Sambaran Banerjee

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), thereby offering insights into their formation mechanisms. The observed population of BBH mergers exhibits two intriguing peculiarities related to BH spins, namely, a positively biased distribution of the effective spin parameter, , and an apparent anti-correlation between merger mass ratio, q , and . 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 distribution and the 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 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.

We analyze the velocity dispersion profiles of nine open clusters in the solar neighborhood using kinematic data from Gaia DR 3, aiming to identify potential dynamical signatures of stellar-mass black holes through a comparison of theoretical and observed dispersion profiles. The selected clusters include LP 2373 gp4, NGC 1980, NGC 2451A, NGC 2516, NGC 3532, NGC 6475, UBC 7, Praesepe, and Pleiades. We refine the center positions of the clusters with the Meanshift algorithm. Using the Markov Chain Monte Carlo method, we calculate the velocity dispersion for each cluster and construct 1D velocity dispersion profiles. NGC 2516, NGC 3532, and NGC 6475 show potential central cusps in their radial velocity dispersion profiles, which may indicate the presence of stellar-mass black holes. LP 2373 gp4, NGC 6475, and Praesepe all display a negative correlation between velocity dispersion and stellar mass, indicating these clusters are approaching energy equipartition or expanding. NGC 2516 and NGC 3532 exhibit a positive dependence between velocity dispersion and stellar mass, which may be attributed to the preferential ejection of massive stars following dynamical interactions involving binaries or black holes. These two clusters are the only two that are dynamical, not relaxed, and are closest to virial equilibrium. We compare the observations with N -body simulations of star clusters. A comparison of observed and simulated velocity dispersion profiles reveals that NGC 2516 and NGC 3532 exhibit lower proper motion dispersions than model clusters. Better agreement with the observed profiles is achieved for model clusters with larger ages. This suggests that the observed clusters may have undergone rapid dynamical evolution. Our results suggest that NGC 2516 and NGC 3532 may host at least two stellar-mass black holes each.

We analyze the velocity dispersion profiles of nine open clusters in the solar neighborhood using kinematic data from Gaia DR 3, aiming to identify potential dynamical signatures of stellar-mass black holes through a comparison of theoretical and observed dispersion profiles. The selected clusters include LP2373 gp4, NGC 1980, NGC 2451A, NGC 2516, NGC 3532, NGC 6475, UBC 7, Praesepe, and Pleiades. We refine the center positions of the clusters with the Meanshift algorithm. Using the Markov Chain Monte Carlo method, we calculate the velocity dispersion for each cluster and construct one-dimensional velocity dispersion profiles. NGC 2516, NGC 3532, and NGC 6475 show potential central cusps in their radial velocity dispersion profiles, which may indicate the presence of stellar-mass black holes. LP2373 gp4, NGC 6475, and Praesepe all display a negative correlation between velocity dispersion and stellar mass, indicating these clusters are approaching energy equipartition or expanding. NGC 2516 and NGC 3532 exhibit a positive dependence between velocity dispersion and stellar mass, which may be attributed to the preferential ejection of massive stars following dynamical interactions involving binaries or black holes. These two clusters are the only two that are dynamical not relaxed and are closest to virial equilibrium. We compare the observations with N-body simulations of star clusters. A comparison of observed and simulated velocity dispersion profiles reveals that NGC 2516 and NGC 3532 exhibit lower proper motion dispersions than model clusters. Better agreement with the observed profiles is achieved for model clusters with larger ages. This suggests that the observed clusters may have undergone rapid dynamical evolution. Our results suggest that NGC 2516 and NGC 3532 may host at least two stellar-mass black holes each.

Context. Tidally induced spin-up of stripped helium stars in short-period (< 1 d) binaries with black holes (BHs) has been presented as a possible mechanism for reproducing the high-spin tail of the BH spin distribution derived from gravitational wave (GW) merger observations. With such short periods, a fraction of the strong stellar wind from the stripped helium stars could be accreted by the BHs, and its gravitational potential energy could be released as observable radiation in the X-ray regime. Aims. We estimated the X-ray luminosity and its observability from the population of BHs in orbit with stripped helium stars that evolve into BH–BH or BH–neutron star binaries and merge within a Hubble time. Methods. We post-processed recent advancements in estimating X-ray luminosities (via wind accretion onto stellar-mass BHs) into the rapid population synthesis codes BSE and StarTrack. We derived lower limits on the X-ray luminosity distribution from the population of stripped helium star–BH binaries at four metallicities (0.01, 0.1, 0.5, and 1 Z_⊙) and two mass transfer stability criteria. Results. We find that a large fraction (0.1–0.5) of stripped helium stars in the above population transfer enough wind matter onto the BH to produce X-ray luminosities above 10³⁵ erg s⁻¹, up to ∼10³⁹ erg s⁻¹. Such binaries should be observable as X-ray-bright systems at 0.1 Z_⊙, 0.5 Z_⊙, and Z_⊙, that is, in Sextans A, the Large Magellanic Cloud (LMC), and the solar neighbourhood, respectively. We show that most of these X-ray-bright systems also have the shortest orbital periods, and tides can spin up the stripped helium star component. The formation efficiency of these systems increases with decreasing metallicity. However, accounting for the local star formation rates, our population synthesis predicts that ∼2 and ∼1 such X-ray-bright helium star–BH binaries in the Milky Way and LMC, respectively, will produce a binary compact object merger within a Hubble time. Conclusions. Ongoing high-sensitivity X-ray surveys and high-resolution optical surveys of low-metallicity environments such as Sextans A are important stepping stones towards identifying the population of short-period helium star–BH binaries and possibly constraining the contribution of isolated binary evolution to the high spin tail of the BH spin distribution in GW mergers.

