Authors: Reinhold Willcox; Morgan MacLeod; Ilya Mandel; Ryosuke Hirai
arXiv: 2308.06666

e use the rapid binary population synthesis code COMPAS to investigate commonly used prescriptions for the determination of mass transfer stability in close binaries and the orbital separations after stable mass transfer. The degree of orbital tightening during non-conservative mass transfer episodes is governed by the poorly-constrained angular momentum carried away by the ejected material. Increased orbital tightening drives systems towards unstable mass transfer leading to a common envelope. We find that the fraction of interacting binaries that will undergo only stable mass transfer throughout their lives fluctuates between a few and $\sim 20\%$ due to uncertainty in the angular momentum loss alone. If mass transfer is significantly non-conservative, stability prescriptions that rely on the assumption of conservative mass transfer under-predict the number of systems which experience unstable mass transfer and stellar mergers. This may substantially impact predictions about the rates of various transients, including luminous red novae, stripped-envelope supernovae, X-ray binaries, and the progenitors of coalescing compact binaries.

Authors: Markus R. Mosbech; Alexander C. Jenkins; Sownak Bose; Celine Boehm; Mairi Sakellariadou; Yvonne Y. Y. Wong
arXiv: 2207.14126

e show that gravitational waves have the potential to unravel the microphysical properties of dark matter due to the dependence of the binary black hole merger rate on cosmic structure formation, which is itself highly dependent on the dark matter scenario. In particular, we demonstrate that suppression of small-scale structure -- such as that caused by interacting, warm, or fuzzy dark matter -- leads to a significant reduction in the rate of binary black hole mergers at redshifts $z\gtrsim5$. This shows that future gravitational-wave observations will provide a new probe of physics beyond the $\Lambda$CDM model.

Authors: Aldana Grichener
arXiv: 2302.06663

e perform population synthesis of massive binaries to study the mergers of neutron stars (NSs) and black holes (BHs) with the cores of their giant secondaries during common envelope evolution (CEE). We use different values of the efficiency parameter $\alpha_{\rm CE}$ in the framework of the energy formalism for traditional CEE ($\alpha_{\rm CE} \leq 1$) and including additional energy sources to unbind the envelope ($\alpha_{\rm CE} > 1$). We constrain the possible values of $\alpha_{\rm CE}$ by comparing the results of our simulations with local rate densities of binary compact object mergers as inferred from gravitational-wave observations. We find two main evolutionary pathways of binary systems that result in NS-core mergers, while only one of them can also lead to the merger of a BH with the core of the giant star. We explore the zero age main sequence (ZAMS) statistical properties of systems that result in NS/BH-core mergers and find that the two evolutionary channels correspond to a bimodal distribution of orbital separations. We estimate the percentage of the mergers' event rates relative to core collapse supernovae (CCSNe). We include the effect of mass accreted by the NS/BH during CEE in a separate set of simulations and find it does not affect the mergers' event rates.

Authors: I. Agudo; L. Amati; T. An; F. E. Bauer; S. Benetti; M. G. Bernardini; R. Beswick; K. Bhirombhakdi; T. de Boer; M. Branchesi; S. J. Brennan; M. D. Caballero-García; E. Cappellaro; N. Castro Rodríguez; A. J. Castro-Tirado; K. C. Chambers; E. Chassande-Mottin; S. Chaty; T.-W. Chen; A. Coleiro; S. Covino; F. D'Ammando; P. D'Avanzo; V. D'Elia; A. Fiore; A. Flörs; M. Fraser; S. Frey; C. Frohmaier; L. Galbany; C. Gall; H. Gao; J. García-Rojas; G. Ghirlanda; S. Giarratana; J. H. Gillanders; M. Giroletti; B. P. Gompertz; M. Gromadzki; K. E. Heintz; Y.-D. Hu; M. E. Huber; A. Inkenhaag; L. Izzo; Z. P. Jin; P. G. Jonker; D. A. Kann; E. C. Kool; R. Kotak; G. Leloudas; A. J. Levan; C.-C. Lin; J. D. Lyman; E. A. Magnier; K. Maguire; I. Mandel; B. Marcote; D. Mata Sánchez; S. Mattila; A. Melandri; M. J. Michałowski; J. Moldon; M. Nicholl; A. Nicuesa Guelbenzu; S. R. Oates; F. Onori; M. Orienti; R. Paladino; Z. Paragi; M. Perez-Torres; E. Pian; G. Pignata; S. Piranomonte; J. Quirola-Vásquez; F. Ragosta; A. Rau; S. Ronchini; A. Rossi; R. Sánchez-Ramírez; O. S. Salafia; S. Schulze; S. J. Smartt; K. W. Smith; J. Sollerman; S. Srivastav; R. L. C. Starling; D. Steeghs; H. F. Stevance; V. Testa; M. A. P. Torres; S. D. Vergani; D. Vescovi; R. Wainscost; D. Watson; K. Wiersema; Ł. Wyrzykowski; J. Yang; S. Yang; D. R. Young
arXiv: 2208.09000

e present the results from multi-wavelength observations of a transient discovered during the follow-up of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance ($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of $\sim 0.1\,M_\odot$, with $^{56}$Ni comprising $\sim 20\%$ of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that $\sim 1$ such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500\,Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.

Authors: Enrique Moreno Méndez; Fabio De Colle; Diego López Cámara; Alejandro Vigna-Gómez
arXiv: 2207.03514

ydrodynamic studies of stellar-mass compact objects (COs) in a common envelope (CE)have shown that the accretion rate onto the CO is a few orders of magnitude below the Bondi-Hoyle-Lyttleton (BHL) estimate. This is several orders of magnitude above the Eddington limit and above the limit for neutrino-cooled accretion (i.e., hypercritical accretion, or HCA). Considering that a binary system inside the CE of a third star accretes material at nearly the same rate as a single object of the same total mass, we propose stellar-evolution channels which form binary black hole (BBH) systems with its component masses within the pair-instability supernova (PISN) mass gap. Our model is based on HCA onto the BBH system engulfed into the CE of a massive tertiary star. Furthermore, we propose a mass transfer mode which allows to store mass lost by the binary onto a third star. Through the use of population synthesis simulations for the evolution of BBHs and standard binary-evolution principles for the interaction with a tertiary star, we are able to produce BBHs masses consistent with those estimated for GW190521. We also discuss the massive binary system Mk34 as a possible progenitor of BBHs in the PISN gap, as well as the spin distribution of the observed mergers in the gravitational-wave catalog.

Authors: Chiaki Kobayashi; Ilya Mandel; Krzysztof Belczynski; Stephane Goriely; Thomas H. Janka; Oliver Just; Ashley J. Ruiter; Dany Van Beveren; Matthias U. Kruckow; Max M. Briel; Jan J. Eldridge; Elizabeth Stanway
arXiv: 2211.04964

omparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole-neutron star mergers, depending on the black-hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this paper, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.

Authors: Isobel M. Romero-Shaw; Ryosuke Hirai; Arash Bahramian; Reinhold Willcox; Ilya Mandel
arXiv: 2303.05375

We conduct a binary population synthesis study to investigate the formation of wind-fed high-mass X-ray binaries containing black holes (BH-HMXBs). We evolve multiple populations of high-mass binary stars and consider BH-HMXB formation rates, masses, spins and separations. We find that systems similar to Cygnus X-1 likely form after stable Case A mass transfer (MT) from the main sequence progenitors of black holes, provided such MT is characterised by low accretion efficiency, $\beta \lesssim 0.1$, with modest orbital angular momentum losses from the non-accreted material. Additionally, BH-HMXB formation relies on a new simple treatment for Case A MT that allows donors to retain larger core masses compared to traditional rapid population-synthesis procedures. At solar metallicity, our Preferred model yields $\mathcal{O}(1)$ observable BH-HMXBs in the Galaxy today, consistent with observations. In this simulation, $8\%$ of BH-HMXBs go on to merge as binary black holes or neutron star--black hole binaries within a Hubble time. Compact-object mergers that evolve via a BH-HMXB phase may contribute $\gtrsim20$~Gpc$^{-3}$~yr$^{-1}$ to the merger rate inferred from gravitational-wave observations. We also suggestthat MT efficiency is higher during stable Case B MT than during Case A MT.

