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Giacomo Fragione

Publications and source records attributed to Giacomo Fragione.

At least 19 recordsLinked to original sources

Profiling stellar environments of gravitational wave sources

Gravitational waves (GWs) have enabled direct detections of compact binary coalescences (CBCs). However, their poor sky localisation and the typical lack of observable electromagnetic (EM) counterparts make it difficult to confidently identify their hosts, and study the environments that nurture their evolution. In this work, we show that $\textit{detailed}$ information of the host environment (e.g. the mass and steepness of the host potential) can be directly inferred by measuring the kinematic parameters (acceleration and its time-derivatives) of the binary's center of mass using GWs alone, without requiring an EM counterpart. We consider CBCs in various realistic environments such as globular clusters, nuclear star clusters, and active galactic nuclei disks to demonstrate how orbit and environment parameters can be extracted for CBCs detectable by ground- and space-based observatories, including the LIGO detector at A+ sensitivity, Einstein Telescope of the XG network, LISA, and DECIGO, $\textit{on a single-event basis}$. These constraints on host stellar environments promise to shed light on our understanding of how CBCs form, evolve, and merge.

astro-ph.HE

Remnant masses from 1D+ core-collapse supernovae simulations: bimodal neutron star mass distribution and black holes in the low-mass gap

The explosion of core-collapse supernovae (CCSNe) is an extremely challenging problem, and there are still large uncertainties regarding which stars lead to successful explosions that leave behind a neutron star, and which ones will form a black hole instead. In this paper, we simulate 341 progenitors at three different metallicities using spherically symmetric simulations that include neutrino-driven convection via a mixing-length theory. We use these simulations to improve previously derived explosion criteria based on the density and entropy profiles of the pre-supernova progenitor. We also provide numerical fits to calculate the final mass of neutron stars based on either compactness, the location of the Si/Si-O interface, or the Chandrasekhar mass. The neutron star birth mass distribution derived from our 1D+ simulations is bimodal, contrary to what the most popular 1D CCSN simulations have shown so far. We compare the theoretically derived neutron star mass distributions with the observed ones and discuss potential implications for population synthesis studies. We also analyze the black hole mass distribution predicted by our simulations. To be consistent with current models of matter ejection in failed SNe, a large fraction of the envelope must be expelled, leading to small black holes in the low-mass gap. One black hole in this mass region has recently been observed in the GW230529 event by the LIGO-Virgo-KAGRA collaboration. Our results naturally agree with this detection, which the most popular prescriptions for explodability and remnant masses are not able to reproduce. In general, we find that the explosion outcome and mass of the remnant strongly depend on the pre-collapse structure of the progenitor. However, their dependence on the initial mass of the star and the mass of the CO core is highly uncertain and non-linear.

astro-ph.HE

IMBH Progenitors from Stellar Collisions in Dense Star Clusters

Very massive stars (VMSs) formed via a sequence of stellar collisions in dense star clusters have been proposed as the progenitors of massive black hole seeds. VMSs could indeed collapse to form intermediate-mass black holes (IMBHs), which would then grow by accretion to become the supermassive black holes observed at the centers of galaxies and powering high-redshift quasars. Previous studies have investigated how different cluster initial conditions affect the formation of a VMS, including mass segregation, stellar collisions, and binaries, among others. In this study, we investigate the growth of VMSs with a new grid of Cluster Monte Carlo (CMC) star cluster simulations -- the most expansive to date. The simulations span a wide range of initial conditions, varying the number of stars, cluster density, stellar initial mass function (IMF), and primordial binary fraction. We find a gradual shift in the mass of the most massive collision product across the parameter space; in particular, denser clusters born with top-heavy IMFs provide strong collisional regimes that form VMSs with masses easily exceeding 1000 solar masses. Our results are used to derive a fitting formula that can predict the typical mass of a VMS formed as a function of the star cluster properties. Additionally, we study the stochasticity of this process and derive a statistical distribution for the mass of the VMS formed in one of our models, recomputing the model 50 times with different initial random seeds.