Properties of the to-date observed binary black hole (BBH) merger events suggest a preference toward spin–orbit aligned mergers. Naturally, this has caused widespread interest and speculations regarding implications on various merger formation channels. Here we show that (i) not only the BBH merger population from isolated binaries but also (ii) BBH population formed in young massive clusters (YMCs) would possess an asymmetry in favor of aligned mergers, in the distribution of the events’ effective spin parameter (χ _eff). In our analysis, we utilize BBH merger outcomes from state-of-the-art N-body evolutionary models of YMCs and isolated binary population synthesis. We incorporate, for the first time in such an analysis, misalignments due to both natal kicks and dynamical encounters. The YMC χ _eff distribution has a mean (an antialigned merger fraction) of 〈χ _eff〉 ≤ 0.04 (f _X − ≈ 40%), which is smaller (larger) than but consistent with the observed asymmetry of 〈χ _eff〉 ≈ 0.06 (f _X − ≈ 28%) as obtained from the population analysis by the LIGO–Virgo–KAGRA collaboration. In contrast, isolated binaries alone tend to produce a much stronger asymmetry; for the tested physical models, 〈χ _eff〉 ≈ 0.25 and f _X − ≲ 7%. Although the YMC χ _eff distribution is more similar to the observed counterpart, none of the channels correctly reproduce the observed distribution. Our results suggest that further extensive model explorations for both isolated binary and dynamical channels as well as better observational constraints are necessary to understand the physics of “the symmetry breaking” of the BBH merger population.

Context. The recent discovery of the binary black hole (BBH) merger event GW190521, between two black holes (BHs) of ≈100 M_samp, in addition to other massive BBH merger events involving BHs within the pair-instability supernova (PSN) mass gap have sparked widespread debate on the origin of such extreme gravitational-wave (GW) events. GW190521 simultaneously triggers two critical questions: how BHs can appear within the ‘forbidden’ PSN gap and, if they do, how they get to participate in general-relativistic (GR) mergers. Aims. In this study, I investigate whether dynamical interactions in young massive clusters (YMCs) serve as a viable scenario for assembling PSN-gap BBH mergers. Methods. To that end, I explore a grid of 40 new evolutionary models of a representative YMC of initial mass and size M_cl = 7.5 × 10⁴ M_samp (N ≈ 1.28 × 10⁵) and r_h = 2 pc, respectively. The model grid ranges over metallicity 0.0002 ≤ Z ≤ 0.02 and comprises initial cluster configurations of King central concentration parameters W₀ = 7 and 9. In each model, all BH progenitor stars are initially in primordial binaries following observationally motivated distributions. All cluster models are evolved with the direct, relativistic N-body code NBODY7, incorporating up-to-date remnant formation, BH natal spin, and GR merger recoil schemes. Results. Binary black hole mergers from these model cluster computations agree well with the masses and effective spin parameters, χ_eff, of the events from the latest gravitational-wave transient catalogue (GWTC). In particular, GW190521-like, that is to say ≈200 M_samp, low χ_eff events are produced via a dynamical merger among BHs derived from star-star merger products. GW190403_051519-like, that is PSN-gap, highly asymmetric, high χ_eff events result from mergers involving BHs that are spun up via matter accretion or a binary interaction. The resulting present-day, differential intrinsic merger rate density, within the PSN gap, accommodates that from GWTC well. Conclusions. This study demonstrates that, subject to model uncertainties, the tandem of massive binary evolution and dynamical interactions in ≲100 Myr-old, low metallicity YMCs in the Universe can plausibly produce GR mergers involving PSN-gap BHs and in rates consistent with that from up-to-date GW observations. Such clusters can produce extreme events similar to GW190521 and GW190403_051519. The upper limit of the models’ GW190521-type event rate is within the corresponding LIGO-Virgo-KAGRA (LVK)-estimated rate limits, although the typical model rate lies below LVK’s lower limit. The present YMC models yield a merger rate density of 0−3.8 × 10⁻² yr⁻¹ Gpc⁻³ for GW190521-type events. They produce GW190403_051519-like events at a rate within 0−1.6 × 10⁻¹ yr⁻¹ Gpc⁻³ and their total BBH-merger yield within the PSN gap is 0−8.4 × 10⁻¹ yr⁻¹ Gpc⁻³.