Authors: Alejandro Vigna-Gómez; Enrico Ramirez-Ruiz
arXiv: 2203.08478

The Milky Way is believed to host hundreds of millions of quiescentstellar-mass black holes (BHs). In the last decade, some of these objects havebeen potentially uncovered via gravitational microlensing events. All thesedetections resulted in a degeneracy between the velocity and the mass of thelens. This degeneracy has been lifted, for the first time, with the recentastrometric microlensing detection of OB110462. However, two independentstudies reported very different lens mass for this event. Sahu et al. (2022)inferred a lens mass of 7.1 $\pm$ 1.3 Msun, consistent with a BH, while Lam etal. (2022) inferred 1.6-4.2 Msun, consistent with either a neutron star or aBH. Here, we study the landscape of isolated BHs formed in the field. Inparticular, we focus on the mass and center-of-mass speed of foursub-populations: isolated BHs from single-star origin, disrupted BHs ofbinary-star origin, main-sequence stars with a compact object companion, anddouble compact object mergers. Our model predicts that most ($\gtrsim$ 70%)isolated BHs in the Milky Way are of binary origin. However, non-interactionslead to most massive BHs ($\gtrsim$ 15-20 Msun) being predominantly of singleorigin. Under the assumption that OB110462 is a free-floating compact object weconclude that it is more likely to be a BH originally belonging to a binarysystem. Our results suggest that low-mass BH microlensing events can be usefulto understand binary evolution of massive stars in the Milky Way, whilehigh-mass BH lenses can be useful to probe single stellar evolution.

Authors: Jeff Riley; Ilya Mandel
arXiv: 2303.00508

Rapidly growing catalogs of compact binary mergers from advanced gravitational-wave detectors allow us to explore the astrophysics of massive stellar binaries. Merger observations can constrain the uncertain parameters that describe the underlying processes in the evolution of stars and binary systems in population models. In this paper, we demonstrate that binary black hole populations - namely, detection rates, chirp masses, and redshifts - can be used to measure cosmological parameters describing the redshift-dependent star formation rate and metallicity distribution. We present a method that uses artificial neural networks to emulate binary population synthesis computer models, and construct a fast, flexible, parallelisable surrogate model that we use for inference.

Authors: Liana Rauf; Cullan Howlett; Tamara M. Davis; Claudia D. P. Lagos
arXiv: 2302.08172

In this work, we explore the connection between the gravitational wave (GW)merger rates of stellar-mass binary black holes (BBH) and galaxy properties. Wedo this by generating populations of stars using the binary synthesis codeCOMPAS and evolving them in galaxies from the semi-analytic galaxy formationmodel {\sc Shark}, to determine the number of mergers occurring in eachsimulation time-step. We find that large, metal-rich galaxies with high starformation rates are more likely to have gravitational wave (GW) events comparedto younger, more metal poor galaxies. Our simulation with the default inputparameters predicts a higher local merger rate density compared to the thirdgravitational wave transient catalogue (GWTC-3) prediction from LIGO, VIRGO andKAGRA. This is due to short coalescence times, low metallicities and an highformation rate of stars at low redshift in the galaxy simulation, whichproduces more BBHs that merge within the age of the Universe compared toobservations. We identify alternate remnant mass models that more accuratelyreproduce the observed volumetric rate and provide updated fits to the mergerrate distribution as a function of redshift. We then investigate the relativefraction of GW events in our simulation volume that are in observable hostgalaxies from different upcoming photometric and spectroscopic surveys,determining which of those are more ideal for tracing host galaxies with highmerger rates. The implications of this work can be utilised for numerousapplications, including for constraining stellar evolution models, betterinforming follow-up programs, and placing more informative priors on potentialhost galaxies when measuring cosmological parameters such as the Hubbleconstant.

Authors: Aldana Grichener
arXiv: 2302.06663

We perform population synthesis of massive binaries to study the mergers ofneutron stars (NSs) and black holes (BHs) with the cores of their giantsecondaries during common envelope evolution (CEE). We use different values ofthe efficiency parameter $\alpha_{\rm CE}$ in the framework of the energyformalism for traditional CEE ($\alpha_{\rm CE} \leq 1$) and includingadditional energy sources to unbind the envelope ($\alpha_{\rm CE} > 1$). Weconstrain the possible values of $\alpha_{\rm CE}$ by comparing the results ofour simulations with local rate densities of binary compact object mergers asinferred from gravitational waves observations. We find two primaryevolutionary pathways of binary systems that result in NS-core mergers, whileonly one of them can also lead to the merger of a BH with the core of the giantstar. We explore the zero age main sequence (ZAMS) statistical properties ofsystems that result in NS/BH-core mergers and find that the two evolutionarychannels correspond to a bimodal distribution of orbital separations. Weestimate the percentage of the mergers' event rates relative to core collapsesupernovae (CCSNe). We include the effect of mass accreted by the NS/BH duringCEE in a separate set of simulations and find it does not affect the mergers'event rates.

Authors: L. M. de Sá; A. Bernardo; R. R. A. Bachega; L. S. Rocha; J. E. Horvath
arXiv: 2211.02714

Binary population synthesis provides a direct way of studying the effects ofdifferent choices of binary evolution models and initial parameterdistributions on present-day binary compact merger populations, which can thenbe compared to empirical properties such as observed merger rates. Samples ofzero-age main sequence binaries to be evolved by such codes are typicallygenerated from an universal IMF and simple, uniform, distributions for orbitalperiod $P$, mass ratio $q$ and eccentricity $e$. More recently, however,mounting observational evidence has suggested the non-universality of the IMFand the existence of correlations between binary parameters. In this study, weimplement a metallicity- and redshift-dependent IMF alongside correlateddistributions for $P$, $q$ and $e$ in order to generate representativepopulations of binaries at varying redshifts, which are then evolved with theCOMPAS code in order to study the variations in merger rates and overallpopulation properties.

Authors: Simon Stevenson; Teagan Clarke
arXiv: 2210.05040

Gravitational waves from merging binary black holes can be used to shed lighton poorly understood aspects of massive binary stellar evolution, such as theevolution of massive stars (including their mass-loss rates), the commonenvelope phase, and the rate at which massive stars form throughout the cosmichistory of the Universe. In this paper we explore the \emph{correlated} impactof these phases on predictions for the merger rate and chirp mass distributionof merging binary black holes, aiming to identify possible degeneracies betweenmodel parameters. In many of our models, a large fraction (more than 70% ofdetectable binary black holes) arise from the chemically homogeneous evolutionscenario; these models tend to over-predict the binary black hole merger rateand produce systems which are on average too massive. Our preferred modelsfavour enhanced mass-loss rates for helium rich Wolf--Rayet stars, in tensionwith recent theoretical and observational developments. We identifycorrelations between the impact of the mass-loss rates of Wolf--Rayet stars andthe metallicity evolution of the Universe on the rates and properties ofmerging binary black holes. Based on the observed mass distribution, we arguethat the $\sim 10\%$ of binary black holes with chirp masses greater than $40$M$_\odot$ (the maximum predicted by our models) are unlikely to have formedthrough isolated binary evolution, implying a significant contribution (> 10%)from other formation channels such as dense star clusters or active galacticnuclei. Our models will enable inference on the uncertain parameters governingbinary evolution in the near future.

Authors: L. A. C. van Son; S. E. de Mink; M. Renzo; S. Justham; E. Zapartas; K. Breivik; T. Callister; W. M. Farr; C. Conroy
arXiv: 2209.13609

Gravitational-wave (GW) detections are starting to reveal features in themass distribution of double compact objects. The lower end of the black hole(BH) mass distribution is especially interesting as few formation channelscontribute here and because it is more robust against variations in the cosmicstar formation than the high mass end. In this work we explore the stable masstransfer channel for the formation of GW sources with a focus on the low-massend of the mass distribution. We conduct an extensive exploration of theuncertain physical processes that impact this channel. We note that, forfiducial assumptions, this channel reproduces the peak at $\sim9\mathrm{M_{\odot}}$ in the GW-observed binary BH mass distribution remarkablywell, and predicts a cutoff mass that coincides with the upper edge of thepurported neutron star BH mass gap. The peak and cutoff mass are a consequenceof unique properties of this channel, namely (1) the requirement of stabilityduring the mass transfer phases, and (2) the complex way in which the finalcompact object masses scale with the initial mass. We provide an analyticalexpression for the cutoff in the primary component mass and show that thisadequately matches our numerical results. Our results imply that selectioneffects resulting from the formation channel alone can provide an explanationfor the purported neutron star--BH mass gap in GW detections. This provides analternative to the commonly adopted view that the gap emerges during BHformation.

