arXiv subjects

Simon Stevenson

Publications and source records attributed to Simon Stevenson.

At least 19 recordsLinked to original sources

arXiv2026

Formation of heavy double neutron stars II: the role of heavy first-born neutron stars and low metallicity

The high total mass of GW190425 challenges our understanding of double neutron star (DNS) formation, as no such heavy ($\ge 3$\, M$_\odot$) DNS system has been observed in the Milky Way disk. Numerous formation scenarios have been proposed to explain its formation. We test these within a self-consistent binary evolution framework calibrated to the Galactic DNS population. In Paper~I of this series, we studied the evolution of a $1.4$\,M$_\odot$ neutron star (NS) in a binary with a $2.5$--$10$\,M$_\odot$ helium star at solar metallicity ($Z = Z_\odot$), assuming Eddington-limited accretion onto the NS using \texttt{MESA}. In this paper, we consider the evolution of NS-He star binaries with a broad range of NS masses from $1.1$--$1.9$\,M$_\odot$ at $Z=Z_\odot$, 0.1\,$Z_\odot$ and 0.01\,$Z_\odot$. We find that the formation of heavy DNSs is rare, accounting for only $\sim 0.5$ per cent of DNSs. The majority of these are formed through the `standard formation' channel, widely believed to form GW170817 and observed Galactic DNSs. We find no contribution through the `fast-merger' channel at solar metallicity, but systems at low metallicity predominantly form through unstable mass transfer. Furthermore, we find no significant dependence of the heavy DNS formation fraction on metallicity over the range considered here. We conclude that heavy DNSs do not form a separate subpopulation and merely represent the high-mass tail of the standard DNS population.

astro-ph.SR↗

arXiv2025

A subpopulation of low-mass, spinning black holes: signatures of dynamical assembly

Gravitational-wave observations of massive, rapidly spinning binary black holes mergers provide increasing evidence for the dynamical origin of some mergers. Previous studies have interpreted the mergers with primary mass $\gtrsim45\,M_\odot$ as being dominated by hierarchical, second-generation mergers, with rapidly spinning primaries being the products of previous black hole mergers assembled in dense stellar clusters. In this work, we reveal confident evidence of another subpopulation with rapid and isotropic spins at low mass containing the two exceptional events GW241011 and GW241110, consistent with a hierarchical merger hypothesis. Our result suggests the mass distribution of the second-generation black holes is peaked at low primary masses of $\sim16\,M_\odot$ rather than $\gtrsim45\,M_\odot$ in the pair-instability gap. Such low-mass second-generation black holes must be formed from the merger of even lighter first-generation black holes, implying that dense, metal-rich stellar environments contribute to the binary black hole population. By separating the contamination of higher-generation black holes, our result reveals the primary mass distribution of first-generation black holes formed from stellar collapse, which shows a significant dip between $\sim12\,M_\odot$ to $\sim20\,M_\odot$. This may indicate a dearth of black holes due to variation in the core compactness of the progenitor.

astro-ph.HE↗

arXiv2025

Strategy for identifying Vera C. Rubin Observatory kilonova candidates for targeted gravitational-wave searches