astro-ph.GA

Stellar Escape from Globular Clusters. II. Clusters May Eat Their Own Tails

We apply for the first time the Monte Carlo star cluster modeling method to study tidal tail and stellar stream formation from globular clusters, assuming a circular orbit in a smooth Galactic potential. Approximating energetically unbound bodies (potential escapers; PEs) as collisionless enables this fast but spherically symmetric method to capture asymmetric tidal phenomena with unprecedented detail. Beyond reproducing known stream features, including epicyclic overdensities, we show how 'returning tidal tails' may form after the stream fully circumnavigates the Galaxy back to the cluster, enhancing the stream's velocity dispersion. While a realistically clumpy, time-dependent Galactic potential may disrupt such tails, they warrant scrutiny as potentially excellent constraints on the Galactic potential's history and substructure. Re-examining the escape timescale $Δt$ of PEs, we find new behavior related to chaotic scattering in the three-body problem; the $Δt$ distribution features sharp plateaus corresponding to distinct locally smooth patches of the chaotic saddle separating the phase space basins of escape. We study for the first time $Δt$ in an evolving cluster, finding that $Δt\sim(E_{\rm J}^{-0.1},E_{\rm J}^{-0.4})$ for PEs with (low, high) Jacobi energy $E_{\rm J}$, flatter than for a static cluster ($E_{\rm J}^{-2}$). Accounting for cluster mass loss and internal evolution -- and (roughly) for ongoing relaxation among PEs -- lowers the median $Δt$ from ${\sim}10\,$Gyr to ${\lesssim}100\,$Myr. We finally outline future improvements to escape physics in the Monte Carlo method intended to enable both the first large-parameter-space studies of tidal tail/stellar stream formation from full globular cluster simulations and detailed comparisons to stream observations.

astro-ph.GA

Binary black hole mergers and intermediate-mass black holes in dense star clusters with collisional runaways

Intermediate-mass black holes (IMBHs) are believed to be the missing link between the supermassive black holes (BHs) found at the centers of massive galaxies and BHs formed through stellar core collapse. One of the proposed mechanisms for their formation is a collisional runaway process in high-density young star clusters, where an unusually massive object forms through repeated stellar collisions and mergers, eventually collapsing to form an IMBH. This seed IMBH could then grow further through binary mergers with other stellar-mass BHs. Here we investigate the gravitational-wave (GW) signals produced during these later IMBH--BH mergers. We use a state-of-the-art semi-analytic approach to study the stellar dynamics and to characterize the rates and properties of IMBH--BH mergers. We also study the prospects for detection of these mergers by current and future GW observatories, both space-based (LISA) and ground-based (LIGO Voyager, Einstein Telescope, and Cosmic Explorer). We find that most of the merger signals could be detected, with some of them being multi-band sources. Therefore, GWs represent a unique tool to test the collisional runaway scenario and to constrain the population of dynamically assembled IMBHs.

astro-ph.GA

Accelerated binary black holes in globular clusters: forecasts and detectability in the era of space-based gravitational-wave detectors

The motion of the center of mass of a coalescing binary black hole (BBH) in a gravitational potential imprints a line-of-sight acceleration (LOSA) onto the emitted gravitational wave (GW) signal. The acceleration could be sufficiently large in dense stellar environments, such as globular clusters (GCs), to be detectable with next-generation space-based detectors. In this work, we use outputs of the \textsc{cluster monte carlo (cmc)} simulations of dense star clusters to forecast the distribution of detectable LOSAs in DECIGO and LISA eras. We study the effect of cluster properties -- metallicity, virial and galactocentric radii -- on the distribution of detectable accelerations, account for cosmologically-motivated distributions of cluster formation times, masses, and metallicities, and also incorporate the delay time between the formation of BBHs and their merger in our analysis. We find that larger metallicities provide a larger fraction of detectable accelerations by virtue of a greater abundance of relatively lighter BBHs, which allow a higher number of GW cycles in the detectable frequency band. Conversely, smaller metallicities result in fewer detections, most of which come from relatively more massive BBHs with fewer cycles but larger LOSAs. We similarly find correlations between the virial radii of the clusters and the fractions of detectable accelerations. Our work, therefore, provides an important science case for space-based GW detectors in the context of probing GC properties via the detection of LOSAs of merging BBHs.

