrick, Charles D. ; et al ( November 2022 , The Astrophysical Journal)

   We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using theProspectorstellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of$z={0.64}_{-0.32}^{+0.83}$(68% confidence), including nine photometric redshifts atz≳ 1. We further find a median mass-weighted age oft_m=${0.8}_{-0.53}^{+2.71}$Gyr, stellar mass of log(M_*/M_⊙) =${9.69}_{-0.65}^{+0.75}$, star formation rate of SFR =${1.44}_{-1.35}^{+9.37}$M_⊙yr⁻¹, stellar metallicity of log(Z_*/Z_⊙) =$-{0.38}_{-0.42}^{+0.44}$, and dust attenuation of$A_V={0.43}_{-0.36}^{+0.85}$mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (≈84%), with small fractions that are either transitioning (≈6%) or quiescent (≈10%); however, we observe a much larger fraction (≈40%) of quiescent and transitioning hosts atz≲ 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution,more »with a fast-merging channel atz> 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website.

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3. Cover Your Basis: Comprehensive Data-driven Characterization of the Binary Black Hole Population

   https://doi.org/10.3847/1538-4357/acb5ed  

   Edelman, Bruce ; Farr, Ben ; Doctor, Zoheyr ( March 2023 , The Astrophysical Journal)

   We introduce the first complete nonparametric model for the astrophysical distribution of the binary black hole (BBH) population. Constructed from basis splines, we use these models to conduct the most comprehensive data-driven investigation of the BBH population to date, simultaneously fitting nonparametric models for the BBH mass ratio, spin magnitude and misalignment, and redshift distributions. With GWTC-3, we report the same features previously recovered with similarly flexible models of the mass distribution, most notably the peaks in merger rates at primary masses of ∼10M_⊙and ∼35M_⊙. Our model reports a suppressed merger rate at low primary masses and a mass-ratio distribution consistent with a power law. We infer a distribution for primary spin misalignments that peaks away from alignment, supporting conclusions of recent work. We find broad agreement with the previous inferences of the spin magnitude distribution: the majority of BBH spins are small (a< 0.5), the distribution peaks ata∼ 0.2, and there is mild support for a nonspinning subpopulation, which may be resolved with larger catalogs. With a modulated power law describing the BBH merger rate’s evolution in redshift, we see hints of the rate evolution either flattening or decreasing atz∼ 0.2–0.5, but the full distribution remains entirely consistentmore »with a monotonically increasing power law. We conclude with a discussion of the astrophysical context of our new findings and how nonparametric methods in gravitational-wave population inference are uniquely poised to complement to the parametric approach as we enter the data-rich era of gravitational-wave astronomy.

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4. Impact of massive binary star and cosmic evolution on gravitational wave observations – II. Double compact object rates and properties

   https://doi.org/10.1093/mnras/stac1677  

   Broekgaarden, Floor S. ; Berger, Edo ; Stevenson, Simon ; Justham, Stephen ; Mandel, Ilya ; Chruślińska, Martyna ; van Son, Lieke A. C. ; Wagg, Tom ; Vigna-Gómez, Alejandro ; de Mink, Selma E. ; et al ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   Making the most of the rapidly increasing population of gravitational-wave detections of black hole (BH) and neutron star (NS) mergers requires comparing observations with population synthesis predictions. In this work, we investigate the combined impact from the key uncertainties in population synthesis modelling of the isolated binary evolution channel: the physical processes in massive binary-star evolution and the star formation history as a function of metallicity, Z, and redshift z, $\mathcal {S}(Z,z)$. Considering these uncertainties, we create 560 different publicly available model realizations and calculate the rate and distribution characteristics of detectable BHBH, BHNS, and NSNS mergers. We find that our stellar evolution and $\mathcal {S}(Z,z)$ variations can combined impact the predicted intrinsic and detectable merger rates by factors in the range 102–104. We find that BHBH rates are dominantly impacted by $\mathcal {S}(Z,z)$ variations, NSNS rates by stellar evolution variations and BHNS rates by both. We then consider the combined impact from all uncertainties considered in this work on the detectable mass distribution shapes (chirp mass, individual masses, and mass ratio). We find that the BHNS mass distributions are predominantly impacted by massive binary-star evolution changes. For BHBH and NSNS, we find that both uncertainties are important. Wemore »also find that the shape of the delay time and birth metallicity distributions are typically dominated by the choice of $\mathcal {S}(Z,z)$ for BHBH, BHNS, and NSNS. We identify several examples of robust features in the mass distributions predicted by all 560 models, such that we expect more than 95 per cent of BHBH detections to contain a BH $\gtrsim 8\, \rm {M}_{\odot }$ and have mass ratios ≲ 4. Our work demonstrates that it is essential to consider a wide range of allowed models to study double compact object merger rates and properties. Conversely, larger observed samples could allow us to decipher currently unconstrained stages of stellar and binary evolution.

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5. 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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