Since the observation of the binary neutron star merger GW170817 and the associated kilonova AT2017gfo, the next joint gravitational-wave/optical kilonova has been highly anticipated. Overlapping observations between the Vera C. Rubin Observatory and the international gravitational-wave detector network are expected soon. Wide-field survey facilities, such as Rubin, can serve dual roles in gravitational-wave astronomy: conducting dedicated optical counterpart searches following gravitational-wave triggers and, through surveys such as the Legacy Survey of Space and Time (LSST), providing opportunities for fortuitous kilonova discoveries during routine operations. We use simulations to develop a strategy for identifying kilonova candidates observed by Rubin and processed by the Fink broker. These candidates can be used as astrophysical triggers for a targeted gravitational-wave search. We simulate kilonovae light-curves for the first year of Rubin with the latest observing strategy for the Wide-Fast-Deep and the Deep Drilling Fields. Assuming a kilonova rate of 250 Gpc$^{-3}$ yr$^{-1}$, we find that Rubin brokers should observe $\sim 4$ kilonovae per year with at least one alert above a signal-to-noise ratio of 5 within the gravitational-wave detector horizon ($\sim 350$ Mpc). Most of these will be faint, and detected 1-2 days following the neutron star merger. Photometric and spectroscopic follow-up will be limited to large telescopes. Using archival data from the Zwicky Transient Facility (ZTF) and our proposed selection criteria, we estimate a minimum contamination of at least 30 events per month from other transients and variables, even under our strictest selection criteria. A deep gravitational-wave search targeting Rubin kilonova candidates may lead to the next multi-messenger discovery.

astro-ph.HE↗

arXiv2025

Formation of heavy double neutron stars I: Eddington-limited accretion for a 1.4 $M_{\odot}$ neutron star at solar metallicity

More than 30 Galactic double neutron star (DNS) binaries have now been identified through radio pulsar timing. The 24 DNSs in the Galactic field with measured total masses lie in the narrow range of 2.3--2.9 $M_{\odot}$. In contrast, gravitational-wave observations have detected two DNS mergers: GW170817, with a total mass of 2.7 $M_{\odot}$, and GW190425, with a significantly higher mass of 3.4 $M_{\odot}$. The unusually high mass of GW190425 suggests a non-standard formation channel not represented in the known Galactic population. To investigate the origin of such a massive DNS system, we model the late evolutionary stages of helium stars with initial masses between 2.5 and 9.8 $M_{\odot}$ in binaries with 1.4 $M_{\odot}$ neutron star companions, using the 1D stellar evolution code MESA at solar metallicity. We test alternative formation pathways and calibrate our models to reproduce the observed Galactic DNS mass and orbital distributions. By incorporating a modified natal kick prescription, our population synthesis results are broadly consistent with the observed total mass distribution of known DNS systems. Only a small fraction of DNSs of our model have total masses $\geq$ 3 $M_{\odot}$, insufficient to explain the high rate of massive DNS mergers inferred from GW observations. However, our model rules out the formation of heavy DNS systems like GW190425 via the second unstable mass transfer.

astro-ph.HE↗

arXiv2025

Implications of modern mass-loss rates for massive stars

Massive stars lose a significant fraction of their mass through stellar winds at various stages of their lives, including on the main sequence, during the red supergiant phase, and as helium-rich stripped stars. In stellar population synthesis, uncertainty in the mass-loss rates in these evolutionary stages limits our understanding of the formation of black holes and merging compact binaries. In the last decade, the theoretical predictions, simulation, and direct observation of wind mass-loss rates in massive stars have improved significantly, typically leading to a reduction in the predicted mass-loss rates of massive stars. In this paper we explore the astrophysical implications of an updated treatment of winds in the COMPAS population synthesis code. There is a large amount of variation in predicted mass-loss rates for massive red supergiants; some of the prescriptions we implement predict that massive red supergiants are able to lose their hydrogen envelopes through winds alone (providing a possible solution to the so-called missing red supergiant problem), while others predict much lower mass-loss rates that would not strip the hydrogen envelope. We discuss the formation of the most massive stellar-mass black holes in the Galaxy, including the high-mass X-ray binary Cygnus X-1 and the newly discovered Gaia BH3. We find that formation rates of merging binary black holes are sensitive to the mass-loss rate prescriptions, while the formation rates of merging binary neutron stars and neutron-star black hole binaries are more robust to this uncertainty.