astro-ph.HE

On the Stochastic Gravitational Wave Background from Binary Black Hole Mergers Dynamically Assembled in Dense Star Clusters

With about a hundred binary black hole (BBH) mergers detected by LIGO-Virgo-KAGRA, and with several hundreds expected in the current O4 run, GWs are revolutionizing our understanding of the universe. Some BBH sources are too faint to be individually detected, but collectively they may give rise to a stochastic GW background (SGWB). In this paper, we calculate the SGWB associated with BBH mergers dynamically assembled in dense star clusters, using state-of-the-art numerical models. We discuss the role of modeling the evolution of the mass distribution of BBH mergers, which has significant implications for model selection and parameter estimation, and could be used to distinguish between different channels of BBH formation. We demonstrate how the birth properties of star clusters affect the amplitude and frequency spectrum of the SGWB, and show that upcoming observation runs of ground-based GW detectors may be sensitive enough to detect it. Even in the case of a non-detection, we find that GW data can be used to constrain the highly uncertain cluster birth properties, which can complement direct observations of young massive clusters and proto-star clusters in the early universe by JWST.

gr-qc

Constraining the Properties of Black Hole Seeds from the Farthest Quasars

Over 60 years after the discovery of the first quasar, more than $275$ such sources are identified in the epoch of reionization at $z>6$. JWST is now exploring higher redshifts ($z\gtrsim 8$) and lower mass ($ \lesssim 10^7 \Msun$) ranges. The discovery of progressively farther quasars is instrumental to constraining the properties of the first population of black holes (BHs), or BH seeds, formed at $z \sim 20-30$. For the first time, we use Bayesian analysis of the most comprehensive catalog of quasars at $z>6$ to constrain the distribution of BH seeds. We show that the mass distribution of BH seeds can be effectively described by combining a power law and a lognormal function tailored to the mass ranges associated with light and heavy seeds, assuming Eddington-limited growth and early seeding time. Our analysis reveals a power-law slope of $-0.70^{+0.46}_{-0.46}$ and a lognormal mean of $4.44^{+0.30}_{-0.30}$. The inferred values of the Eddington ratio, the duty cycle, and the mean radiative efficiency are $0.82^{+0.10}_{-0.10}$, $0.66^{+0.23}_{-0.23}$, and $0.06^{+0.02}_{-0.02}$, respectively. Models that solely incorporate a power law or a lognormal distribution within the specific mass range corresponding to light and heavy seeds are statistically strongly disfavored, unlike models not restricted to this specific range. Our results suggest that including both components is necessary to comprehensively account for the masses of high-redshift quasars, and that both light and heavy seeds formed in the early Universe and grew to form the population of quasars we observe.

astro-ph.GA

Neutron star kicks and implications for their rotation at birth

Neutron stars are born out of core-collapse supernovae, and they are imparted natal kicks at birth as a consequence of asymmetric ejection of matter and possibly neutrinos. Unless the force resulting from the kicks is exerted exactly at their center, it will also cause the neutron star to rotate. In this paper, we discuss the possibility that neutron stars may receive off-center natal kicks at birth, which imprint a natal rotation. In this scenario, the observed pulsar spin and transverse velocity in the Galaxy are expected to correlate. We develop a model of the natal rotation imparted to neutron stars and constrain it by the observed population of pulsars in our Galaxy. When considering a single-kick position parameter, we find that the location of the off-center kick is $R_{\rm kick}=2.03^{+3.74}_{-1.69}$\,km at $90\%$ confidence, and is robust when considering pulsars with different observed periods, transverse velocities, and ages. Nonetheless, the model encounters challenges in effectively fitting the data, particularly at small transverse velocities, prompting the exploration of alternative models that include more complex physics. Our framework could be used as a guide for core-collapse simulations of massive stars.

astro-ph.HE

The dominant mechanism(s) for populating the outskirts of star clusters with neutron star binaries

