Wednesday, June 18
7-Dimensional Telescope for Multimessenger Astronomy
Even though gravitational-wave (GW) signal detection sensitivity has been steadily improving, the localization will likely remain poor (10 to 100 deg^2) for most future GW events. We have been building the 7-Dimensional Telescope (7DT), a new multiple telescope system capable of performing a wide-field spectral mapping of the sky to detect optical counterparts and prepare a galaxy catalog for future GW events. In this presentation, we introduce the 7DT and show its scientific potential for multi-messenger astronomy. In October 2024, we installed 16 of the 7DT’s 20 telescopes at the El Sauce Observatory in Chile, and scientific observations have been ongoing. We will show several highlights from early 7DT observations to demonstrate its ability to capture spectra of all objects in 7DT’s 1.2 deg^2 field of view, which should help quickly identify kilonova associated with a GW event and understand the nature of other transient phenomena such as asteroids and exoplanets. We will also show our effort in constructing a galaxy redshift catalog to prepare a list of the likely host galaxy candidates for future GW events. We will briefly introduce other optical GW follow-up facilities in Korea.
Evolutionary Links: From Gaia Neutron-Star Binaries to Pulsar–White Dwarf Endpoints
We investigate neutron-star–main-sequence binaries identified by Gaia by reconstructing their past evolution with the binary-population synthesis code POSYDON and modelling their future evolution using detailed MESA simulations. Anchored to observationally constrained masses, orbital parameters, and metallicities, our models precisely trace each system’s evolutionary pathway. We incorporate updated prescriptions for eccentric mass transfer and magnetic braking and, as a control, compare these to standard circularized mass-transfer scenarios. Within MESA, we also implement detailed treatments of rotationally powered spin-up and accretion-driven pulsar recycling. We find that, regardless of whether mass exchange occurs via eccentric periastron bursts or circularized Roche-lobe overflow, all binaries evolve into recycled-pulsar–white-dwarf systems. However, the assumed mass-transfer geometry—eccentric versus circular—critically influences the intermediate evolutionary tracks and the resulting spin periods, white-dwarf types, and masses.
ECLIPSE DOES NOT HIDE, BUT REVEALS: Comprehensive X-ray Reprocessing Studies in High and Low Mass X-ray Binaries with XMM-Newton
X-ray reprocessing serves as a vital diagnostic tool for gaining insights into the environments of X-ray binary systems. However, the study of X-ray reprocessing encounters challenges arising from the blending of intense primary radiation from the compact star with the reprocessed radiation from the surrounding. Eclipsing X-ray binaries offer a unique opportunity to investigate pure reprocessed X-rays, as the companion star effectively shields the intense primary X-rays. We carried out first comprehensive studies of X-ray reprocessing in most of the eclipsing High Mass X-ray Binary (HMXB) and Low Mass X-ray Binary (LMXB) systems by comparing their X-ray spectra during and outside of eclipse using XMM-Newton. We found ample diversity in the X-ray reprocessing characteristics in HMXBs, which implies significantly dynamic wind structure surrounding the compact objects in HMXBs. Significant differences observed in X-ray reprocessing characteristics in LMXBs despite all being dipping and eclipsing sources, suggest large dependencies of X-ray reprocessing on the inclination angle, scale height of the accretion disk, relative size of the accretion disk with respect to the companion, binary separation, mass ratio between the neutron star, the companion etc. Our studies revealed unexpected X-ray behaviors. For instance, (i) we observed high equivalent widths of Fe emission lines in both SgHMXBs and SFXTs during eclipse, indicating high Fe abundance, contrary to earlier findings showing low equivalent widths in SFXTs outside of eclipse. (ii) Cen X-3 showed a lower Fe Kα equivalent width during eclipse than outside, unlike other SgHMXBs. (iii) In 4U 1538-522, low-energy X-rays were not obscured during eclipse as expected. (iv) LMXBs showed a smaller out-of-eclipse to eclipse flux ratio than HMXBs, suggesting greater reprocessing despite less dense stellar winds. Overall the studies deepen our understanding of the intricate interplay between X-ray reprocessing and the diverse mechanisms within X-ray binary systems.