astro-ph.SR↗

arXiv2025

Rapid stellar and binary population synthesis with COMPAS: methods paper II

The COMPAS public rapid binary population synthesis code has undergone a number of key improvements since the original COMPAS methods paper (Team COMPAS: Riley et al., 2022) was published. These include more sophisticated and robust treatments of binary interactions: mass transfer physics, common-envelope events, tides and gravitational-wave radiation reaction; and updated prescriptions for stellar evolution, winds and supernovae. The code structure and outputs have also been updated, with a focus on improving resolution without sacrificing computational speed. This paper describes the substantive changes in the code between the previous methods paper and COMPAS v03.22.01.

astro-ph.SR↗

arXiv2024

The birth mass function of neutron stars

The birth mass function of neutron stars encodes rich information about supernova explosions, double star evolution, and properties of matter under extreme conditions. To date, it has remained poorly constrained by observations, however. Applying probabilistic corrections to account for mass accreted by recycled pulsars in binary systems to mass measurements of 90 neutron stars, we find that the birth masses of neutron stars can be described by a unimodal distribution that smoothly turns on at $1.1 M_{\odot}$, peaks at $1.27 M_{\odot}$, before declining as a steep power law. Such a ``turn-on" power-law distribution is strongly favoured against the widely-adopted empirical double-Gaussian model at the $3 \sigma$ level. The power-law shape may be inherited from the initial mass function of massive stars, but the relative dearth of massive neutron stars implies that single stars with initial masses greater than $\approx 18 M_{\odot}$ do not form neutron stars, in agreement with the absence of massive red supergiant progenitors to supernovae.

astro-ph.HE↗

arXiv2024

Binary population synthesis of the Galactic canonical pulsar population

Pulsars are rapidly rotating neutron stars that emit radiation across the electromagnetic spectrum, from radio to gamma-rays. We use the rapid binary population synthesis suite COMPAS to model the Galactic population of canonical pulsars. We account for both radio and gamma-ray selection effects, as well as the motion of pulsars in the Galactic potential due to natal kicks. We compare our models to the catalogues of pulsars detected in the radio, and those detected in gamma-rays by Fermi, and find broad agreement with both populations. We reproduce the observed ratio of radio-loud to radio-quiet gamma-ray pulsars. We further examine the possibility of low spin-down luminosity (Edot) pulsars emitting weak, unpulsed gamma-ray emission and attempt to match this with results from a recent gamma-ray stacking survey of these pulsars. We confirm the correlation between the latitude of a pulsar and its Edot arises due to natal kicks imparted to pulsars at birth, assuming that all pulsars are born in the Galactic disk.

astro-ph.HE↗

arXiv2024

A Trifecta of Modelling Tools: A Bayesian Binary Black Hole Model Selection combining Population Synthesis and Galaxy Formation Models

Gravitational waves (GWs) have revealed surprising properties of binary black hole (BBH) populations, but there is still mystery surrounding how these compact objects evolve. We apply Bayesian inference and an efficient method to calculate the BBH merger rates in the Shark host galaxies, to determine the combination of COMPAS parameters that outputs a population most like the GW sources from the LVK transient catalogue. For our COMPAS models, we calculate the likelihood with and without the dependence on the predicted number of BBH merger events. We find strong correlations between hyper-parameters governing the specific angular momentum (AM) of mass lost during mass transfer, the mass-loss rates of Wolf-Rayet stars via winds and the chemically homogeneous evolution (CHE) formation channel. We conclude that analysing the marginalised and unmarginalised likelihood is a good indicator of whether the population parameters distribution and number of observed events reflect the LVK data. In doing so, we see that the majority of the models preferred in terms of the population-level parameters of the BBHs greatly overpredict the number of events we should have observed to date. Looking at the smaller number of models which perform well with both likelihoods, we find that those with no CHE, AM loss occurring closer to the donor during the first mass-transfer event, and/or higher rates of mass-loss from Wolf-Rayet winds are generally preferred by current data. We find these conclusions to be robust to our choice of selection criteria.

astro-ph.HE↗

arXiv2024

Are all models wrong? Falsifying binary formation models in gravitational-wave astronomy

