arXiv subjects

Aleksandra Olejak

Publications and source records attributed to Aleksandra Olejak.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of Main Sequence Stars Transferring Mass to a Supermassive Black Hole

We consider main-sequence stars orbiting close to a supermassive black hole (SMBH), which are potential gravitational wave (GW) sources for LISA if their orbital periods are of the order of an hour. At such a short orbital period, mass transfer from the star to the SMBH occurs. The evolution of the semi-major axis and GW frequency depends on the evolution of the stellar mass and radius. We use MESA to study stars that transfer mass to SMBHs similar to Sagittarius A* starting on the zero-age main sequence (MS). We identify 4 evolutionary phases. (I) Stars initially with mass $>2 M_\odot$ remain on the MS as their mass and radius decrease. (II) Below $2 M_\odot$, the separation is sufficiently small so that the GW timescale is too short for the stars to maintain thermal equilibrium. They evolve adiabatically off the MS and shrink rapidly as they lose their high-entropy envelope. (III) Below $0.5-1 M_\odot$, depending on the initial mass, the uniform low-entropy core is exposed and stars expand adiabatically. (IV) Below $0.15-0.4 M_\odot$, the thermal and GW timescales become comparable, and stars cool and shrink while maintaining this balance. Overall, mass transfer causes the orbit to expand, slowing down the orbital evolution and leading to GW emission at lower frequencies making the GW signal harder to detect. If located around Sagittarius A*, mass transferring stars spend most of the time relatively close to the LISA sensitivity curve, with maximal SNR reaching around 600 during the transition between stages II and III.

astro-ph.HE↗

arXiv2026

Binary Evolution Can Mimic the Pair-Instability Mass Gap in Black Hole Mergers

The recent O4 catalogs from the LIGO-Virgo-KAGRA collaboration, which significantly increased the number of gravitational-wave (GW) detections, reveal features with potentially important astrophysical implications. One notable example is a hint of the so-called pair-instability mass gap. In particular, the observed decline in the number of black holes (BHs) with mass above 45Msun, together with indications of possibly higher spins for BHs above this threshold, has been interpreted by Antonini et al. and Tong et al. as evidence for pair-instability supernovae.In this work, we investigate whether mass transfer in binary systems can produce features in the BH component-mass distributions that mimic the inferred pair-instability limit. We use both the population synthesis code StarTrack and a simple semi-analytical framework to highlight the impact of mass transfer efficiency on the BH masses. We find that mass-ratio reversal during the first Roche-lobe overflow, followed by a highly non-conservative second mass-transfer phase, naturally limits the mass of the first-born BH and produces an apparent cutoff that mimics the lower edge of a mass gap. While the upper mass limit for the more massive BH is increased through accretion during the first mass-transfer phase and is ultimately set by the pair-instability limit, the less massive BH is limited to the stripped primary mass. As a result, the fraction of systems in which the less massive BH exceeds 45Msun is negligible, below 0.0001 in our default models. While pair instability remains a possible interpretation of current GW data, similar features can naturally arise from binary evolution without invoking a low pair-instability cutoff. The emerging evidence for predominantly positive effective spins extending into the putative upper mass gap may further support a contribution from isolated binary evolution to the high-mass tail.

astro-ph.HE↗

arXiv2026

Remnant recoil and host environments of GWTC-4.0 binary black-hole mergers

Determining the astrophysical origin of binary black holes and whether merger remnants are retained in their birth environments is essential for understanding hierarchical mergers and the growth of intermediate-mass black holes. We identified gravitational-wave events most consistent with dense-cluster origin and assessed whether their merger remnants are retained in globular clusters, nuclear star clusters, or galactic potentials. We considered 84 events consistent with binary-black-hole mergers from the first part of the fourth observing run (O4a) of the LIGO-Virgo-KAGRA detector network, and 3 selected events from the second part (O4b). We compared parameter-estimation posteriors with synthetic population models for field and cluster binaries using Bayes factors, accounting for the relative abundances of these formation channels. We computed recoil-velocity posteriors for all events using the IMRPhenomXPNR waveform model. We identified five events whose intrinsic parameters show preference for the adopted dense-cluster models over the considered field-binary populations, including the most massive O4a event GW231123_135430, while finding no robust preference for a dense-cluster origin for the high-spinning O4b event GW241011_233834. Typical recoil velocities are a few hundred km/s, with extended high-velocity tails. These kicks suggest merger remnants are likely ejected from typical globular clusters, while retention in nuclear star clusters remains possible but not guaranteed. Within the adopted models, efficient hierarchical growth may be challenging in typical globular clusters, whereas nuclear star clusters remain viable environments for repeated mergers. Although results depend on the adopted population models, this analysis highlights the importance of improved population models and higher-quality detections enabled by future GW detectors.

