Friday, June 20
Gravitational-wave mergers from star clusters and isolated binaries
Dynamical interactions among stellar-remnant black holes (BH) in dense star clusters is widely conceived as a formation mechanism of merging binary black holes (BBH) that the LIGO-Virgo-KAGRA (LVK) gravitational-wave (GW) interferometer network detects in numbers. In this talk, I shall present results from an in-preparation grid of evolutionary models of star clusters, focussing on BBH mergers occurring in them. The homogeneous grid comprises long-term-evolved (up to at least a few Gyr age) star clusters with initial conditions as follows: mass range over 10^4 Msun – 10^5 Msun, size over 1 – 3 pc, 100% primordial binary fraction for massive stars, 10% primordial binary fraction for low-mass stars. The simulations incorporate stellar and binary evolution, that include recent prescriptions for stellar remnant formation, and post-Newtonian treatment of compact binaries, and are performed with an updated version of the direct N-body code NBODY7. I shall focus on (a) BBH merger mass spectrum, (b) redshift evolution of BBH merger rate, (c) properties of dynamically assembled versus primordially assembled BBH mergers, as obtained from these star cluster models. In the second part of the talk, I shall focus on BBH mergers from isolated massive binary evolution – another widely debated formation channel of GW events. In particular, I shall present results from a comprehensive suite of binary population synthesis models that demonstrate the occurrence of an anti-correlation between the merging BBHs’ spin and mass ratio. Existence of such an “Xeff-q” anti-correlation is hinted by the observed GW-event population. I shall discuss the origin of this anti-correlation in these binary-evolution models.
Expanding the Family Tree: Refined Population Inference of Hierarchical Mergers in Gravitational-Wave Data
In the decade since LIGO’s first direct detection of gravitational waves (GWs), the LIGO-Virgo-KAGRA (LVK) collaboration has seen its catalog of GW events grow exponentially. Studying these events on both individual and population levels has provided valuable insights into the properties of binary BH mergers. Despite these advances, key questions about how and where binary BHs form remain unresolved. For instance, BHs with masses up to ~45 times the mass of our Sun are typically expected to form through the direct collapse of their progenitor stars (i.e., standard stellar evolution). Above this mass range, theoretical models predict a range of masses where BHs are not expected to form due to the pair instability process in massive-star cores. However, past observations of mergers involving BHs within this gap challenge these theoretical expectations. One possible explanation for the formation of such heavy, highly-spinning BHs is the hierarchical merger scenario, in which upper mass gap BHs arise from the repeated mergers of lower-mass BHs in dynamical environments such as globular clusters. In this talk, I will discuss ongoing work to refine the population modeling of hierarchical mergers from observed GW events, informed by the latest theoretical insights into dense stellar systems. Advancements in modeling will be crucial for more accurately interpreting the origins of past and future GW events, especially in the context of a growing GW catalog and upcoming results from LVK’s fourth observing run. Collaborators: Mike Zevin, Debatri Chattopadhyay, Anarya Ray, Vicky Kalogera, et al.
The dynamical origin of Gaia BH3
Last year, the Gaia Collaboration announced the discovery of a black hole (BH) of 33 M⊙ in a wide binary system with a giant star, located in the ED-2 stream. The peculiarities of this system, including the component masses, semimajor axis and the low metallicity of the star, have been argued to be at tension with current binary evolution models. We showed with Cluster Monte Carlo models that Gaia BH3 most likely formed dynamically in the progenitor cluster of the ED-2 stream. In this talk, I present new detailed N-body models of this progenitor cluster and the ED-2 stream, and compare the resulting stream and star-BH systems to Gaia observations, to learn more about the formation of these dormant star-BH binaries and what to expect for future Gaia data releases.