As the catalogue of gravitational-wave transients grows, several entries appear "exceptional" within the population. Tipping the scales with a total mass of $\approx 150 M_\odot$, GW190521 likely contained black holes in the pair-instability mass gap. The event GW190814, meanwhile, is unusual for its extreme mass ratio and the mass of its secondary component. A growing model-building industry has emerged to provide explanations for such exceptional events, and Bayesian model selection is frequently used to determine the most informative model. However, Bayesian methods can only take us so far. They provide no answer to the question: does our model provide an adequate explanation for the data? If none of the models we are testing provide an adequate explanation, then it is not enough to simply rank our existing models - we need new ones. In this paper, we introduce a method to answer this question with a frequentist $p$-value. We apply the method to different models that have been suggested to explain GW190521: hierarchical mergers in active galactic nuclei and globular clusters. We show that some (but not all) of these models provide adequate explanations for exceptionally massive events like GW190521.

astro-ph.HE↗

arXiv2023

Rapid localization and inference on compact binary coalescences with the Advanced LIGO-Virgo-KAGRA gravitational-wave detector network

We present a rapid parameter estimation framework for compact binary coalescence (CBC) signals observed by the LIGO-Virgo-KAGRA (LVK) detector network. The goal of our framework is to enable optimal source localization of binary neutron star (BNS) signals in low latency, as well as improve the overall scalability of full CBC parameter estimation analyses. Our framework is based on the reduced order quadrature (ROQ) technique, and resolves its shortcomings by utilizing multiple ROQ bases in a single parameter estimation run. We have also developed sets of compact ROQ bases for various waveform models, IMRPhenomD, IMRPhenomPv2, IMRPhenomPv2$\_$NRTidalv2, and IMRPhenomXPHM. We benchmark our framework with hundreds of simulated observations of BNS signals by the LIGO-Virgo detector network, and demonstrate that it provides accurate and unbiased estimates on BNS source location, with a median analysis time of $6$ minutes. The median searched area is reduced by around 30$\%$ compared to estimates produced by BAYESTAR: from $21.8\,\mathrm{deg^2}$ to $16.6\,\mathrm{deg^2}$. Our framework also enables detailed parameter estimation taking into account gravitational-wave higher multipole moments, the tidal deformation of colliding objects, and detector calibration errors of amplitude and phase with the time scale of hours. Our rapid parameter estimation technique has been implemented in one of the LVK parameter estimation engines, BILBY, and is being employed by the automated parameter estimation analysis of the LVK alert system.

gr-qc↗

arXiv2023

Modelling stellar evolution in mass-transferring binaries and gravitational-wave progenitors with METISSE

Massive binaries are vital sources of various transient processes, including gravitational-wave mergers. However, large uncertainties in the evolution of massive stars, both physical and numerical, present a major challenge to the understanding of their binary evolution. In this paper, we upgrade our interpolation-based stellar evolution code METISSE to include the effects of mass changes, such as binary mass transfer or wind-driven mass loss, not already included within the input stellar tracks. METISSE's implementation of mass loss (applied to tracks without mass loss) shows excellent agreement with the SSE fitting formulae and with detailed MESA tracks, except in cases where the mass transfer is too rapid for the star to maintain equilibrium. We use this updated version of METISSE within the binary population synthesis code BSE to demonstrate the impact of varying stellar evolution parameters, particularly core overshooting, on the evolution of a massive (25M$_\odot$ and 15M$_\odot$) binary system with an orbital period of 1800 days. Depending on the input tracks, we find that the binary system can form a binary black hole or a black hole-neutron star system, with primary(secondary) remnant masses ranging between 4.47(1.36)M$_\odot$ and 12.30(10.89)M$_\odot$, and orbital periods ranging from 6 days to the binary becoming unbound. Extending this analysis to a population of isolated binaries uniformly distributed in mass and orbital period, we show that the input stellar models play an important role in determining which regions of the binary parameter space can produce compact binary mergers, paving the way for predictions for current and future gravitational-wave observatories.