Authors: Floor S. Broekgaarden; Edo Berger; Simon Stevenson; Stephen Justham; Ilya Mandel; Martyna Chruślińska; Lieke A. C. van Son; Tom Wagg; Alejandro Vigna-Gómez; Selma E. de Mink; Debatri Chattopadhyay; Coenraad J. Neijssel
arXiv: 2112.05763

Making the most of the rapidly increasing population of gravitational-wavedetections of black hole (BH) and neutron star (NS) mergers requires comparingobservations with population synthesis predictions. In this work we investigatethe combined impact from the key uncertainties in population synthesismodelling of the isolated binary evolution channel: the physical processes inmassive binary-star evolution and the star formation history as a function ofmetallicity, $Z$, and redshift $z, \mathcal{S}(Z,z)$. Considering theseuncertainties we create 560 different publicly available model realizations andcalculate 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 impact the predicted intrinsic and detectable merger rates byfactors $10^2$-$10^4$. We find that BHBH rates are dominantly impacted by$\mathcal{S}(Z,z)$ variations, NSNS rates by stellar evolution variations andBHNS rates by both. We then consider the combined impact from all uncertaintiesconsidered in this work on the detectable mass distribution shapes (chirp mass,individual masses and mass ratio). We find that the BHNS mass distributions arepredominantly impacted by massive binary-star evolution changes. For BHBH andNSNS we find that both uncertainties are important. We also find that the shapeof the delay time and birth metallicity distributions are typically dominatedby the choice of $\mathcal{S}(Z,z)$ for BHBH, BHNS and NSNS. We identifyseveral examples of robust features in the mass distributions predicted by all560 models, such that we expect more than 95% of BHBH detections to contain aBH $\gtrsim 8\,\rm{M}_{\odot}$ and have mass ratios $\lesssim 4$. Our workdemonstrates that it is essential to consider a wide range of allowed models tostudy double compact object merger rates and properties.

Authors: Simon Stevenson; Teagan Clarke
arXiv: 2210.05040

Gravitational waves from merging binary black holes can be used to shed lighton poorly understood aspects of massive binary stellar evolution, such as theevolution of massive stars (including their mass-loss rates), the commonenvelope phase, and the rate at which massive stars form throughout the cosmichistory of the Universe. In this paper we explore the \emph{correlated} impactof these phases on predictions for the merger rate and chirp mass distributionof merging binary black holes, aiming to identify possible degeneracies betweenmodel parameters. In many of our models, a large fraction (more than 70% ofdetectable binary black holes) arise from the chemically homogeneous evolutionscenario; these models tend to over-predict the binary black hole merger rateand produce systems which are on average too massive. Our preferred modelsfavour enhanced mass-loss rates for helium rich Wolf--Rayet stars, in tensionwith recent theoretical and observational developments. We identifycorrelations between the impact of the mass-loss rates of Wolf--Rayet stars andthe metallicity evolution of the Universe on the rates and properties ofmerging binary black holes. Based on the observed mass distribution, we arguethat the $\sim 10\%$ of binary black holes with chirp masses greater than $40$M$_\odot$ (the maximum predicted by our models) are unlikely to have formedthrough isolated binary evolution, implying a significant contribution (> 10%)from other formation channels such as dense star clusters or active galacticnuclei. Our models will enable inference on the uncertain parameters governingbinary evolution in the near future.

Authors: Tom Wagg; Floor S. Broekgaarden; Selma E. de Mink; Lieke A. C. van Son; Neige Frankel; Stephen Justham
arXiv: 2111.13704

Future searches for gravitational waves from space will be sensitive todouble compact objects (DCOs) in our Milky Way. We present new simulations ofthe populations of double black holes (BHBHs), black hole neutron stars (BHNSs)and double neutron stars (NSNSs) that will be detectable by the plannedspace-based gravitational wave detector LISA. For our estimates, we use anempirically-informed model of the metallicity dependent star formation historyof the Milky Way. We populate it using an extensive suite of binarypopulation-synthesis predictions for varying assumptions relating to masstransfer, common-envelope, supernova kicks, remnant masses and wind mass lossphysics. For a 4(10)-year 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 discuss how the variationsin the physics assumptions alter the distribution of properties of thedetectable systems, even when the detection rates are unchanged. In particularwe discuss the observable characteristics such as the chirp mass, eccentricityand sky localisation and how the BHBH, BHNS and NSNS populations can bedistinguished, both from each other and from the more numerous double whitedwarf population. We further discuss the possibility of multi-messengerobservations of pulsar populations with the Square Kilometre Array (SKA) andassess the benefits of extending the LISA mission.

Authors: Veome Kapil; Ilya Mandel; Emanuele Berti; Bernhard Müller
arXiv: 2209.09252

Current prescriptions for supernova natal kicks in rapid binary populationsynthesis simulations are based on fits of simple functions to single pulsarvelocity data. We explore a new parameterization of natal kicks received byneutron stars in isolated and binary systems developed by Mandel & M\"uller,which is based on 1D and 3D supernova simulations and accounts for the physicalcorrelations between progenitor properties, remnant mass, and the kickvelocity. We constrain two free parameters in this model using very longbaseline interferometry velocity measurements of Galactic single pulsars. Wefind that the inferred values of natal kick parameters do not differsignificantly between single and binary evolution scenarios. The best-fitvalues of these parameters are $v_{\rm ns} = 520$ km s$^{-1}$ for the scalingpre-factor for neutron star kicks, and $\sigma_{\rm ns}=0.3$ for the fractionalstochastic scatter in the kick velocities.

Authors: L. A. C. van Son; S. E. de Mink; M. Chruslinska; C. Conroy; R. Pakmor; L. Hernquist
arXiv: 2209.03385

New observational facilities are probing astrophysical transients such asstellar explosions and gravitational wave (GW) sources at ever increasingredshifts, while also revealing new features in source property distributions.To interpret these observations, we need to compare them to predictions fromstellar population models. Such models require the metallicity-dependent cosmicstar formation history ($\mathcal{S}(Z,z)$) as an input. Large uncertaintiesremain in the shape and evolution of this function. In this work, we propose asimple analytical function for $\mathcal{S}(Z,z)$. Variations of this functioncan be easily interpreted, because the parameters link to its shape in anintuitive way. We fit our analytical function to the star-forming gas of thecosmological TNG100 simulation and find that it is able to capture the mainbehaviour well. As an example application, we investigate the effect ofsystematic variations in the $\mathcal{S}(Z,z)$ parameters on the predictedmass distribution of locally merging binary black holes (BBH). Our mainfindings are: I) the locations of features are remarkably robust againstvariations in the metallicity-dependent cosmic star formation history, and II)the low mass end is least affected by these variations. This is promising as itincreases our chances to constrain the physics that governs the formation ofthese objects.

Authors: Alejandro Vigna-Gómez; Enrico Ramirez-Ruiz
arXiv: 2203.08478

The Milky Way is believed to host hundreds of millions of quiescentstellar-mass black holes (BHs). In the last decade, some of these objects havebeen potentially uncovered via gravitational microlensing events. All thesedetections resulted in a degeneracy between the velocity and the mass of thelens. This degeneracy has been lifted, for the first time, with the recentastrometric microlensing detection of OB110462. However, two independentstudies reported very different lens mass for this event. Sahu et al. (2022)inferred a lens mass of 7.1 $\pm$ 1.3 Msun, consistent with a BH, while Lam etal. (2022) inferred 1.6-4.2 Msun, consistent with either a neutron star or aBH. Here, we study the landscape of isolated BHs formed in the field. Inparticular, we focus on the mass and center-of-mass speed of foursub-populations: isolated BHs from single-star origin, disrupted BHs ofbinary-star origin, main-sequence stars with a compact object companion, anddouble compact object mergers. Our model predicts that most ($\gtrsim$ 50%)isolated BHs in the Milky Way are of binary origin. Moreover, the origin oflow-speed (< 50 km/s) isolated BHs depends on their mass: at least $\approx$70% of low-mass ($\lesssim$ 10 Msun) BHs are from binary origin. Under theassumption that OB110462 is a free-floating compact object, we conclude that itis more likely to be a BH originally belonging to a binary system. Our resultssuggest that low-speed BH microlensing events can be useful to understandbinary evolution of massive stars in the Milky Way.

Authors: Floor S. Broekgaarden; Simon Stevenson; Eric Thrane
arXiv: 2205.01693

The spins of merging binary black holes offer insights into their formationhistory. Recently it has been argued that in isolated binary evolution of twomassive stars the firstborn black hole is slowly rotating, whilst theprogenitor of the second-born black hole can be tidally spun up if the binaryis tight enough. Naively, one might therefore expect that only the less massiveblack hole in merging binaries exhibits non-negligible spin. However, if themass ratio of the binary is "reversed" (typically during the first masstransfer episode), it is possible for the tidally spun up second-born to becomethe more massive black hole. We study the properties of such mass-ratioreversed (MRR) binary black hole mergers using a large set of 560 populationsynthesis models. We find that the more massive black hole is formed second in$\gtrsim 70\%$ of binary black holes observable by LIGO, Virgo, and KAGRA formost model variations we consider, with typical total masses $\gtrsim 20$M$_{\odot}$ and mass ratios $q = m_2 / m_1 \sim 0.7$ (where $m_1 > m_2$). Theformation history of these systems typically involves only stable mass transferepisodes. The second-born black hole has non-negligible spin ($\chi > 0.05$) inup to $25\%$ of binary black holes, with among those the more (less) massiveblack hole spinning in $0\%$--$80\%$ ($20\%$--$100\%$) of cases, varyinggreatly in our models. We discuss our models in the context of several observedgravitational-wave events and the observed mass ratio - effective spincorrelation.