The redshift-evolving eccentricity distribution of merging binary black holes observed by ground-based gravitational-wave detectors
The formation of merging binary black holes (BBHs) form remains a key unresolved issue in astrophysics, despite nearly 100 detections by the LIGO-Virgo-KAGRA collaboration. Detectable eccentricity offers one of the most promising ways to distinguish different formation channels. However, detecting a sufficient number of eccentric mergers to reliably carry out such a task is expected to be feasible only with third-generation GW detectors, such as the Einstein Telescope or Cosmic Explorer. As these instruments will detect BBH mergers up to redshift z~6, it is critical to understand how the eccentricity distribution evolves with redshift. We predict the evolution of eccentricity distributions over redshift for merging BBHs from two key channels: the globular cluster (GC) channel and the hierarchical triple channel, where three-body dynamics induce high eccentricities in the inner binary. Our population synthesis method shows that mergers from the GC channel dominate in the local universe (z~0) by an order of magnitude, in broad agreement with previous studies. However, if we focus only on mergers that have detectable eccentricity with third generation detectors (e >10^-4 – 10^-3 at 10 Hz), this picture considerably changes: at z~0, 40% of eccentric mergers arise from hierarchical triples, and this fraction rises to 70% at z~2-3. Therefore, high-redshift eccentric mergers may be dominated by field triples, challenging the view that such mergers primarily occur in dense environments. We also explore the impact of uncertainties in GC and stellar evolution and find that the relative contribution of eccentric mergers from hierarchical triple remains at least ~30-40%. Finally, I will show that the merger rate density and eccentricity distribution of GW sources from the GC channel does not evolve with redshift significantly. This implies that that GW sources formed in GCs could potentially be used to constrain the CG formation history as well as initial parameters of GCs.
Mergers all the way down: building massive star clusters in dwarf galaxy starbursts
JWST has revealed that massive star clusters, the possible progenitors of globular clusters (GCs), formed with high mean stellar surface densities. Their internal structure and kinematics, however, still remain a mystery. A number of phenomena only possible at such high densities have been theorized to occur: for instance, massive stars and black holes can grow in mass due to collisions and star-forming gas can be retained, self-enriched by the massive stars, in the cluster for extended periods of time. In this talk I will explore the formation of star clusters in such extreme environments by presenting the results of star-by-star hydrodynamical simulations of low-metallicity dwarf galaxy starbursts. The simulations account for a multiphase interstellar medium, stellar radiation, winds and supernovae, and accurate small-scale gravitational dynamics near massive stars. The latest simulation includes prescriptions for stellar collisions and tidal disruption events by black holes. I will discuss the galactic population of star clusters, concentrating on the internal structure and chemical contents of massive clusters where the hierarchical formation history leaves a kinematic imprint in their stellar populations.
The MandelZoom project I: modelling black hole accretion through an 𝛼-disc with a resolved interstellar medium in dwarf galaxies
While there is a mounting observational evidence that intermediate mass black holes (IMBHs) may be important in shaping the properties of dwarf galaxies both at high redshifts and in the local Universe, our theoretical understanding of how these IMBHs grow is largely incomplete. To address this issue, we perform high-resolution simulations of an isolated dwarf galaxy with a virial mass of $10^{10} M_{\odot}$ harbouring a $10^4 M_{\odot}$ IMBH at its centre at a peak spatial resolution of $\lesssim 0.01$~pc. Within the fully multi-phase interstellar medium (ISM) we incorporate explicit sampling of stars from the IMF, photoionization, photoelectric heating, individual supernovae explosions as well as a Shakura-Sunyaev accretion disc model to track the evolution of BH mass and spin. We find that a nuclear star cluster (NSC) effectively captures the ISM gas and promotes formation a circumnuclear disc (CND) on scales of $\lesssim 7$~pc. Simultaneously, gravitational torques from the NSC reduce CND angular momentum on (sub-)parsec scales, circularizing the gas onto the $\alpha$-accretion disc and promoting sustained IMBH growth at $\sim 0.01$ of the Eddington rate. While in the innermost regions ($\lesssim 0.5$~pc), star formation is highly suppressed, CND is susceptible to fragmentation, leading to the formation of massive, young stars. Interestingly, despite an in-situ supernova rate of $0.3$~Myr$^{-1}$, the dense CND persists, sustaining BH accretion and leading to a net spin-up. Our study demonstrates the complexity of IMBH accretion with the resolved ISM, and paves the way to next-generation studies where growth of IMBH in full cosmological context can be captured.