astro-ph.SR↗

arXiv2022

Constraints on the contributions to the observed binary black hole population from individual evolutionary pathways in isolated binary evolution

Gravitational waves from merging binary black holes can be used to shed light on poorly understood aspects of massive binary stellar evolution, such as the evolution of massive stars (including their mass-loss rates), the common envelope phase, and the rate at which massive stars form throughout the cosmic history of the Universe. In this paper we explore the \emph{correlated} impact of these phases on predictions for the merger rate and chirp mass distribution of merging binary black holes, aiming to identify possible degeneracies between model parameters. In many of our models, a large fraction (more than 70% of detectable binary black holes) arise from the chemically homogeneous evolution scenario; these models tend to over-predict the binary black hole merger rate and produce systems which are on average too massive. Our preferred models favour enhanced mass-loss rates for helium rich Wolf--Rayet stars, in tension with recent theoretical and observational developments. We identify correlations between the impact of the mass-loss rates of Wolf--Rayet stars and the metallicity evolution of the Universe on the rates and properties of merging binary black holes. Based on the observed mass distribution, we argue that 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 formed through isolated binary evolution, implying a significant contribution (> 10%) from other formation channels such as dense star clusters or active galactic nuclei. Our models will enable inference on the uncertain parameters governing binary evolution in the near future.

astro-ph.HE↗

arXiv2022

Wide binary pulsars from electron-capture supernovae

Neutron stars receive velocity kicks at birth in supernovae. Those formed in electron-capture supernovae from super asymptotic giant branch stars -- the lowest mass stars to end their lives in supernovae -- may receive significantly lower kicks than typical neutron stars. Given that many massive stars are members of wide binaries, this suggests the existence of a population of low-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 to the mass range of (effectively) single stars leading to electron capture supernovae, the amount of mass lost prior to the supernova, and the magnitude of any natal kick imparted on the neutron star. We estimate that one such binary pulsar should be observable in the Milky Way for every 10,000 isolated pulsars, assuming that the width of the mass range of single stars leading to electron-capture supernovae is $\lesssim 0.2$\,M$_\odot$, and that neutron stars formed in electron-capture supernovae receive typical kicks less than 10\,km s$^{-1}$. We have searched the catalog of observed binary pulsars, but find no convincing candidates that could be formed through this channel, consistent with this low predicted rate. Future observations with the Square Kilometre Array may detect this rare sub-class of binary pulsar and provide strong constraints on the properties of electron-capture supernovae and their progenitors.

astro-ph.HE↗

arXiv2022

Signatures of mass ratio reversal in gravitational waves from merging binary black holes

The spins of merging binary black holes offer insights into their formation history. Recently it has been argued that in isolated binary evolution of two massive stars the firstborn black hole is slowly rotating, whilst the progenitor of the second-born black hole can be tidally spun up if the binary is tight enough. Naively, one might therefore expect that only the less massive black hole in merging binaries exhibits non-negligible spin. However, if the mass ratio of the binary is "reversed" (typically during the first mass transfer episode), it is possible for the tidally spun up second-born to become the more massive black hole. We study the properties of such mass-ratio reversed (MRR) binary black hole mergers using a large set of 560 population synthesis 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 for most model variations we consider, with typical total masses $\gtrsim 20$ M$_{\odot}$ and mass ratios $q = m_2 / m_1 \sim 0.7$ (where $m_1 > m_2$). The formation history of these systems typically involves only stable mass transfer episodes. The second-born black hole has non-negligible spin ($\chi > 0.05$) in up to $25\%$ of binary black holes, with among those the more (less) massive black hole spinning in $0\%$--$80\%$ ($20\%$--$100\%$) of cases, varying greatly in our models. We discuss our models in the context of several observed gravitational-wave events and the observed mass ratio - effective spin correlation.

astro-ph.HE↗

arXiv2022

Modelling the formation of the first two neutron star-black hole mergers, GW200105 and GW200115: metallicity, chirp masses and merger remnant spins