It has been argued that heavy binaries composed of neutron stars (NSs) and millisecond pulsars (MSPs) can end up in the outskirts of star clusters via an interaction with a massive black hole (BH) binary expelling them from the core. We argue here, however, that this mechanism will rarely account for such observed objects. Only for primary masses $\lesssim$ 100 M$_{\odot}$ and a narrow range of orbital separations should a BH-BH binary be both dynamically hard and produce a sufficiently low recoil velocity to retain the NS binary in the cluster. Hence, BH binaries are in general likely to eject NSs from clusters. We explore several alternative mechanisms that would cause NS/MSP binaries to be observed in the outskirts of their host clusters after a Hubble time. The most likely mechanism is a three-body interaction involving the NS/MSP binary and a normal star. We compare to Monte Carlo simulations of cluster evolution for the globular clusters NGC 6752 and 47 Tuc, and show that the models not only confirm that normal three-body interactions involving all stellar-mass objects are the dominant mechanism for putting NS/MSP binaries into the cluster outskirts, they also reproduce the observed NS/MSP binary radial distributions without needing to invoke the presence of a massive BH binary. Higher central densities and an episode of core-collapse can broaden the radial distributions of NSs/MSPs and NS/MSP binaries due to three-body interactions, making these clusters more likely to host NSs in the cluster outskirts.

astro-ph.GA

On the Tidal Capture of White Dwarfs by Intermediate-mass Black Holes in Dense Stellar Environments

Intermediate-mass black holes (IMBHs) are the missing link between stellar-mass and supermassive black holes, widely believed to reside in at least some dense star clusters, but not yet observed directly. Tidal disruptions of white dwarfs (WDs) are luminous only for black holes less massive than $\sim 10^5\,M_{\odot}$, therefore providing a unique smoking gun that could finally prove the existence of IMBHs beyond any reasonable doubt. Here, we investigate the tidal captures of WDs by IMBHs in dense star clusters, and estimate a typical rate of $\sim 1\,{\rm Myr}^{-1}$ for galactic nuclei and $\sim 0.01\,{\rm Myr}^{-1}$ for globular clusters. Following the capture, the WD inspirals onto the IMBH producing gravitational waves detectable out to $\sim100$ Mpc by LISA for $\sim 10^4\,M_{\odot}$ IMBHs. The subsequent tidal stripping/disruption of the WD can also release bright X-ray and gamma-ray emission with luminosities of at least $\gtrsim10^{40}\,\rm{erg\,s^{-1}}$, detectable by \textit{Chandra}, \textit{Swift}, and upcoming telescopes, such as the \textit{Einstein Probe}.

astro-ph.HE

Growing Black Holes through Successive Mergers in Galactic Nuclei: I. Methods and First Results

We present a novel, few-body computational framework designed to shed light on the likelihood of forming intermediate-mass (IM) and supermassive (SM) black holes (BHs) in nuclear star clusters (NSCs) through successive BH mergers, initiated with a single BH seed. Using observationally motivated NSC profiles, we find that the probability of a ${\sim}100 \, M_\odot$ BH to grow beyond ${\sim}1000 \, M_\odot$ through successive mergers ranges from ${\sim}0.1\%$ in low-density, low-mass clusters to nearly $90\%$ in high-mass, high-density clusters. However, in the most massive NSCs, the growth timescale can be very long ($\gtrsim 1\,$Gyr); vice versa, while growth is least likely in less massive NSCs, it is faster there, requiring as little as ${\sim}0.1\,$Gyr. The increased gravitational focusing in systems with lower velocity dispersions is the primary contributor to this behavior. We find that there is a simple "7-strikes-and-you're-in" rule governing the growth of BHs: our results suggest that if the seed survives 7 to 10 successive mergers without being ejected (primarily through gravitational wave recoil kicks), the growing BH will most likely remain in the cluster and will then undergo runaway, continuous growth all the way to the formation of an SMBH (under the simplifying assumption adopted here of a fixed background NSC). Furthermore, we find that rapid mergers enforce a dynamically-mediated "mass gap" between about ${50-300 \, M_\odot}$ in an NSC.