Simulations of the dynamical evolution of lower mass binary stars in the Milky Way Nuclear Stellar Cluster
Most massive galaxies, including the Milky Way typically host both a supermassive black hole (SMBH) and a nuclear stellar cluster (NSC). This environment affects the evolution of binaries via frequent close encounters with the surrounding stars of the NSC as well as secular processes related to the SMBH, such as Eccentric-Kozai-Lidov (EKL) oscillations. Due to the distance and extinction from dust, solar and sub-solar mass main-sequence (MS) stars are not visible for observations and therefore constitute an invisible population of stars in the NSC who’s binaries could have a key role in formation of dusty cloud-like G-objects. This is supported by previous numerical simulations and the recent observation of a $\sim 2\mathrm{M}_\odot$ spectroscopic binary in the vicinity of the S-cluster. We aim to understand how MS stars at the hard/soft boundary (H/SB) evolve dynamically, and how a region where dynamical processes operate on similar timescales shape their outcome. We simulate more than $6\times10^7$ three-body encounters (binary + tertiary) for a total of 5736 binaries, which incorporate EKL oscillations, tidal circularisation and stellar collisions. In this way we follow the dynamical evolution of binaries ($\leq 2\mathrm{M}_\odot$) near the H/SB limit at $0.1\mathrm{pc}$ ($0.3\mathrm{pc}$) from the SMBH in a steady-state model of the NSC. Consistent with previous studies, a majority, 80\% (60\%), of binaries still merge or evaporate within $1\mathrm{Gyr}$ ($2\mathrm{Gyr}$) but a substantial fraction, 10\% (23\%), of our binaries remain intact throughout the simulation, $\geq10 \mathrm{Gyr}$, due to outward migration. Furthermore we find that \textit{i)} $\sim 1\%$ of our binary merger products’ orbits originating at $0.1\mathrm{pc}$ contaminate the orbits of the S-cluster stars, \textit{ii)} $\sim1\%$ of the mergers occur late enough ($\lesssim 10\mathrm{Gyr}$) that they should not have evolved significantly at present day, and as such may appear as young metal poor main sequence stars or red giant stars, \textit{iii)} $80\%$ of collisions between surrounding stars and our binaries leads to three-body pile ups (all three stars merge), virtually all of these occur within the first few $10^8$ years with a maximum rate of $\sim 0.1 \mathrm{Gyr}^{-1}$. Our work demonstrates the importance of studying the details of low-mass binary evolution and dynamics in galactic nuclei in order to reveal the full complexity of this environment and its peculiar objects.
Modeling Tidal Disruption Events and Compact Object Plunges in Nuclear Star Clusters
Nuclear star clusters and supermassive black holes (SMBHs) often coes xist at the centers of galaxies, engaging in complex dynamical interactions that influence SMBH growth. Stars traveling too close to the SMBH are disrupted by tidal forces, with a fraction of the resulting debris accreted onto the black hole. Orbiting compact objects, in contrast, may undergo relativistic inspirals, emitting gravitational waveas they lose orbital energy. In the coming years it is expected that tens of thousands of TDE events will be observed with new instruments for optical and UV (e.g. EinsteinProbe, ULTRASAT, LSST, ELT, GMT) and SKA in the radio band. Our work aims to improve the theoretical basis to understand light curves and other observational characteristics of TDE by computer simulations of TDE, embedded in a fully self-consistent dynamical model of a nuclear star cluster around the SMBH. Here, we report on the results of some preliminary pilot models. Using the STARDISK version of the NBODY6++GPU code, we perform numerical simulations of a nuclear star cluster hosting a central massive black hole, employing two distinct approaches to study the rates of these dynamical events and their contributions to SMBH growth. The first approach uses (i) an improved tidal radius that depends on the mass and radius of disrupted stars and (ii) a prescription for accretion of compact objects based on the orbit shrinkage timescale. The second approach refines the calculation of mass accretion onto the SMBH following tidal disruption events (TDEs). We start our simulations with a zero-age stellar population, representing the aftermath of a starburst. The results reveal that the accretion rate of compact objects is reduced by an order of magnitude compared to models without the prescription, with most accretion occurring at pericenters ranging from 4 to 27 Schwarzschild radii of the SMBH. The mass accretion rate from TDEs peaks within the first 2 Myr in all simulations but declines more rapidly for higher initial SMBH masses. Additionally, approximately half of the stellar debris from most TDEs is accreted, which is fairly consistent with the hydrodynamical simulations of Law-Smith et al. (2020).