The two neutron star-black hole mergers (GW200105 and GW200115) observed in gravitational waves by advanced LIGO and Virgo, mark the first ever discovery of such binaries in nature. We study these two neutron star-black hole systems through isolated binary evolution, using a grid of population synthesis models. Using both mass and spin observations (chirp mass, effective spin and remnant spin) of the binaries, we probe their different possible formation channels in different metallicity environments. Our models only support LIGO data when assuming the black hole is non spinning. Our results show a strong preference that 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 with GW200115. We also find that chirp mass and remnant spins jointly aid in constraining the models, whilst the effective spin parameter does not add any further information. We also present the observable (i.e. post selection effects) median values of spin and mass distribution from all our models, which maybe used as a reference for future mergers. Further, we show that the remnant spin parameter distribution exhibits distinguishable features in different neutron star-black hole sub-populations. We find that non-spinning, first born black holes dominate significantly the merging neutron star-black hole population, ensuring electromagnetic counterparts to such mergers a rare affair.

astro-ph.HE↗

arXiv2022

Dynamical double black holes and their host cluster properties

We investigate the relationship between the global properties of star clusters and their double black hole (DBH) populations. We use the code {\tt NBODY6} to evolve a suite of star cluster models with an initial mass of $\mathcal{O}(10^4)$M$_\odot$ and varying initial parameters. We conclude that cluster metallicity plays the most significant role in determining the lifespan of a cluster, while the initial half-mass radius is dominant in setting the rate of BH exchange interactions in the central cluster regions. We find that the mass of interacting BHs, rather than how frequently their interactions with other BHs occur, is more crucial in the thermal expansion and eventual evaporation of the cluster. We formulate a novel approach to easily quantify the degree of BH-BH dynamical activity in each model. We report 12 in-cluster and three out-of-cluster (after ejection from the cluster) DBH mergers, of different types (inspiral, eccentric, hierarchical) across the ten $N$-body models presented. Our DBH merger efficiency is 3--4$\times10^{-5}$ mergers per M$_\odot$. We note the cluster initial density plays the most crucial role in determining the number of DBH mergers, with the potential presence of a transitional density point (between 1.2-3.8$\times10^3$M$_\odot$/pc$^3$) below which the number of in-cluster mergers increases with cluster density and above which the increased stellar density acts to prevent in-cluster BH mergers. The importance of the history of dynamical interactions within the cluster in setting up the pathways to ejected DBH mergers is also discussed. Our findings show a broad match with observed LIGO-Virgo DBH mergers.

astro-ph.GA↗

arXiv2022

Biases in estimates of black hole kicks from the spin distribution of binary black holes

A population of more than 50 binary black hole mergers has now been observed by the LIGO and Virgo gravitational-wave observatories. While neutron stars are known to have large velocities associated with impulsive kicks imparted to them at birth in supernovae, whether black holes receive similar kicks, and of what magnitude, remains an open question. Recently, Callister et al. (2021) analysed the binary black hole population under the hypothesis that they were all formed through isolated binary evolution and claimed that large black hole kicks (greater than 260 km/s at 99% confidence) were required for the spin distribution of merging binary black holes to match observations. Here we highlight that a key assumption made by Callister et al. (2021) -- that all secondary black holes can be tidally spun up -- is not motivated by physical models, and may lead to a bias in their estimate of the magnitudes of black hole kicks. We make only minor changes to the Callister et al. (2021) model, accounting for a population of wider merging binaries where tidal synchronisation is ineffective. We show that this naturally produces a bimodal spin distribution for secondary black holes, and that the spin-orbit misalignments observed in the binary black hole population can be explained by more typical black hole kicks of order 100 km/s, consistent with kicks inferred from Galactic X-ray binaries containing black holes. We conclude that the majority of the binary black hole population is consistent with forming through isolated binary evolution.

astro-ph.HE↗