Modelling Millisecond Pulsar Populations in Globular Clusters with NBODY6++GPU
Millisecond pulsars (MSPs) are neutron stars with rotational periods as low as a few milliseconds. They are formed via angular momentum transfer from accreted materials from a companion star. In the high density environment of globular clusters (GCs), MSPs are likely to form through dynamically formed interacting binaries. In fact, over 300 MSPs are detected in GCs, more than half of the known MSP population. In this work, we attempt to model the MSP populations in intermediate mass clusters using the state-of-the-art N-body simulation code, NBODY6++GPU. We update NBODY6++GPU to include a pulsar spin-down mechanism due to magnetic braking, and spin-up from accretion. These results are compared with observed MSP populations in GCs with similar masses. We then correlate the number of observable MSPs to physical conditions of GCs, and also attempt to predict merger events involving neutron stars in GCs. Since the gamma-ray emission from GCs originates from MSPs, we use the results to imply the observed gamma-ray emission. Different gamma-ray emission mechanisms within GCs are discussed, including the direct superposition of MSP gamma-rays, and the inverse Compton scattering of various photon fields, such as cosmic microwave background, intra-cluster star light and galactic star light by relativistic particles in pulsar winds. MSPs ejected from the clusters through dynamical interaction contribute towards the GeV Excess at the Galactic Centre, and this effect is also discussed in this work.
The multimessenger life of intermediate mass-ratio inspirals in globular clusters: The tidal stripping of white dwarfs.
We explore the multimessenger signals produced by intermediate-mass ratio inspirals (IMRIS) consisting of an intermediate mass black hole (IMBH) and a white dwarf (WD). These IMRIs may be found in globular clusters which are systems that can host an intermediate mass black hole according to numerical simulations, theoretical scaling relations, and some observational evidence. We analyze the MOCCA Simulation Survey 1 to identify systems that could generate a periodic electromagnetic signal due to the tidal stripping of a white dwarf. The luminosities of these systems could be comparable to those of quasi-periodic eruptions (QPE), making them detectable by current X-ray telescopes. Although the periods of these systems are generally longer than those of QPE signals, they still hold potential as sources of both electromagnetic radiation and gravitational waves (GW). We compute the event rates and the occurrence of such systems over the evolution of globular clusters, along with their gravitational wave signals to evaluate their detectability by future space-based gravitational wave detectors, such as LISA, TianQin, or Taiji.
Tidal disruptions of supermassive black hole binaries in galaxy mergers
Galaxy mergers are the direct descendents of the Λ cold dark matter cosmology. There are many close connections between central supermassive black holes (SMBHs) and their host galaxies, which indicates that SMBHs play very important roles in the formation and evolution of galaxies. During the merging of galaxies and the formation of the supermassive black hole binary (SMBHB), black holes and surrounding stars undergo a dramatic evolution, which results in very complicated evolution of SMBHs, stars and other compact objects. Due to the gas poor environment and the limited spatial resolution in observations, it is very difficult to find clues of these systems in quiescent galaxies. However, a close encounter of a star with one of the supermassive black holes (SMBHs) may tidally disrupt it to produce a tidal disruption event (TDE) and temporarily light up the SMBH. TDEs by SMBHBs should be significantly different from the cases by single SMBHs. By using direct N-body simulations and scattering experiments, we investigate TDEs by SMBHBs and find that both the statistical event rates and specific light curves from SMBHB systems are significantly different from the cases in isolated galaxies.
Strong Scatterings Invalidate Proposed Models of Enhanced TDE Rates in Post-Starburst Galaxies
Stars wandering too close to supermassive black holes (SMBHs) can be ripped apart by the tidal forces of the black hole. Recent optical surveys have revealed that E+A galaxies are overrepresented by a factor $\sim $ 30, while green galaxies are overrepresented in both optical and infrared surveys. Different stellar models have been proposed to explain this Tidal Disruption Event (TDE) preference: ultra-steep stellar densities in the nuclear cluster, radial velocity anisotropies, and top-heavy Initial Mass Function (IMF). Here we explore these hypotheses in the framework of our revised loss cone theory that accounts for both weak and strong scattering, i.e., a scattering strong enough to eject a star from the nuclear cluster. We find that, when accounting for weak and strong scatterings, both ultra-steep densities and radial velocity anisotropies fail to explain the post-starburst preference of TDEs except when considering a high anisotropy factor together with a high SMBH mass and a shallow density profile of stellar mass black holes $\gamma_{\rm bh} =7/4$. Our findings hold when combining either model with top-heavy IMFs. Hence, our work emphasizes both the importance of taking into account strong scatterings and the need for new hypotheses to explain the post-starburst preference of TDEs.