Formation and Evolution of New Primordial Open Cluster Groups: Insights from Feedback-Driven Star Formation
Open cluster (OC) groups are collections of spatially close open clusters (OCs) that originate from the same giant molecular cloud. However, the formation mechanisms of OC groups remain unclear due to limited sample sizes and data precision. Recent advancements in Gaia astrometric data provide new opportunities for a more detailed study of OC groups. In this study, we extend the sample of OC groups and explore the formation and evolution mechanisms of newly identified primordial OC groups. We focus on the role of stellar feedback events, particularly supernovae (SNe), in triggering star formation within these groups. We use Gaia data to identify OC groups based on close correlations in three-dimensional (3D) spatial distribution, velocity, and age. We discover four OC groups, with each group’s member OCs being spatially close and exhibiting similar velocities. The age spread of these groups is within 30 Myr, consistent with continuous star formation. The age distribution and spatial extent of the OC group are consistent with the timescale of continuous star formation events and the typical scale of the primordial OC group, indicating the primordial sequential formation of member clusters. To trace the dynamical evolution of these groups, we conduct N-body simulations, which reveal that the groups will disperse over time, eventually evolving into independent OCs. Furthermore, we determine the birthplace of the target OCs and predict regions around the OC group birthplaces where SNe are likely to have occurred, using the correlation between OC ages and their separation from potential SNe sites. We find strong correlations between OC ages and the predicted SNe regions, with notable age gradients outward from these sites. Since pulsars (PSRs) are remnants of SNe, We further trace the orbits of pulsars using the Galactic potential model to investigate their association with these predicted SNe regions.Additionally, 3 PSRs near Group 1 and 26 PSRs near Group 2 are detected, with their birthplaces aligning with the predicted SNe regions. Our results suggest that the member OCs within each group formed from the same molecular cloud through sequential star formation, likely triggered by multiple SNe explosions. These findings support the supernova-triggered star formation process and reinforce the hierarchical star formation model, emphasizing the multi-scale interactions that drive star and cluster formation.
Formation of intermediate-mass black holes in forming star clusters
The formation process of star clusters has not been fully understood yet. Recent numerical simulations of star-cluster formation are reaching the mass of globular clusters (1e6 Msun). However, in hydrodynamics simulation with N-body, the stars have been treated as super particles, representing several stars as one particle. We have developed a new N-body/smoothed-particle hydrodynamics (SPH) code, ASURA+BRIDGE, and have performed N-body/SPH simulations of forming star clusters with almost 1e6 Msun by resolving individual stars. Our code can integrate the orbits of stars without gravitational softening using a direct-tree hybrid N-body code, PeTar (Wang et al. 2020), combined with our N-body/SPH simulation code, ASURA+BRIGE (Fujii et al. 2021). In our simulations, runaway collisions of stars occurred in the forming globular clusters, and the mass of the very massive stars (VMSs) formed via runaway collisions reached a maximum of 1e4 Msun. According to a stellar evolution model, they can collapse to IMBHs with a mass of a few thousand Msun. Our results suggest that some globular clusters may host an IMBH more massive than 1000 Msun. In addition, such VMSs quickly lose their mass via stellar wind and pollute the surrounding gas. In our simulations, so-called second population stars are also formed from the polluted gas.
Simulating intermediate-mass black holes in the first star clusters
Population III (Pop. III) stars are ideal candidates for the formation of intermediate-mass black holes (IMBHs, m = 10^2 – 10^5 Msun) because of their small mass loss and top-heavy initial mass function. Nevertheless, the mass of these IMBHs is usually limited to a few hundred solar masses. However, star cluster dynamics can boost the growth of IMBHs to higher masses. In my talk, I will explore the properties of binary black holes and IMBHs in Pop. III star clusters forming at z>15. To simulate these clusters, I used the N-body code PeTar-bseEmp, a state-of-the-art tool that precisely models star cluster dynamics within an external potential while integrating single and binary stellar evolution. I will show how the mass and density of the simulated clusters, and their subsequent evolution, have a significant impact on the formation channels and features of massive black hole seeds. Finally, I will examine the role of star cluster mergers in enhancing the binary black hole merger rate and driving the growth of central IMBHs.