astro-ph.HE↗

arXiv2026

Distinct spin properties and astrophysical origin of low mass binary black holes in gravitational wave data

We analyze the effective-spin distribution of binary black hole mergers in GWTC-5.0 as a function of primary black hole mass using hierarchical Bayesian inference. We model the population as a mixture of two spin components separated by a transition mass scale inferred directly from the data. We find strong evidence for a transition at $\tilde{m} = 15.2^{+4.3}_{-3.6}\, M_\odot$. Mock-catalog analyses show that such a transition is unlikely to arise from finite-sample fluctuations of a mass-independent $χ_{\rm eff}$ population and the posterior predictive distributions of $χ_{\rm eff}$ inferred below and above the transition are clearly distinct. Below the transition mass, the effective-spin distribution is narrow, peaks at a small positive value $χ_{\rm eff}>0$, but also shows significant support for negative $χ_{\rm eff}$. Above the transition, the distribution is broader and its peak shifts to values consistent with $χ_{\rm eff}\simeq0$, making its support at both positive and negative $χ_{\rm eff}$ roughly similar. These findings suggest that the dominant merger population concentrated around $10\,M_{\odot}$ is statistically distinct from the rest and that it arises from a different formation channel. We show that this low-mass population is broadly consistent with formation from massive stellar multiples in the field: it may either arise from isolated binary star evolution but only if black hole natal kicks below $\tilde{m}$ are generally very large ($\gtrsim100\,\rm km/s$) or be caused by the dynamical evolution of hierarchical triples. In contrast, isolated binary evolution with standard fallback kick models cannot reproduce the support for negative $χ_{\rm eff}$.

astro-ph.HE↗

arXiv2026

On the effective spin-mass ratio relation of binary black hole mergers that evolved in isolation

The ground-based measurement of gravitational waves (GW) from merging binary black holes (BBH) allows independent determination of spins of stellar-remnant black holes (BH). The observed population of BBH mergers exhibits two intriguing peculiarities related to BH spins, namely, a positively biased distribution of the effective spin parameter, $χ_{\rm eff}$, and an apparent anti-correlation between merger mass ratio, $q$, and $χ_{\rm eff}$. Here we investigate the potential mechanisms for such observed properties, in BBH mergers via isolated binary evolution. We synthesise BBH mergers with the fast binary evolution code BSE. The role of various physical assumptions is explored, including tidal spin-up, compact remnant mass, and mass transfer physics. We compare the properties of BBHs that form through stable mass transfer (SMT) and common envelope evolution (CE). We find that both the asymmetry in the $χ_{\rm eff}$ distribution and the $χ_{\rm eff}$ anti-correlation can be natural outcomes of isolated-binary BBH formation. The anti-correlation is especially pronounced for SMT-channel BBH mergers that experience a mass-ratio reversal, i.e., those where the second-born BH is the more massive one. The anti-correlation arises from the dependence of orbital shrinking during mass transfer and the Roche lobe size on the system's mass ratio. This characteristic $χ_{\rm eff}-q$ trend diminishes with increasing metallicity and when the isolated-binary BBH merger population is mixed with a significant contribution of dynamically formed BBH mergers or the newly formed BH's spin is misaligned relative to the parent star's spin. Our results demonstrate that isolated massive binary evolution via the SMT sub-channel can reproduce trends in the observable BBH merger population, with the characteristic signatures in mass, mass ratio, and spin distributions.