Authors: Simon Stevenson; Reinhold Willcox; Alejandro Vigna-Gomez; Floor Broekgaarden
arXiv: 2205.03989

Neutron stars receive velocity kicks at birth in supernovae. Those formed inelectron-capture supernovae from super asymptotic giant branch stars -- thelowest mass stars to end their lives in supernovae -- may receive significantlylower kicks than typical neutron stars. Given that many massive stars aremembers of wide binaries, this suggests the existence of a population oflow-mass ($1.25 < M_\mathrm{psr} / $M$_\odot < 1.3$), wide ($P_\mathrm{orb}\gtrsim 10^{4}$\,day), eccentric ($e \sim 0.7$), unrecycled ($P_\mathrm{spin}\sim 1$\,s) binary pulsars. The formation rate of such binaries is sensitive tothe mass range of (effectively) single stars leading to electron capturesupernovae, the amount of mass lost prior to the supernova, and the magnitudeof any natal kick imparted on the neutron star. We estimate that one suchbinary pulsar should be observable in the Milky Way for every 10,000 isolatedpulsars, assuming that the width of the mass range of single stars leading toelectron-capture supernovae is $\lesssim 0.2$\,M$_\odot$, and that neutronstars formed in electron-capture supernovae receive typical kicks less than10\,km s$^{-1}$. We have searched the catalog of observed binary pulsars, butfind no convincing candidates that could be formed through this channel,consistent with this low predicted rate. Future observations with the SquareKilometre Array may detect this rare sub-class of binary pulsar and providestrong constraints on the properties of electron-capture supernovae and theirprogenitors.

Authors: Debatri Chattopadhyay; Simon Stevenson; Floor Broekgaarden; Fabio Antonini; Krzysztof Belczynski
arXiv: 2203.05850

The two neutron star-black hole mergers (GW200105 and GW200115) observed ingravitational waves by advanced LIGO and Virgo, mark the first ever discoveryof such binaries in nature. We study these two neutron star-black hole systemsthrough isolated binary evolution, using a grid of population synthesis models.Using both mass and spin observations (chirp mass, effective spin and remnantspin) of the binaries, we probe their different possible formation channels indifferent metallicity environments. Our models only support LIGO data whenassuming the black hole is non spinning. Our results show a strong preferencethat GW200105 and GW200115 formed from stars with sub-solar metallicities$Z\lesssim 0.005$. Only two metal-rich ($Z=0.02$) models are in agreement withGW200115. We also find that chirp mass and remnant spins jointly aid inconstraining the models, whilst the effective spin parameter does not add anyfurther information. We also present the observable (i.e. post selectioneffects) median values of spin and mass distribution from all our models, whichmaybe used as a reference for future mergers. Further, we show that the remnantspin parameter distribution exhibits distinguishable features in differentneutron star-black hole sub-populations. We find that non-spinning, first bornblack holes dominate significantly the merging neutron star-black holepopulation, ensuring electromagnetic counterparts to such mergers a rareaffair.

Authors: Team COMPAS; Jeff Riley; Poojan Agrawal; Jim W. Barrett; Kristan N. K. Boyett; Floor S. Broekgaarden; Debatri Chattopadhyay; Sebastian M. Gaebel; Fabian Gittins; Ryosuke Hirai; George Howitt; Stephen Justham; Lokesh Khandelwal; Floris Kummer; Mike Y. M. Lau; Ilya Mandel; Selma E. de Mink; Coenraad Neijssel; Tim Riley; Lieke van Son; Simon Stevenson; Alejandro Vigna-Gomez; Serena Vinciguerra; Tom Wagg; Reinhold Willcox
arXiv: 2109.10352

Compact Object Mergers: Population Astrophysics and Statistics (COMPAS;https://compas.science) is a public rapid binary population synthesis code.COMPAS generates populations of isolated stellar binaries under a set ofparametrized assumptions in order to allow comparisons against observationaldata sets, such as those coming from gravitational-wave observations of mergingcompact remnants. It includes a number of tools for population processing inaddition to the core binary evolution components. COMPAS is publicly availablevia the github repository https://github.com/TeamCOMPAS/COMPAS/, and isdesigned to allow for flexible modifications as evolutionary models improve.This paper describes the methodology and implementation of COMPAS. It is aliving document which will be updated as new features are added to COMPAS; thecurrent document describes COMPAS v02.21.00.

Authors: Floor S. Broekgaarden; Edo Berger; Simon Stevenson; Stephen Justham; Ilya Mandel; Martyna Chruślińska
arXiv: 2112.05763

Making the most of the rapidly increasing population of gravitational-wavedetections of black hole (BH) and neutron star (NS) mergers requires comparingobservations with population synthesis predictions. In this work we investigatethe combined impact from the key uncertainties in population synthesismodelling of the isolated binary evolution channel: the physical processes inmassive binary-star evolution and the star formation history as a function ofmetallicity, $Z$, and redshift $z, \mathcal{S}(Z,z)$. Considering theseuncertainties we create 560 different publicly available model realizations andcalculate 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 impact the predicted intrinsic and detectable merger rates byfactors $10^2$-$10^4$. We find that BHBH rates are dominantly impacted by$\mathcal{S}(Z,z)$ variations, NSNS rates by stellar evolution variations andBHNS rates by both. We then consider the combined impact from all uncertaintiesconsidered in this work on the detectable mass distribution shapes (chirp mass,individual masses and mass ratio). We find that the BHNS mass distributions arepredominantly impacted by massive binary-star evolution changes. For BHBH andNSNS we find that both uncertainties are important. We also find that the shapeof the delay time and birth metallicity distributions are typically dominatedby the choice of $\mathcal{S}(Z,z)$ for BHBH, BHNS and NSNS. We identifyseveral examples of robust features in the mass distributions predicted by all560 models, such that we expect more than 95% of BHBH detections to contain aBH $\gtrsim 8\,\rm{M}_{\odot}$ and have mass ratios $\lesssim 4$. Our workdemonstrates that it is essential to consider a wide range of allowed models tostudy double compact object merger rates and properties.

Authors: Tom Wagg; Floor S. Broekgaarden; Selma E. de Mink; Lieke A. C. van Son; Neige Frankel; Stephen Justham
arXiv: 2111.13704

Future searches for gravitational waves from space will be sensitive todouble compact objects (DCOs) in our Milky Way. We present new simulations ofthe populations of double black holes (BHBHs), black hole neutron stars (BHNSs)and double neutron stars (NSNSs) that will be detectable by the plannedspace-based gravitational wave detector LISA. For our estimates, we use anempirically-informed model of the metallicity dependent star formation historyof the Milky Way. We populate it using an extensive suite of binarypopulation-synthesis predictions for varying assumptions relating to masstransfer, common-envelope, supernova kicks, remnant masses and wind mass lossphysics. For a 4(10)-year 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 discuss how the variationsin the physics assumptions alter the distribution of properties of thedetectable systems, even when the detection rates are unchanged. In particularwe discuss the observable characteristics such as the chirp mass, eccentricityand sky localisation and how the BHBH, BHNS and NSNS populations can bedistinguished, both from each other and from the more numerous double whitedwarf population. We further discuss the possibility of multi-messengerobservations of pulsar populations with the Square Kilometre Array (SKA) andassess the benefits of extending the LISA mission.

Authors: L. A. C. van Son, S. E. de Mink, T. Callister, S. Justham, M. Renzo, T. Wagg, F. S. Broekgaarden, F. Kummer, R. Pakmor, I. Mandel
arXiv: 2110.01634

Gravitational wave detectors are starting to reveal the redshift evolution of the binary black hole (BBH) merger rate, RBBH(z). We make predictions for RBBH(z) as a function of black hole mass for systems originating from isolated binaries. To this end, we investigate correlations between the delay time and black hole mass by means of the suite of binary population synthesis simulations, COMPAS. We distinguish two channels: the common envelope (CE), and the stable Roche-lobe overflow (RLOF) channel, characterised by whether the system has experienced a common envelope or not. We find that the CE channel preferentially produces BHs with masses below about 30M⊙ and short delay times (tdelay≲1Gyr), while the stable RLOF channel primarily forms systems with BH masses above 30M⊙ and long delay times (tdelay≳1Gyr). We provide a new fit for the metallicity specific star-formation rate density based on the Illustris TNG simulations, and use this to convert the delay time distributions into a prediction of RBBH(z). This leads to a distinct redshift evolution of RBBH(z) for high and low primary BH masses. We furthermore find that, at high redshift, RBBH(z) is dominated by the CE channel, while at low redshift it contains a large contribution (∼40%) from the stable RLOF channel. Our results predict that, for increasing redshifts, BBHs with component masses above 30M⊙ will become increasingly scarce relative to less massive BBH systems. Evidence of this distinct evolution of RBBH(z) for different BH masses can be tested with future detectors.