Seeds to Success: growing heavy black holes in star clusters
Dense stellar clusters provide ideal conditions for the formation of intermediate-mass black holes (IMBHs), i.e. heavy black holes with masses between 100 and 100,000 solar masses. These objects may arise from (i) runaway stellar collisions in light and compact clusters, or (ii) hierarchical binary black hole (BBH) mergers within massive and dense clusters. Assessing the efficiency of both processes remain computationally challenging for N-body / Monte Carlo codes either due to (i) the sheer number of models required to define statistically robust results or (ii) to the huge number of stars composing the cluster. Semi-analytic population synthesis codes offer an efficient and new alternative to exploring IMBH formation in such environments. In this talk, I will present results from simulations of over 10 million BHs in young, globular, and nuclear clusters at various metallicities performed using the semi-analytic code B-POP. I will identify which cluster conditions favor IMBH formation, discuss how formation timescales relate to the IMBH host properties, and examine possible overlaps with IMBH observations at both low and high redshifts. Finally, I will overview potential implications of IMBH formation in our Galaxy and possible gravitational wave (GW) signatures of their production in massive clusters.
Modeling Massive Black Holes in Globular Clusters with the Cluster Monte Carlo Code
The presence of intermediate-mass black holes (IMBHs) in globular clusters (GCs) has been an open question for decades. Their formation scenarios remain a topic of debate, with proposed models including runaway stellar collisions—enhanced by the presence of binaries—hierarchical BH mergers, and the direct collapse of Population III stars, among others. Though direct observations of IMBHs in star clusters have remained elusive, recent detections of fast-moving stars in the core of Omega Centauri (Omega Cen), one of the most massive GCs in the Milky Way, suggest the presence of a massive compact object at its center. Similarly, 47 Tucanae shows tentative signs of hosting an IMBH, further emphasizing the need to accurately model the role of IMBHs in the evolution of star clusters. In this talk, I will give an overview of what we have learned about the formation and dynamical evolution of IMBHs in GCs, focusing on results obtained using the Cluster Monte Carlo code (CMC)—a state-of-the-art N-body code that has shown to accurately reproduce Milky Way-like GCs. I will also present our current work on modeling an IMBH in Omega Cen, including prescriptions of the loss cone physics for both single and binary stars. Finally, I will discuss how the presence of an IMBH in Omega Cen affects the central dynamics and its resulting observational signatures, along with predictions for the rates of tidal disruption events and compact-object inspirals for different IMBH seeds.
Hierarchical Black Hole Mergers in AGN Disks
Recent discoveries by JWST have shed new light on the population of massive black holes (MBHs), challenging existing theoretical models of MBH formation, growth, and feedback. In this context, I investigate hierarchical black hole (BH) mergers within active galactic nuclei (AGNs), which stand out with respect to other dynamical environments for three main reasons: enhanced binary formation due to migration traps, accelerated binary inspiral due to gas hardening and high merger remnant retention due to the deep gravitational potential of the SMBH. I will present the main results of my new semi-analytical model, which allows me to effectively explore the parameter space while capturing all the main physical processes involved. I will show that the mass of the SMBH has a relevant impact on the properties of binary BH mergers, potentially leading to the formation of MBHs (up to 10.000 solar masses). We model the SMBH mass function across cosmic time by combining observational data from the local universe with simulated data from the semi-analytical code CAT. We explore two scenarios for growth of SMBHs: either the SMBH growth happens via Bondi-Hoyle-Lyttleton spherical accretion with a maximum allowed limit at the Eddington rate, or occasional super-Eddington accretion can be triggered by gas-rich galaxy mergers. We construct samples of mock GW observations for the Einstein Telescope, which can be useful to constrain the SMBH mass distribution at high redshift and constrain their seeding mechanism.