astro-ph.HE↗

arXiv2026

Non-conservative Mass Transfer as a Formation Channel for Gaia Black Hole System

The detected Gaia systems hosting compact objects challenge standard models of binary star evolution. In particular, if the observed black hole (BH) systems evolved in isolation, they are expected to have undergone a mass transfer phase. Given their highly unequal masses, such mass transfer is dynamically unstable within standard models, leading to a stellar merger or a short-period binary. In contrast, the observed systems have much wider orbits than predicted, making their formation within conventional evolutionary frameworks difficult to reconcile. Using detailed binary evolution calculations, we test whether non-conservative mass transfer, in which most of the mass is lost from the system carrying the specific angular momentum of the donor's center of mass, can explain the properties of two Gaia BH systems. This mass-loss geometry differs from standard isotropic re-emission from the accretor's vicinity. We find that our mass-loss geometry model reproduces the orbital periods of the two Gaia BH systems remarkably well over a wide range of initial conditions, offering a plausible formation pathway. We speculate this may point to enhanced eruptive mass loss, potentially driven by high-opacity subsurface layers in the donor prior to Roche-lobe overflow, consistent with preferentially bipolar outflows observed in luminous blue variables. Alternatively, it may indicate the need for more sophisticated mass-transfer prescriptions that account for highly unequal Roche-lobe sizes, sub-synchronous rotation, and possible self-accretion. Similar mechanisms may operate in other post-mass-transfer systems facing analogous evolutionary challenges, including Gaia neutron-star and white-dwarf binaries, stripped-envelope Wolf-Rayet stars, and low-mass X-ray binaries.

astro-ph.HE↗

arXiv2026

Gravitational-wave Observations Suggest Most Black Hole Mergers Form in Triples

The spin-orbit tilt angles $θ_{1(2)}$ of merging stellar-mass black holes provide key insights into their astrophysical origin. Non-parametric population modelling of The LIGO, Virgo, and KAGRA Collaborations (2025a, arXiv:2508.18083) shows that the spin-orbit tilt distribution of mergers in the latest Gravitational-Wave Transient Catalog 4.0 exhibits a global peak at near-perpendicular directions $\cosθ_{1(2)}\approx0$. Here, we recover this feature using hierarchical Bayesian inference with parametric models that are tailored to enhance the diagnostic power about astrophysical formation channels. We find that the spin distribution of the low-mass bulk of the binary black hole merger population ($m_1\lesssim 44.3^{+8.7}_{-4.6}\,\rm M_\odot$) can be well-modelled by a dominant Gaussian component that peaks at $\cosθ_{1(2)}\approx0$, possibly mixed with a subdominant isotropic component. Models that include a component with spins preferentially aligned with the orbit are disfavoured by current data (with Bayes factors $|Δ\ln\mathcal{B}|\approx1$ to $3$) and constrain its contribution to be likely small ($ξ\sim\mathcal{O}(1)\,\%$), although large contributions cannot yet ruled out with certainty. If these findings are reinforced by more detections, they would challenge any major contribution from the traditional isolated-binary formation scenario yielding closely aligned spins. Instead, the dominant component with near-perpendicular spins matches expectations from the evolution of isolated massive stellar triples in the galactic field, where the Lidov-Kozai effect naturally produces a unique overabundance of mergers with $\cosθ_{1(2)}\approx0$.

astro-ph.HE↗

arXiv2026

Spins of Black Holes in X-ray Binaries and the Tension with the Gravitational Wave Measurements