Authors: Team COMPAS: Riley, Jeff ; Agrawal, Poojan ; Barrett, Jim W. ; Boyett, Kristan N. K. ; Broekgaarden, Floor S. ; Chattopadhyay, Debatri ; Gaebel, Sebastian M. ; Gittins, Fabian ; Hirai, Ryosuke ; Howitt, George ; Justham, Stephen ; Khandelwal, Lokesh ; Kummer, Floris ; Lau, Mike Y. M. ; Mandel, Ilya ; de Mink, Selma E. ; Neijssel, Coenraad ; Riley, Tim van Son, Lieke ; Stevenson, Simon ; Vigna-Gomez, Alejandro ; Vinciguerra, Serena ; Wagg, Tom ; Willcox, Reinhold
arXiv: 2109.10352

COMPAS (Compact Object Mergers: Population Astrophysics and Statistics, https://compas.science) is a public rapid binary population synthesis code. COMPAS generates populations of isolated stellar binaries under a set of parametrised assumptions in order to allow comparisons against observational data sets, such as those coming from gravitational-wave observations of merging compact remnants. It includes a number of tools for population processing in addition to the core binary evolution components. COMPAS is publicly available via the github repository https://github.com/TeamCOMPAS/COMPAS/, and is designed to allow for flexible modifications as evolutionary models improve. This paper describes the methodology and implementation of COMPAS. It is a living document which will be updated as new features are added to COMPAS; the current document describes COMPAS v02.21.00.

Authors: Belczynski, K. ; Romagnolo, A. ; Olejak, A. ; Klencki, J. ; Chattopadhyay, D. ; Stevenson, S. ; Miller, M. Coleman ; Lasota, J. -P. ; Crowther, P. A.
arXiv: 2108.10885

The LIGO/Virgo gravitational--wave observatories have detected 50 BH-BH coalescences. This sample is large enough to have allowed several recent studies to draw conclusions about the branching ratios between isolated binaries versus dense stellar clusters as the origin of double BHs. It has also led to the exciting suggestion that the population is highly likely to contain primordial black holes. Here we demonstrate that such conclusions cannot yet be robust, because of the large current uncertainties in several key aspects of binary stellar evolution. These include the development and survival of a common envelope, the mass and angular momentum loss during binary interactions, mixing in stellar interiors, pair-instability mass loss and supernova outbursts. Using standard tools such as the population synthesis codes StarTrack and COMPAS and the detailed stellar evolution code MESA, we examine as a case study the possible future evolution of Melnick 34, the most massive known binary star system. We show that, despite its well-known orbital architecture, various assumptions regarding stellar and binary physics predict a wide variety of outcomes: from a close BH-BH binary (which would lead to a potentially detectable coalescence), through a wide BH-BH binary (which might be seen in microlensing observations), or a Thorne-Zytkow object, to a complete disruption of both objects by pair-instability supernovae. Thus since the future of massive binaries is inherently uncertain, sound predictions about the properties of BH-BH systems are highly challenging at this time. Consequently, drawing conclusions about the formation channels for the LIGO/Virgo BH-BH merger population is premature.

Authors: Broekgaarden, F. ; Berger, E.
arXiv: 2108.05763

In this work we study the formation of the first two black hole-neutron star (BHNS) mergers detected in gravitational waves (GW200115 and GW200105) from massive stars in wide isolated binary systems - the isolated binary evolution channel. We use 560 BHNS binary population synthesis model realizations from Broekgaarden et al. (2021a) and show that the system properties (chirp mass, component masses and mass ratios) of both GW200115 and GW200105 match predictions from the isolated binary evolution channel. We also show that most model realizations can account for the local BHNS merger rate densities inferred by LIGO-Virgo. However, to simultaneously also match the inferred local merger rate densities for BHBH and NSNS systems we find we need models with moderate kick velocities (σ≲102kms−1) or high common-envelope efficiencies (αCE≳2) within our model explorations. We conclude that the first two observed BHNS mergers can be explained from the isolated binary evolution channel for reasonable model realizations.

Authors: Reinhold Willcox, Ilya Mandel, Eric Thrane, Adam Deller, Simon Stevenson, Alejandro Vigna-Gomez
arXiv: 2107.04251

Observations of binary pulsars and pulsars in globular clusters suggest that at least some pulsars must receive weak natal kicks at birth. If all pulsars received strong natal kicks above \unit[50]{\kms}, those born in globular clusters would predominantly escape, while wide binaries would be disrupted. On the other hand, observations of transverse velocities of isolated radio pulsars indicate that only $5\pm2\%$ have velocities below \unit[50]{\kms}. We explore this apparent tension with rapid binary population synthesis modelling. We propose a model in which supernovae with characteristically low natal kicks (e.g., electron-capture supernovae) only occur if the progenitor star has been stripped via binary interaction with a companion. We show that this model naturally reproduces the observed pulsar speed distribution and without reducing the predicted merging double neutron star yield. We estimate that the zero-age main sequence mass range for non-interacting progenitors of electron-capture supernovae should be no wider than ${\approx}0.2 M_\odot$.

Authors: Luyao Lin, Derek Bingham, Floor Broekgaarden, Ilya Mandel
arXiv: 2106.01552

In this paper, a fast and parallelizable method based on Gaussian Processes (GPs) is introduced to emulate computer models that simulate the formation of binary black holes (BBHs) through the evolution of pairs of massive stars. Two obstacles that arise in this application are the a priori unknown conditions of BBH formation and the large scale of the simulation data. We address them by proposing a local emulator which combines a GP classifier and a GP regression model. The resulting emulator can also be utilized in planning future computer simulations through a proposed criterion for sequential design. By propagating uncertainties of simulation input through the emulator, we are able to obtain the distribution of BBH properties under the distribution of physical parameters.

Authors: Floor S. Broekgaarden, Edo Berger, Coenraad J. Neijssel, Alejandro Vigna-Gómez, Debatri Chattopadhyay, Simon Stevenson, Martyna Chruslinska, Stephen Justham, Selma E. de Mink, Ilya Mandel
arXiv: 2103.02608

Mergers of black hole-neutron star (BHNS) binaries are expected to be observed by gravitational wave (GW) detectors in the coming years. Such observations will not only provide confirmation that these systems exist, but will also give unique insights into the death of massive stars, the evolution of binary systems and their possible association with gamma-ray bursts, 𝑟-process enrichment and kilonovae. Here we present binary population synthesis of isolated BHNS systems in order to predict their merger rate and characteristics for ground-based GW observatories. We present the results for 420 different model permutations that explore key uncertainties in our assumptions about massive binary star evolution (e.g., mass transfer, common-envelope evolution, supernovae), and in our assumptions for the metallicity-specific star formation rate density, and characterize their relative impacts on our predictions. We predict intrinsic local BHNS merger rates in the range R^0_m ≈ 4–830 Gpc−3 yr−1 and detected rates in the range Rdet ≈ 1–180 yr−1 for a GW network consisting of LIGO, Virgo and KAGRA at design sensitivity. We find that the binary evolution and metallicity-specific star formation rate density each impact the predicted merger rates by order O (10). We also present predictions for the GW detected BHNS merger properties and find that all 420 model variations predict that . 5% of the BHNS mergers have BH masses 𝑚BH & 18 M, total masses 𝑚tot & 20 M, chirp masses Mc & 5.5 M, mass ratios 𝑞f & 12 or 𝑞f . 2. Moreover, we find that massive NSs with 𝑚NS > 2 M are expected to be commonly detected in BHNS mergers in almost all our model variations. Finally, a wide range of ∼ 0%–70% of the BHNS mergers are predicted to eject mass during the merger. Our results highlight the importance of considering variations in binary evolution and cosmological models when predicting, and eventually evaluating, populations of BHNS mergers.

Authors: Coenraad J. Neijssel, Serena Vinciguerra, Alejandro Vigna-Gomez, Ryosuke Hirai, James C. A. Miller-Jones, Arash Bahramian, Thomas J. Maccarone, Ilya Mandel
arXiv: 2102.09092

Recent observations of the high-mass X-ray binary Cygnus X-1 have shown that both the companion star (41 solar masses) and the black hole (21 solar masses) are more massive than previously estimated. Furthermore, the black hole appears to be nearly maximally spinning. Here we present a possible formation channel for the Cygnus X-1 system that matches the observed system properties. In this formation channel, we find that the orbital parameters of Cygnus X-1, combined with the observed metallicity of the companion, imply a significant reduction in mass loss through winds relative to commonly used prescriptions for stripped stars.