Gravitational-Wave Signatures of Highly Eccentric Stellar Binary Black-Holes in Galactic Nuclei
A significant fraction of gravitational-wave (GW) mergers in galactic nuclei are expected to be eccentric in the Laser- Interferometer-Space-Antenna (LISA) frequency sensitivity range, 10^−4 − 10^−1 Hz. Several GW mergers detected by the LIGO-Virgo-KAGRA Collaboration have been suggested to have evidence for residual eccentricity at 10 Hz, suggesting dynam- ical or triple origins for a significant fraction of the detected population. In triple systems, von-Zeipel-Lidov-Kozai oscillations perturb both the eccentricity and the argument of pericentre, 𝜔, of the inner black hole binary, which could be fully circulating, where 𝜔 cycles through 2𝜋, or may librate, with 𝜔 ranges about a fixed value with small or large variation. We use TSUNAMI, a regularised N-body code with up to 3.5 post-Newtonian (PN) term corrections, to identify four different families of orbits: (i) circulating, (ii) small and (iii) large amplitude librating, and (iv) merging orbits. We develop and demonstrate a new method to construct gravitational-wave waveforms using the quadrupole formula utilising the instantaneous total acceleration of each binary component in TSUNAMI. We show that the four orbital families have distinct waveform phenomenologies, enabling them to be distinguished if observed in LISA. In particular, the properties of the tertiary companion can be inferred and serve as an independent mass measurement and distinguish field triple dynamics from galactic nuclei dynamics.
Investigating the Origins of Hypervelocity Stars in the Galactic Center
Understanding the origin of hypervelocity stars (HVSs) in the Galactic Center is a critical question, particularly regarding the dynamic processes that have shaped this region. In this study, we assert that HVSs are likely formed as counterparts to S stars, specifically through the Hills mechanism originating in the young stellar disk. By employing advanced numerical simulations, we thoroughly analyze the statistical likelihood of HVS production in our models and robustly compare these results with observational data of HVSs in the Galactic halo. Our findings provide significant insights into the intricate interactions that drive stellar dynamics in the Galactic Center.
Resonant Relaxation in the Galactic Center : Multi-Component Stellar System Cases
At the center of the Galaxy, it is believed that there is a massive black hole (MBH). Also it is thought that the MBH is surrounded by stars and compact objects. A few tens of stars are observed and called as S stars. S stars are orbiting around the black hole along their Kepler orbits. Using observational result of orbits of S stars, the enclosed mass within their orbits is estimated (e.g. THE GRAVITY COLLABORATION 2024). From a theoretical point of view, Rauch & Tremaine (1996) predicted precession of stars around an SMBH due to resonant relaxation (RR). In our previous studies, we show the change of pericenter of S stars due to RR may be observed using direct $N$-body simulations, which is the first direct $N$-body simulation of the center of GC. In the previous work, we assume that the stellar system around the MBH is composed of equal mass bodies. In the present work, we performed a series of $N$-body simulations in which the stellar system is composed of various mass bodies including an intermediate mass black hole (IMBH). We found that the change of position of pericenter due to RR of the environment of the various mass system is as comparable as that of the equal mass system composed of the same average mass of stars. Our result implies that the observed change of orbits of S stars due to RR will tell us the enclosed and the average mass of stars. It won’t directly tell us about the composition, however, it may be a clue to know it. As an additional outcome, we found that close encounters between the IMBH and an S star caused drastic change of star’s orbit. This drive the evolution of distribution of inclination and eccentricity of the S stars. This may also produce some high-speed stars ejected from the GC though the probability is small. These subjects will be discussed, too.