We review current challenges in understanding the values and origin of the spins of black holes in binaries. Thanks to recent advances in astrophysical instrumentation, the spins can now be measured using both gravitational waves emitted by merging black holes and electromagnetic radiation from accreting X-ray binaries containing black holes. A key finding of the gravitational-wave observatories is that premerger black holes in binaries have low spin values, with an average dimensionless spin parameter of $a_*\sim$0.1--0.2, with 90\% having $a_*\lesssim 0.6$. This implies that the natal spins of black holes are generally low, and the angular momentum transport in massive stars is efficient. On the other hand, most of the published spins in X-ray binaries are very high. In particular, this is the case for binaries with high-mass donors (potential progenitors of mergers), where their published spins range from 0.8 to 1.0. At the same time, their short lifetimes prevent significant spin-up by accretion. Those with low-mass donors could be spun-up to $a_*\gtrsim 0.7$ by accretion only if the donor initial masses were more than several solar masses, which remains unproven. However, the existing methods of spin measurements suffer from significant systematic errors. The method relying on relativistic X-ray line broadening is based on the separation of the observed spectra into incident and reflected ones, which is highly uncertain. The method relying on spectral fitting of accretion disk continua uses models that predict the disk to be highly unstable, while stability is observed. Improved stable models predict lower spins. The published spin measurements in X-ray binaries are uncertain. The spins of the binaries with high-mass donors may be low, while those with low-mass donors have a broader spin distribution, ranging from low to high.

astro-ph.HE↗

arXiv2026

Resolving Black Hole Family Issues Among the Massive Ancestors of Very High-Spin Gravitational-Wave Events Like GW231123

The latest detection of GW231123, a binary black hole (BH) merger with exceptionally large masses and high spins for the incoming components, has been suggested as a smoking gun for hierarchical formation. In this scenario, a first generation of BHs resulting from collapsing stars form in a dense environment. Here they can assemble dynamically and undergo subsequent mergers. We discuss three challenges for the formation of a GW231123-like event inside a star cluster: 1) The high masses of the incoming BHs appear to be in the predicted pair-instability mass gap and thus suggest that second-generation or higher-order generation BHs are involved. 2) Very high spins ($χ_f \gtrsim 0.8$) are very unlikely for dynamically assembled BHs because of the isotropic distribution of spin vectors. 3) Hierarchically formed BHs are susceptible to receive large recoils, which could kick them out of their cluster. We simulate this scenario and show that only a few percent of mergers recover remnants within GW231123's primary spin estimate $χ_1=0.9^{+0.10}_{-0.19}$ and are retained inside typical star clusters. A large fraction of very rapidly spinning second-generation BHs (including $χ_f>0.9$) can only form if the first-generation BHs merges with aligned spins. This is a natural outcome of massive binary star evolution scenarios, such as a chemically homogeneous evolution. This scenario also predicts equal masses for the components, implying that the resulting BHs tend to receive very low recoil kicks and would therefore likely be retained inside a cluster. We conclude that GW231123-like events, if formed in a star cluster, could require first-generation BHs with large aligned spins that evolved through stellar binary interaction, followed by the dynamical assembly for a subsequent merger. We discuss the implications for the uncertain lower edge of the putative mass gap 60-130 $\rm M_\odot$.

astro-ph.HE↗

arXiv2025

Self-lensing binaries as probes of Supernova physics

Self-lensing (SL) in binary systems has the potential to provide a unique observational window into the Galactic population of compact objects. Using the $\mathtt{startrack}$ and COSMIC population synthesis codes, we investigate how different supernova mechanisms affect the observable population of SL systems, with particular attention to the mass gap (2$\mathrm{-}$5 M$_\odot$) in compact object distributions. We test three supernova remnant formation models with different convective growth timescales ($f_{\rm mix}$ = 0.5, 1.0, and 4.0), simulating SL binary systems across the Galactic disk and bulge. We identify distinct groupings of SL sources based on lens mass and Einstein crossing time, clearly differentiating neutron star from black hole systems and close from wide orbits. Notably, the delayed $f_{\rm mix} = 0.5$ model predicts a significantly higher fraction of systems with lens masses in the mass gap region (up to $\sim10$ times more for certain surveys), suggesting that SL observations could help constrain this controversial population. Our analysis reveals a strong preference for systems with low centre-of-mass velocities ($v_{\rm cm}\leq20$ km/s) across all models, resulting primarily from physical processes governing compact object formation and binary survival. While many potential detections will have limited observational coverage, ZTF is predicted to yield several dozen well-covered systems that should enable detailed characterization. When applying simple detection criteria including photometric precision and signal-to-noise requirements, predicted rates decrease by approximately two orders of magnitude, but still yield up to a few tens of expected detections for LSST and ZTF in the Galactic disk population.