Authors: James C.A. Miller-Jones, Arash Bahramian, Jerome A. Orosz, Ilya Mandel, Lijun Gou, Thomas J. Maccarone, Coenraad J. Neijssel, Xueshan Zhao, Janusz Ziółkowski, Mark J. Reid (10), Phil Uttley (11), Xueying Zheng, Do-Young Byun (12,13), Richard Dodson (14), Victoria Grinberg (15), Taehyun Jung (12,13), Jeong-Sook Kim (12), Benito Marcote (16), Sera Markoff (11,17), María J. Rioja (14,18,19), Anthony P. Rushton (20,21), David M. Russell (22), Gregory R. Sivakoff (23), Alexandra J. Tetarenko (24), Valeriu Tudose (25), Joern Wilms
arXiv: 2102.09091

The evolution of massive stars is influenced by the mass lost to stellar winds over their lifetimes. These winds limit the masses of the stellar remnants (such as black holes) that the stars ultimately produce. We use radio astrometry to refine the distance to the black hole X-ray binary Cygnus X-1, which we find to be 2.22+0.18 −0.17 kiloparsecs. When combined with previous optical data, this implies a black hole mass of 21.2±2.2 solar masses, higher than previous measurements. The formation of such a high-mass black hole in a high-metallicity system constrains wind mass loss from massive stars.

Authors: D. Chattopadhyay, S. Stevenson, J. R. Hurley, M. Bailes, F. Broekgaarden
arXiv: 2011.13503

Binaries comprised of a neutron star (NS) and a black hole (BH) have so far eluded observations as pulsars and with gravitational waves (GWs). We model the formation and evolution of these NS+BH binaries including pulsar evolution using the binary population synthesis code COMPAS. We predict the presence of a total of 50-1300 binaries containing a pulsar and a BH (PSR+BHs) in the Galactic field. We find the population observable by the next-generation of radio telescopes, represented by the SKA and MeerKAT, current (LIGO/Virgo) and future (LISA) GW detectors. We conclude that the SKA will observe 1-60 PSR+BHs, with 0-4 binaries containing millisecond pulsars. MeerKAT is expected to observe 0-30 PSR+BH systems. Future radio detections of NS+BHs will constrain uncertain binary evolution processes such as BH natal kicks. We show that systems in which the NS formed first (NSBH) can be distinguished from those where the BH formed first (BHNS) by their pulsar and binary properties. We find 40% of the LIGO/Virgo observed NS+BHs from a Milky-Way like field population will have a chirp mass ≥3.0 M⊙. We estimate the spin distributions of NS+BHs with two models for the spins of BHs. The remnants of BHNS mergers will have a spin of ∼0.4, whilst NSBH merger remnants can have a spin of ∼0.6 or ∼0.9 depending on the model for BH spins. We estimate that approximately 25-930 PSR+BHs will be radio alive whilst emitting GWs in the LISA frequency band, raising the possibility of joint observation by the SKA and LISA.

Authors: A. Vigna-Gomez, S. Toonen, E. Ramirez-Ruiz, N. Leigh, J. Riley, C. Haster
arXiv: 2010.13669

Stellar triples with massive stellar components are common, and can lead to sequential binary black-hole mergers. Here, we outline the evolution towards these sequential mergers, and explore these events in the context of gravitational-wave astronomy and the pair-instability mass gap. We find that binary black-hole mergers in the pair-instability mass gap can be of triple origin and therefore are not exclusively formed in dense dynamical environments. We discuss the sequential merger scenario in the context of the most massive gravitational-wave sources detected to date: GW170729 and GW190521. We propose that the progenitor of GW170729 is a low-metallicity field triple. We support the premise that GW190521 could not have been formed in the field. We conclude that triple stellar evolution is fundamental in the understanding of gravitational-wave sources, and likely, other energetic transientsas well.

Authors: J. Riley, I. Mandel, P. Marchant, E. Butler, K. Nathaniel, C. Neijssel, S. Shortt, A. Vigna-Gomez
arXiv: 2010.00002

We explore chemically homogeneous evolution (CHE) as a formation channel for massive merging binary black holes (BBHs). We develop methods to include CHE in a rapid binary population synthesis code, Compact Object Mergers: Population Astrophysics and Statistics (COMPAS), which combines realistic models of binary evolution with cosmological models of the star-formation history of the Universe. For the first time, we simultaneously explore conventional isolated binary star evolution under the same set of assumptions. This approach allows us to constrain population properties and make simultaneous predictions about the gravitational-wave detection rates of BBH mergers for the CHE and conventional formation channels. The overall mass distribution of detectable BBHs is consistent with existing gravitational-wave observations. We find that the CHE channel may yield up to ~70% of all gravitational-wave detections of BBH mergers coming from isolated binary evolution.

Authors: I. Mandel, B. Muller, J. Riley, S. E. de Mink, A. Vigna-Gomez, D. Chattopadhyay
arXiv: 2007.03890

We report on the impact of a probabilistic prescription for compact remnant masses and spins on massive binary population synthesis. We find that this prescription populates the putative mass gap between neutron stars and black holes with low-mass black holes. However, evolutionary effects reduce the number of X-ray binary candidates with low-mass black holes, consistent with the dearth of such systems in the observed sample. We further find that this prescription is consistent with the formation of heavier binary neutron stars such as GW190425, but over-predicts the masses of Galactic double neutron stars. The revised natal kicks, particularly increased ultra-stripped supernova kicks, do not directly explain the observed Galactic double neutron star orbital period--eccentricity distribution. Finally, this prescription allows for the formation of systems similar to the recently discovered extreme mass ratio binary GW190814, but only if we allow for the survival of binaries in which the common envelope is initiated by a donor crossing the Hertzsprung gap, contrary to our standard model.

Authors: I. Mandel, B. Muller
arXiv: 2006.08360

Based on recent results from three-dimensional supernova simulations and semianalytical parametrised models, we develop analytical prescriptions for the dependence of the mass of neutron stars and black holes and the natal kicks, if any, on the presupernova carbon-oxygen core and helium shell masses. Our recipes are probabilistic rather than deterministic in order to account for the intrinsic stochasticity of stellar evolution and supernovae. We anticipate that these recipes will be particularly useful for rapid population synthesis, and we illustrate their application to distributions of remnant masses and kicks for a population of single stars.

Authors: L.A.C. van Son, S. E. de Mink, F. S. Broekgaarden, M. Renzo, S. Justham, E. Laplace, J. Moran-Fraile, D. D. Hendriks, R. Farmer
arXiv: 2004.05187

The theory for single stellar evolution predicts a gap in the mass distribution of black holes (BHs) between approximately 45--130\Msun, the so-called ``the pair-instability mass gap". We examine whether BHs can pollute the gap after accreting from a stellar companion. To this end, we simulate the evolution of isolated binaries using a population synthesis code, where we allow for super-Eddington accretion. Under our most extreme assumptions, we find that at most about 2\% of all merging binary BH systems contains a BH with a mass in the pair-instability mass gap, and we find that less than 0.5\% of the merging systems has a total mass larger than 90\Msun. We find no merging binary BH systems with a total mass exceeding 100\Msun. We compare our results to predictions from several dynamical pathways to pair-instability mass gap events and discuss the distinguishable features. We conclude that the classical isolated binary formation scenario will not significantly contribute to the pollution of the pair-instability mass gap. The robustness of the predicted mass gap for the isolated binary channel is promising for the prospective of placing constraints on (i) the relative contribution of different formation channels, (ii) the physics of the progenitors including nuclear reaction rates, and (iii), tentatively, the Hubble parameter.

Authors: Serena Vinciguerra, Coenraad J. Neijssel, Alejandro Vigna-Gomez, Ilya Mandel, Philipp Podsiadlowski, Thomas J. Maccarone, Matt Nicholl, Samuel Kingdon, Alice Perry, Francesco Salemi
arXiv: 2003.00195

Be X-ray binaries (BeXRBs) consist of rapidly rotating Be stars with neutron star companions accreting from the circumstellar emission disk. We compare the observed population of BeXRBs in the Small Magellanic Cloud with simulated populations of BeXRB-like systems produced with the COMPAS population synthesis code. We focus on the apparently higher minimal mass of Be stars in BeXRBs than in the Be population at large. Assuming that BeXRBs experienced only dynamically stable mass transfer, their mass distribution suggests that at least ~30% of the mass donated by the progenitor of the neutron star is typically accreted by the B-star companion. We expect these results to affect predictions for the population of double compact object mergers. A convolution of the simulated BeXRB population with the star formation history of the Small Magellanic Cloud shows that the excess of BeXRBs is most likely explained by this galaxy's burst of star formation approximately 25-40 Myr ago, rather than by its low metallicity.