astro-ph.GA

LISA Constraints on an Intermediate-Mass Black Hole in the Galactic Centre

Galactic nuclei are potential hosts for intermediate-mass black holes (IMBHs), whose gravitational field can affect the motion of stars and compact objects. The absence of observable perturbations in our own Galactic Centre has resulted in a few constraints on the mass and orbit of a putative IMBH. Here, we show that the Laser Interferometer Space Antenna (LISA) can further constrain these parameters if the IMBH forms a binary with a compact remnant (a white dwarf, a neutron star, or a stellar-mass black hole), as the gravitational-wave signal from the binary will exhibit Doppler-shift variations as it orbits around Sgr A$^\star$. We argue that this method is the most effective for IMBHs with masses $10^3\,M_\odot\lesssim M_{\rm IMBH}\lesssim 10^5\,M_\odot$ and distances of $0.1$ mpc to $2$ mpc with respect to the supermassive black hole, a region of the parameter space partially unconstrained by other methods. We show that in this region the Doppler shift is most likely measurable whenever the binary is detected in the LISA band, and it can help constrain the mass and orbit of a putative IMBH in the centre of our Galaxy. We also discuss possible ways for an IMBH to form a binary in the Galactic Centre, showing that gravitational-wave captures of stellar-mass black holes and neutron stars are the most efficient channel.

astro-ph.HE

Roman CCS White Paper: Characterizing the Galactic population of isolated black holes

Although there are estimated to be 100 million isolated black holes (BHs) in the Milky Way, only one has been found so far, resulting in significant uncertainty about their properties. The Galactic Bulge Time Domain Survey provides the only opportunity in the coming decades to grow this catalog by order(s) of magnitude. This can be achieved if 1) Roman's astrometric potential is fully realized in the observation strategy and software pipelines, 2) Roman's observational gaps of the Bulge are minimized, and 3) observations with ground-based facilities are taken of the Bulge to fill in gaps during non-Bulge seasons. A large sample of isolated BHs will enable a broad range of astrophysical questions to be answered, such as massive stellar evolution, origin of gravitational wave sources, supernova physics, and the growth of supermassive BHs, maximizing Roman's scientific return.

astro-ph.IM

On the Likely Dynamical Origin of GW191109 and of Binary Black Hole Mergers with Negative Effective Spin

With the growing number of binary black hole (BBH) mergers detected by LIGO/Virgo/KAGRA, several systems have become difficult to explain via isolated binary evolution, having components in the pair-instability mass gap, high orbital eccentricities, and/or spin-orbit misalignment. Here, we focus on GW191109\_010717, a BBH merger with component masses of $65^{+11}_{-11}$ and $47^{+15}_{-13}$ $\rm M_{\odot}$, and effective spin $-0.29^{+0.42}_{-0.31}$, which could imply a spin-orbit misalignment of more than $π/2$ radians for at least one of its components. Besides its component masses being in the pair-instability mass gap, we show that isolated binary evolution is unlikely to reproduce the proposed spin-orbit misalignment of GW191109 with high confidence. On the other hand, we demonstrate that BBHs dynamically assembled in dense star clusters would naturally reproduce the spin-orbit misalignment and the masses of GW191109, and the rates of GW191109-like events, if at least one of the components were to be a second-generation BH. Finally, we generalize our results to all the events with a measured negative effective spin, arguing that GW200225 also has a likely dynamical origin.

astro-ph.HE

Demographics of Hierarchical Black Hole Mergers in Dense Star Clusters

With about a hundred mergers of binary black holes (BBHs) detected via gravitational waves by the LIGO-Virgo-KAGRA (LVK) Collaboration, our understanding of the darkest objects in the Universe has taken unparalleled steps forward. While most of the events are expected to consist of BHs directly formed from the collapse of massive stars, some may contain the remnants of previous BBH mergers. In the most massive globular clusters and in nuclear star clusters, successive mergers can produce second- (2G) or higher-generation BHs, and even form intermediate-mass BHs. Overall, we predict that up to $\sim 10\%$, $\sim 1\%$ or $\sim 0.1\%$ of the BBH mergers have one component being a 2G, 3G, or 4G BH, respectively. Assuming that $\sim 500$ BBH mergers will be detected in O4 by LVK, this means that $\sim 50$, $\sim 5$, or $\sim 0.5$ events, respectively, will involve a 2G, 3G, or 4G BH, if most sources are produced dynamically in dense star clusters. With their distinctive signatures of higher masses and spins, such hierarchical mergers offer an unprecedented opportunity to learn about the BH populations in the densest stellar systems and to shed light on the elusive intermediate-mass BHs that may form therein.