astro-ph.HE↗

arXiv2025

Prompt stellar and binary black hole mergers in tight triples: Insights from chemically homogeneous evolution

Short-period massive binary stars are predicted to undergo chemically homogeneous evolution (CHE), making them prime candidates for producing binary black holes (BBHs) that may merge within the age of the Universe. Most of these binaries have a tertiary companion, and here we explore how a nearby third body possibly influences this evolutionary channel. Our analysis combines analytic treatments of triple dynamics with insights from detailed stellar evolution models, focusing on the role of the von Zeipel-Lidov-Kozai mechanism while also accounting for tidal and general relativistic apsidal precession. We examine the dynamics of triples at three critical evolutionary stages: the zero-age main sequence, shortly after the main sequence, and at the time of BBH formation. We find that, for triples having outer orbital periods less than 70 d(120 d), the inner binary can merge during(or after) the main-sequence stage, leading to a hydrogen-rich(helium-rich) stellar merger. If a stellar merger is avoided, the inner binary may eventually form a BBH. In mildly hierarchical triples, with outer periods of around 100 d, the tertiary component can trigger a rapid merger of the BBH on timescales comparable to the outer orbital period. Stellar tides play a crucial role in determining the fate of the inner binary in such tight triple systems, as they can suppress the perturbative effects of the third star. When tidal forces damp the oscillations induced by the tertiary, the BBH merger may occur soon after stellar collapse. Notably, these outcomes are not restricted to CHE binaries but can also apply to any BBH formed from stars in tight orbits. Mergers in these systems are characterized by the proximity of a tertiary companion and the presence of recently ejected gas, making them promising candidates for electromagnetic counterparts and gravitational-wave signals influenced by nearby tertiary objects.

astro-ph.SR↗

arXiv2025

Estimating the binary neutron star merger rate density evolution with Einstein Telescope

The Einstein Telescope (ET) is a proposed third-generation, wide-band gravitational wave (GW) detector which will have an improved detection sensitivity in low frequencies, leading to a longer observation time in the detection band and higher detection rate for binary neutron stars (BNSs). Despite the fact that ET will have a higher detection rate, a large fraction of BNSs will remain undetectable. We present a scheme to estimate accurate detection efficiency and to reconstruct the true merger rate density of the population of the BNSs, as a function of redshift. We show that with ET as a single instrumnet, for a population of BNSs with $R_{mer} \sim 100 (300)$ $\rm Gpc^{-3} yr^{-1}$ at $z\sim 0(2)$, we can reconstruct the merger rate density uptil $z \sim 2$ , with a relative error of $12\%$ at ($z \sim 2$), despite the loss in detection of the bulk of the BNS population.

astro-ph.HE↗

arXiv2025

A fundamental limit to how close binary systems can get via stable mass transfer shapes the properties of binary black hole mergers

Mass transfer in binary systems is the key process in the formation of various classes of objects, including merging binary black holes (BBHs) and neutron stars. Orbital evolution during mass transfer depends on how much mass is accreted and how much angular momentum is lost $-$ two of the main uncertainties in binary evolution. Here, we demonstrate that, despite these unknowns, a fundamental limit exists to how close binary systems can get via stable mass transfer (SMT), that is robust against uncertainties in orbital evolution. Based on detailed evolutionary models of interacting systems with a BH accretor and a massive star companion, we show that the post-interaction orbit is always wider than $\sim10R_{\odot}$, even with extreme shrinkage due to L2 outflows. Systems evolving towards tighter orbits become unstable and result in stellar mergers. This separation limit has direct implications for the properties of BBH mergers: long delay times ($\gtrsim1 \rm Gyr$), and no high BH spins from the tidal spin-up of helium stars. At high metallicity, the SMT channel may be severely quenched due to Wolf-Rayet winds. The reason for the separation limit lies in the stellar structure, not in binary physics. If the orbit gets too narrow during mass transfer, a dynamical instability is triggered by a rapid expansion of the remaining donor envelope due to its near-flat entropy profile. The closest separations can be achieved from core-He burning ($\sim8-15R_{\odot}$) and Main Sequence donors ($\sim15-30R_{\odot}$), while Hertzsprung Gap donors lead to wider orbits ($\gtrsim30-50R_{\odot}$) and non-merging BBHs. These outcomes and mass transfer stability are governed by the internal composition profiles of donor stars. Thus, the formation of compact binaries is a sensitive probe of chemical mixing in stars. We propose a simple treatment of mass transfer stability to reproduce the detailed results.