Authors: Debatri Chattopadhyay, Simon Stevenson, Jarrod R. Hurley, Luca J. Rossi, Chris Flynn
arXiv: 1912.02415

We have implemented prescriptions for modelling pulsars in the rapid binary population synthesis code COMPAS. We perform a detailed analysis of the double neutron star (DNS) population, accounting for radio survey selection effects. The surface magnetic field decay timescale (~1000 Myr) and mass scale (~0.02 solar masses) are the dominant uncertainties in our model. Mass accretion during common envelope evolution plays a non-trivial role in recycling pulsars. We find a best-fit model that is in broad agreement with the observed Galactic DNS population. Though the pulsar parameters (period and period derivative) are strongly biased by radio selection effects, the observed orbital parameters (orbital period and eccentricity) closely represent the intrinsic distributions. The number of radio observable DNSs in the Milky Way at present is ~2500 in our model, only ~10% of the predicted total number of DNSs in the galaxy. Using our model calibrated to the Galactic DNS population, we make predictions for DNS mergers observed in gravitational waves. The DNS chirp mass distribution varies from ~1.2 to ~2.1 solar masses and median is obtained to be 1.14 solar masses. The expected effective spin chi_eff for isolated DNSs is less than approximately 0.03 from our model. We predict that ~34% of the current Galactic isolated DNSs will merge within a Hubble time, and have a median total mass of 2.7 solar masses. Finally, we discuss implications for fast radio bursts and post-merger remnant gravitational-waves.

Authors: Alejandro Vigna-Gomez, Morgan MacLeod, Coenraad J. Neijssel, Floor S. Broekgaarden, Stephen Justham, George Howitt, Selma E. de Mink, Ilya Mandel
arXiv: 2001.09829

Close Double Neutron Stars (DNSs) have been observed as Galactic radio pulsars, while their mergers have been detected as gamma-ray bursts and gravitational-wave sources. They are believed to have experienced at least one common-envelope episode (CEE) during their evolution prior to DNS formation. In the last decades there have been numerous efforts to understand the details of the common-envelope phase, but its computational modelling remains challenging. We present and discuss the properties of the donor and the binary at the onset of the Roche-lobe overflow (RLOF) leading to these CEEs as predicted by rapid binary population synthesis models. These properties can be used as initial conditions for detailed simulations of the common-envelope phase. There are three distinctive populations, classified by the evolutionary stage of the donor at the moment of the onset of the RLOF: giant donors with fully-convective envelopes, cool donors with partially-convective envelopes, and hot donors with radiative envelopes. We also estimate that, for standard assumptions, tides would not circularise a large fraction of these systems by the onset of RLOF. This makes the study and understanding of eccentric mass transferring systems relevant for DNS populations.

Authors: George Howitt, Simon Stevenson, Alejandro Vigna-Gomez, Stephen Justham, Natasha Ivanova, Tyrone E. Woods, Coenraad J. Neijssel, Ilya Mandel
arXiv: 1912.07771

A class of optical transients known as Luminous Red Novae (LRNe) have recently been associated with mass ejections from binary stars undergoing common-envelope evolution. We use the population synthesis code COMPAS to explore the impact of a range of assumptions about the physics of common-envelope evolution on the properties of LRNe. In particular, we investigate the influence of various models for the energetics of LRNe on the expected event rate and light curve characteristics, and compare with the existing sample. We find that the Galactic rate of LRNe is ~0.2-1 per year in agreement with the observed rate. In our models, the luminosity function of Galactic LRNe covers multiple decades in luminosity and is dominated by signals from stellar mergers, consistent with observational constraints from iPTF and the Galactic sample of LRNe. We discuss how observations of the brightest LRNe may provide indirect evidence for the existence of massive (>40 solar mass) red supergiants. Such LRNe could be markers along the evolutionary pathway leading to the formation of double compact objects. We make predictions for the population of LRNe observable in future transient surveys with the Large Synoptic Survey Telescope and the Zwicky Transient Facility. In all plausible circumstances, we predict a selection-limited observable population dominated by bright, long-duration events caused by common envelope ejections. We show that the Large Synoptic Survey Telescope will observe 20--750 LRNe per year, quickly constraining the luminosity function of LRNe and probing the physics of common-envelope events.

Authors: Lau, M. Y. M., Mandel, I., Vigna-Gomez, A., Neijssel, C., Stevenson, S., Sesana, A.
arXiv: 1910.12422 [astro-ph.HE]

We estimate the properties of the double neutron star (DNS) population that will be observable by the planned space-based interferometer LISA. By following the gravitational radiation driven evolution of DNSs generated from rapid population synthesis of massive binary stars, we estimate that around 35 DNSs will accumulate a signal-to-noise ratio above 8 over a four-year LISA mission. The observed population mainly comprises Galactic DNSs (94 per cent), but detections in the LMC (5 per cent) and SMC (1 per cent) may also be expected. The median orbital frequency of detected DNSs is expected to be 0.8 mHz, and many of them will be eccentric (median eccentricity of 0.11). The orbital properties will provide insights into DNS progenitors and formation channels. LISA is expected to localise these DNSs to a typical angular resolution of 2∘, with best-constrained sources localised to a few arcminutes. These localisations may allow neutron star natal kick magnitudes to be constrained through the Galactic distribution of DNSs, and make it possible to follow up the sources with radio pulsar searches. However, LISA is also expected to resolve ∼104 Galactic double white dwarfs, many of which may have binary parameters that resemble DNSs; we discuss how the combined measurement of binary eccentricity, chirp mass, and sky location may aid the identification of a DNS. We expect the best-constrained DNSs to have eccentricities known to a few parts in a thousand, chirp masses measured to better than 1 per cent fractional uncertainty, and sky localisation at the level of a few arcminutes.

Authors: Stevenson, S., Sampson, M., Powell, J., Vigna-Gomez, A., Neijssel, C., Szecsi, D., Mandel, I.
arXiv: 1904.02821 [astro-ph.HE]

A population of binary black hole mergers has now been observed in gravitational waves by Advanced LIGO and Virgo. The masses of these black holes appear to show evidence for a pile-up between $30$--$45$\,\Msol{} and a cut-off above $\sim 45$\,\Msol. One possible explanation for such a pile-up and subsequent cut-off are pulsational pair-instability supernovae (PPISNe) and pair-instability supernovae (PISNe) in massive stars. We investigate the plausibility of this explanation in the context of isolated massive binaries. We study a population of massive binaries using the rapid population synthesis software COMPAS, incorporating models for PPISNe and PISNe. Our models predict a maximum black hole mass of $40$\,\Msol{}. We expect $\sim 0.5$--$4$\% of all binary black hole mergers at redshift z = 0 will include at least one component that went through a PPISN (with mass $30$--$40$\,\Msol{}), constituting $\sim 5$--$25$\% of binary black hole mergers observed during the first two observing runs of Advanced LIGO and Virgo. Empirical models based on fitting the gravitational-wave mass measurements to a combination of a power law and a Gaussian find a fraction too large to be associated with PPISNe in our models. The rates of PPISNe and PISNe track the low metallicity star formation rate, increasing out to redshift $z = 2$. These predictions may be tested both with future gravitational-wave observations and with observations of superluminous supernovae.

Authors: Neijssel, C., Vigna-Gomez, A., Stevenson, S., Barrett, J., Gaebel, S., Broekgaarden, F., de Mink, S., Szecsi, D., Vinciguerra, S., Mandel, I.
arXiv: 1906.08136 [astro-ph.HE]

We investigate the impact of uncertainty in the metallicity-specific star formation rate over cosmic time on predictions of the rates and masses of double compact object mergers observable through gravitational waves. We find that this uncertainty can change the predicted detectable merger rate by more than an order of magnitude, comparable to contributions from uncertain physical assumptions regarding binary evolution, such as mass transfer efficiency or supernova kicks. We statistically compare the results produced by the COMPAS population synthesis suite against a catalog of gravitational-wave detections from the first two Advanced LIGO and Virgo observing runs. We find that the rate and chirp mass of observed binary black hole mergers can be well matched under our default evolutionary model with a star formation metallicity spread of 0.39 dex around a mean metallicity ⟨Z⟩ that scales with redshift z as ⟨Z⟩=0.035×10−0.23z, assuming a star formation rate of 0.01×(1+z)2.77/(1+((1+z)/2.9)4.7)M⊙ Mpc−3 yr−1. Intriguingly, this default model predicts that 80\% of the approximately one binary black hole merger per day that will be detectable at design sensitivity will have formed through isolated binary evolution with only dynamically stable mass transfer, i.e., without experiencing a common-envelope event.