astro-ph.GA

Partial Tidal Disruption of Main-Sequence Stars by Intermediate-Mass Black Holes

We study close encounters of a $1\,M_{\odot}$ middle-age main-sequence star (modeled using MESA) with massive black holes through hydrodynamic simulations, and explore in particular the dependence of the outcomes on the black hole mass. We consider here black holes in the intermediate-mass range, $M_{\rm BH}= 100-10^4\,M_{\odot}$. Possible outcomes vary from a small tidal perturbation for weak encounters all the way to partial or full disruption for stronger encounters. We find that stronger encounters lead to increased mass loss at the first pericenter passage, in many cases ejecting the partially disrupted star on an unbound orbit. For encounters that initially produce a bound system, with only partial stripping of the star, the fraction of mass stripped from the star increases with each subsequent pericenter passage and a stellar remnant of finite mass is ultimately ejected in all cases. The critical penetration depth that separates bound and unbound remnants has a dependence on the black hole mass when $M_{\rm BH} \lesssim 10^3\,M_{\odot}$. We also find that the number of successive close passages before ejection decreases as we go from the stellar-mass black hole to the intermediate-mass black hole regime. For instance, after an initial encounter right at the classical tidal disruption limit, a $1\,M_{\odot}$ star undergoes 16 (5) pericenter passages before ejection from a $10\,M_{\odot}$ ($100\,M_{\odot}$) black hole. Observations of periodic flares from these repeated close passages could in principle indicate signatures of a partial tidal disruption event.

astro-ph.HE

Globular cluster formation histories, masses and radii inferred from gravitational waves

Globular clusters (GCs) are found in all types of galaxies and harbor some of the most extreme stellar systems, including black holes that may dynamically assemble into merging binaries (BBHs). Uncertain GC properties, including when they formed, their initial masses and sizes, affect their production rate of BBH mergers. Using the gravitational-wave catalog GWTC-3, we measure that dynamically-assembled BBHs -- those that are consistent with isotropic spin directions -- make up ${61^{+29}_{-44}\%}$ of the total merger rate, with a local merger rate of ${10.9^{+16.8}_{-9.3}}$ Gpc$^{-3}$ yr$^{-1}$ rising to ${58.9^{+149.4}_{-46.0}}$ Gpc$^{-3}$ yr$^{-1}$ at $z = 1$. We assume this inferred rate describes the contribution from GCs and compare it against the Cluster Monte Carlo (CMC) simulation catalog to directly fit for the GC initial mass function, virial radius distribution, and formation history. We find that GC initial masses are consistent with a Schechter function with slope ${β_m = -1.9^{+0.8}_{-0.8}}$. Assuming a mass function slope of $β_m = -2$ and a mass range between $10^4$--$10^8\,M_\odot$, we infer a GC formation rate at $z = 2$ of ${5.0^{+9.4}_{-4.0}}$ Gpc$^{-3}$ yr$^{-1}$, or ${2.1^{+3.9}_{-1.7}}\times 10^6\,M_\odot$ Gpc$^{-3}$ yr$^{-1}$ in terms of mass density. We find that the GC formation rate probably rises more steeply than the global star formation rate between $z = 0$ and $z = 3$ ({82\%} credibility) and implies a local number density that is ${f_\mathrm{ev} = 22.6^{+29.9}_{-16.2}}$ times higher than the observed density of survived GCs. This is consistent with expectations for cluster evaporation, but may suggest that other environments contribute to the rate of BBH mergers with significantly tilted spins.

astro-ph.GA