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arXiv2025

Supermassive black holes stripping a subgiant star down to its helium core: a new type of multi-messenger source for LISA

Some stars orbiting supermassive black holes (SMBH) are expected to undergo a gravitational-wave (GW)-driven inspiral and initiate mass transfer on nearly circular orbits. However, the stability and duration of such phases remain unexplored. In this work, we focus on the evolution of a low-mass, radiative-envelope subgiant star being stripped by an SMBH. We find that such systems can undergo a long-lasting, stable mass-transfer phase, even if none of the angular momentum of the transferred material returns to the orbit to counterbalance the GW-driven decay. We show an example where a 2 Msun subgiant is stripped before entering the LISA band and loses almost its entire hydrogen envelope. The remaining helium core undergoes a prolonged GW-driven inspiral, becoming a loud LISA source. If formed in our galaxy, the system would be detectable for several hundred thousand years, ultimately reaching extreme signal-to-noise ratios of a million. Hydrogen shell flashes in the residual envelope cause temporary radial expansions of the stripped star. As a result, a few additional phases of rapid mass transfer occur at orbital periods of 20 - 30 hours. Eventually, the core possibly undergoes circular partial tidal disruption at an orbital period of ~10 minutes, corresponding to a GW emission frequency of a few mHz. We estimate a chance of about 1% that such a detectable LISA source exists in our own galactic center. The loud final GW transient may lead to a few detections reaching as far as ~1 Gpc, including, e.g., the Abell clusters.

astro-ph.HE↗

arXiv2025

X-ray emission from helium star+black hole binaries as probes of tidally induced spin-up of second-born black holes

Tidally induced spin-up of stripped helium stars in short-period (<\,1\,d) binaries with black holes has been presented as one of the possible mechanisms to reproduce the high-spin tail of the black hole spin distribution derived from gravitational wave (GW) merger observations. At such short periods, a fraction of the strong stellar wind from the stripped helium stars may be accreted by the black holes, and its gravitational potential energy may be released as observable radiation in the X-ray regime. We estimate the X-ray luminosity and its observability from black holes in orbit with stripped helium stars, which evolve into binary black hole or black hole+neutron star binaries that merge within Hubble time. We post-process recent advancements for estimating X-ray luminosities (via wind accretion onto stellar mass black holes) into two rapid population synthesis codes, BSE and StarTrack. We derive lower limits on the X-ray luminosity distribution from populations of stripped helium+black hole binaries at four metallicities (0.01, 0.1, 0.5, 1 $Z_{\odot}$) and two mass transfer stability criteria. We find that a large fraction (0.1-0.5) of stripped-helium stars that get spun up by tides also transfer enough wind matter onto the black hole to produce X-ray luminosities above $10^{35}$\,erg\,s$^{-1}$, up to $\sim10^{39}$\,erg\,s$^{-1}$. Such binaries should be observable as X-ray bright systems at 0.1\,$Z_{\odot}$, 0.5\,$Z_{\odot}$ and $Z_{\odot}$, representative of Sextans A, the Large Magellanic Cloud (LMC) and the Solar neighbourhood, respectively. The formation efficiency of these systems increases with decreasing metallicity. However, accounting for the local star formation rates, our population synthesis predicts $\sim$2 and $\sim$1 such observable binaries in the Milky Way and LMC, respectively, that will produce a binary compact object merger within a Hubble time. (Abridged)

astro-ph.HE↗

arXiv2024

Stripped helium-star and compact object binaries in coeval populations -- predictions based on detailed binary evolution models