Authors: Bavera, S., Fragos, T., Qin, Y., Zapartas, E., Neijssel, C., Mandel, I., Batta, A., Gaebel, S., Kimball, C., Steveson, S.
arXiv: 1906.12257 [astro-ph.HE]

We study the formation of coalescing binary black holes via the evolution of isolated field binaries that go through the common envelope phase in order to obtain the combined distributions of the main observables of Advanced LIGO. We use a hybrid technique that combines the parametric binary population synthesis code COMPAS with detailed binary evolution simulations performed with the MESA code. We then convolve our binary evolution calculations with the redshift- and metallicity-dependent star-formation rate and the selection effects of gravitational-wave detectors to obtain predictions of observable properties. By assuming efficient angular momentum transport, we are able to present a model capable of predicting simultaneously the three main gravitational-wave observables: the effective inspiral spin parameter χeff, the chirp mass Mchirp and the cosmological redshift of merger zmerger. We find an excellent agreement between our model and the ten events from the first two advanced detector observing runs. We make predictions for the third observing run O3 and for Advanced LIGO design sensitivity. We expect 59% of events with χeff<0.1, while the remaining 41% of events with χeff≥0.1 are split into 9% with Mchirp<15 M⊙ and 32% with Mchirp≥15 M⊙. In conclusion, the favorable comparison of the existing LIGO/Virgo observations with our model predictions gives support to the idea that the majority, if not all of the observed mergers, originate from the evolution of isolated binaries. The first-born black hole has negligible spin because it lost its envelope after it expanded to become a giant star, while the spin of the second-born black hole is determined by the tidal spin up of its naked helium star progenitor by the first-born black hole companion after the binary finished the common-envelope phase.

Authors: Broekgaarden, F., Justham, S., de Mink, S., Gair, J., Mandel, I., Stevenson, S., Barrett, J., Vigna-Gomez, A., Neijssel, C.
arXiv: 1905.00910 [astro-ph.HE]
Data Release: Zenodo.com

Gravitational-wave observations of double compact object (DCO) mergers are providing new insights into the physics of massive stars and the evolution of binary systems. Making the most of expected near-future observations for understanding stellar physics will rely on comparisons with binary population synthesis models. However, the vast majority of simulated binaries never produce DCOs, which makes calculating such populations computationally inefficient. We present an importance sampling algorithm, STROOPWAFEL, that improves the computational efficiency of population studies of rare events, by focusing the simulation around regions of the initial parameter space found to produce outputs of interest. We implement the algorithm in the binary population synthesis code COMPAS, and compare the efficiency of our implementation to the standard method of Monte Carlo sampling from the birth probability distributions. STROOPWAFEL finds $\sim$25-200 times more DCO mergers than the standard sampling method with the same simulation size, and so speeds up simulations by up to two orders of magnitude. Finding more DCO mergers automatically maps the parameter space with far higher resolution than when using the traditional sampling. This increase in efficiency also leads to a decrease of a factor $\sim$3-10 in statistical sampling uncertainty for the predictions from the simulations. This is particularly notable for the distribution functions of observable quantities such as the black hole and neutron star chirp mass distribution, including in the tails of the distribution functions where predictions using standard sampling can be dominated by sampling noise.

Authors:
Schroeder, S., MacLeod, M., Loeb, A., Vigna-Gomez, A., Mandel, I.
arXiv: 1906.04189 [astro-ph.HE]

We model explosions driven by the coalescence of a black hole or neutron star with the core of its massive-star companion. Upon entering a common envelope phase, a compact object may spiral all the way to the core. The concurrent release of energy is likely to be deposited into the surrounding common envelope, powering a merger-driven explosion. We use hydrodynamic models of binary coalescence to model the common envelope density distribution at the time of coalescence. We find toroidal profiles of material, concentrated in the binary's equatorial plane and extending to many times the massive star's original radius. We use the spherically-averaged properties of this circumstellar material (CSM) to estimate the emergent light curves that result from the interaction between the blast wave and the CSM. We find that typical merger-driven explosions are brightened by up to three magnitudes by CSM interaction. From population synthesis models we discover that the brightest merger-driven explosions, MV∼−18 to −19, are those involving black holes because they have the most massive and extended CSM. Black hole coalescence events are also common; they represent about 50% of all merger-driven explosions and approximately 0.3% of the core-collapse rate. Merger-driven explosions offer a window into the highly-uncertain physics of common envelope interactions in binary systems by probing the properties of systems that merge rather than eject their envelopes.

Authors: Vigna-Gomez, A., Neijssel, C., Stevenson, S., Barrett, J., Belczynski, K., Justham, S., de Mink, S., Muller, B., Podsiadlowski, P., Benzo, M., Szecsi, D., Mandel, I.
Journal: MNRAS
arXiv: 1805.07974 [astro-ph.HE]

Double neutron stars (DNSs) have been observed as Galactic radio pulsars, and the recent discovery of gravitational waves from the DNS merger GW170817 adds to the known DNS population. We perform rapid population synthesis of massive binary stars and discuss model predictions, including DNS formation rates, mass distributions, and delay time distributions. We vary assumptions and parameters of physical processes such as mass transfer stability criteria, supernova natal kick distributions, remnant mass prescriptions, and common-envelope energetics. We compute the likelihood of observing the orbital period-eccentricity distribution of the Galactic DNS population under each of our population synthesis models, allowing us to quantitatively compare the models. We find that mass transfer from a stripped post-helium-burning secondary (case BB) on to a neutron star is most likely dynamically stable. We also find that a natal kick distribution composed of both low (Maxwellian σ =30 km s^{-1}) and high (σ =265 km s^{-1}) components is preferred over a single high-kick component. We conclude that the observed DNS mass distribution can place strong constraints on model assumptions.

Authors: Barrett, J. W., Gaebel, S. M., Neijssel, C. J., Vigna-Gómez, A., Stevenson, S., Berry, C. P. L., Farr, W. M. & Mandel, I.
Journal: MNRAS
arXiv: 1711.06287 [astro-ph.HE]

Gravitational waves give us a unique insight into the properties of binary black holes. The information from gravitational waves should help us figure out how these black holes form—in this paper we investigate exactly how accurately we will be able to determine details of binary evolution. We consider populations of binary black holes simulated using COMPAS, and how sensitive the distribution of chirp masses and merger rate (which will be measured through gravitational waves) are to changes in the input physics. In particular, we consider four of the most uncertain parameters: the supernova kick (σ_kick), the common-envelope efficiency (α_CE), and the mass loss rates during the Wolf–Rayet and luminous blue variable phases (f_WR and f_LBV). We quantify the information we can gain from observations using the Fisher matrix, which includes correlations between parameters. The plot above shows (fractional) measurement uncertainties for many realisations of the binary black hole population after 1000 observations (the uncertainties scale inversely with the square root of the number of observations). We find that we can distinguish populations which differ by just a few percent in these parameters! The measurements are much better when adding in the chirp masses as well as the rates, so perhaps adding in more information from gravitational-wave (or other complementary) observations will improve things even further.

Authors: Barrett, J., Mandel, I., Neijssel, C., Stevenson, S., Vigna-Gomez, A.
Journal: Proceedings of the International Astronomical Union
arXiv: 1704.03781 [astro-ph.HE]

As we enter the era of gravitational wave astronomy, we are beginning to collect observations which will enable us to explore aspects of astrophysics of massive stellar binaries which were previously beyond reach. In this paper we describe COMPAS (Compact Object Mergers: Population Astrophysics and Statistics), a new platform to allow us to deepen our understanding of isolated binary evolution and the formation of gravitational-wave sources. We describe the computational challenges associated with their exploration, and present preliminary results on overcoming them using Gaussian process regression as a simulation emulation technique.

Authors: Stevenson, S., Vigna-Gomez, A., Mandel, I., Barrett, J., Neijssel, C., Perkins, D., de Mink., S. Journal: Nature
arXiv: 1704.01352 [astro-ph.HE]
Data Release: Zenodo.com

Figure 1 from Stevenson et al., Nature Communications 8, 14906 (2017) updated to include GW170104

In a paper published in Nature Communications, we have have shown that all three observed events can be formed via the same formation channel: isolated binary evolution via a common-envelope phase. In this channel, two massive progenitor stars start out at quite wide separations. The stars interact as they expand, engaging in several episodes of mass transfer. The latest of these is typically a common envelope - a very rapid, dynamically unstable mass transfer that envelops both stellar cores in a dense cloud of hydrogen gas. Ejecting this gas from the system takes energy away from the orbit. This brings the two stars sufficiently close together for gravitational-wave emission to be efficient, right at the time when they are small enough that such closeness will no longer put them into contact. The whole process takes a few million years to form two black holes, with a possible subsequent delay of billions of years before the black holes merge and form a single black hole.

The simulations have also helped the team to understand the typical properties of the stars that can go on to form such pairs of merging black holes and the environments where this can happen.