Massive stars mainly form in close binaries, where their mutual interactions can profoundly alter their evolutionary paths. Evolved binaries consisting of a massive OB-type main-sequence star with a stripped helium star or a compact companion represent a crucial stage in the evolution towards double compact objects, whose mergers are (potentially) detectable via gravitational waves. The recent detection of X-ray quiet OB+black hole binaries and OB+stripped helium star binaries has set the stage for discovering more of these systems in the near future. In this work, based on 3670 detailed binary-evolution models and using empirical distributions of initial binary parameters, we compute the expected population of such evolved massive binaries in coeval stellar populations, including stars in star clusters and in galaxies with starburst activities, for ages up to 100 Myr. Our results are vividly illustrated in an animation that shows the evolution of these binaries in the color-magnitude diagram over time. We find that the number of OB+black hole binaries peaks around 10 Myr, and OB+neutron star binaries are most abundant at approximately 20 Myr. Both black holes and neutron stars can potentially be found in populations with ages up to 90 Myr. Additionally, we analyze the properties of such binaries at specific ages. We find that OB+helium stars and OB+black hole binaries are likely to be identifiable as single-lined spectroscopic binaries. Our research serves as a guide for future observational efforts to discover such binaries in young star clusters and starburst environments.

astro-ph.SR↗

arXiv2024

Residual eccentricity as a systematic uncertainty on the formation channels of binary black holes

Resolving the formation channel(s) of merging binary black holes is a key goal in gravitational-wave astronomy. The orbital eccentricity is believed to be a precious tracer of the underlying formation pathway, but is largely dissipated during the usually long inspiral between black hole formation and merger. Most gravitational-wave sources are thus expected to enter the sensitivity windows of current detectors on configurations that are compatible with quasi-circular orbits. In this paper, we investigate the impact of "negligible" residual eccentricity -- lower than currently detectable by LIGO/Virgo -- on our ability to infer the formation history of binary black holes, focusing in particular on their spin orientations. We trace the evolution of both observed and synthetic gravitational-wave events backward in time, while resampling their residual eccentricities to values that are below the detectability threshold. Eccentricities in-band as low as $\sim 10^{-4}$ can lead to significant biases when reconstructing the spin directions, especially in the case of loud, highly precessing systems. Residual eccentricity thus act like a systematic uncertainty for our astrophysical inference. As a mitigation strategy, one can marginalize the posterior distribution over the residual eccentricity using astrophysical predictions.

astro-ph.HE↗

arXiv2024

Unequal-mass, highly-spinning binary black hole mergers in the stable mass transfer formation channel

The growing database of gravitational-wave (GW) detections with the binary black holes (BHs) merging in the distant Universe contains subtle insights into their formation scenarios. One of the puzzling properties of detected GW sources is the possible (anti)correlation between mass ratio q of BH-BH binaries and their effective spin. We use rapid binary evolution models to demonstrate that the isolated binary evolution followed by efficient tidal spin-up of stripped helium core produces a similar pattern in Xeff vs q distributions of BH-BH mergers. In our models, the progenitors of unequal BH-BH systems in the stable mass transfer formation scenario are more likely to efficiently shrink their orbits during the second Roche-lobe overflow than the binaries that evolve into nearly equal-mass component systems. This makes it easier for unequal-mass progenitors to enter the tidal spin-up regime and later merge due to GW emission. Our results are, however, sensitive to some input assumptions, especially, the stability of mass transfer and the angular momentum loss during non-conservative mass transfer. We note that mass transfer prescriptions widely adopted in rapid codes favor the formation of BH-BH merger progenitors with unequal masses and moderate separations. We compare our results with detailed stellar model grids and find reasonable agreement after appropriate calibration of the physics models. We anticipate that future detections of unequal-mass BH-BH mergers could provide valuable constraints on the role of the stable mass transfer formation channel. A significant fraction of BH-BH detections with mass ratio q in range (0.4 - 0.7) would be consistent with the mass ratio reversal scenario during the first, relatively conservative mass transfer, and a non-enhanced angular momentum loss during the second, highly non-conservative mass transfer phase.

astro-ph.HE↗