arXiv subjects

Giacomo Fragione

Publications and source records attributed to Giacomo Fragione.

At least 37 records · Page 2Linked to original sources

arXiv2023

A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy

The majority of long duration ($>2$ s) gamma-ray bursts (GRBs) are believed to arise from the collapse of massive stars \cite{Hjorth+03}, with a small proportion created from the merger of compact objects. Most of these systems are likely formed via standard stellar evolution pathways. However, it has long been thought that a fraction of GRBs may instead be an outcome of dynamical interactions in dense environments, channels which could also contribute significantly to the samples of compact object mergers detected as gravitational wave sources. Here we report the case of GRB 191019A, a long GRB (T_90 = 64.4 +/- 4.5 s) which we pinpoint close (<100 pc projected) to the nucleus of an ancient (>1~Gyr old) host galaxy at z=0.248. The lack of evidence for star formation and deep limits on any supernova emission make a massive star origin difficult to reconcile with observations, while the timescales of the emission rule out a direct interaction with the supermassive black hole in the nucleus of the galaxy, We suggest that the most likely route for progenitor formation is via dynamical interactions in the dense nucleus of the host, consistent with the centres of such galaxies exhibiting interaction rates up to two orders of magnitude larger than typical field galaxies. The burst properties could naturally be explained via compact object mergers involving white dwarfs (WD), neutron stars (NS) or black holes (BH). These may form dynamically in dense stellar clusters, or originate in a gaseous disc around the supermassive black hole. Future electromagnetic and gravitational-wave observations in tandem thus offer a route to probe the dynamical fraction and the details of dynamical interactions in galactic nuclei and other high density stellar systems.

astro-ph.HE↗

arXiv2023

Stellar Escape from Globular Clusters. I. Escape Mechanisms and Properties at Ejection

The theory of stellar escape from globular clusters (GCs) dates back nearly a century, especially the gradual evaporation of GCs via two-body relaxation coupled with external tides. More violent ejection can also occur via strong gravitational scattering, supernovae, gravitational wave-driven mergers, tidal disruption events, and physical collisions, but comprehensive study of the many escape mechanisms has been limited. Recent exquisite kinematic data from the Gaia space telescope has revealed numerous stellar streams in the Milky Way (MW) and traced the origin of many to specific MWGCs, highlighting the need for further examination of stellar escape from these clusters. In this study, the first of a series, we lay the groundwork for detailed follow-up comparisons between Cluster Monte Carlo (CMC) GC models and the latest Gaia data on the outskirts of MWGCs, their tidal tails, and associated streams. We thoroughly review escape mechanisms from GCs and examine their relative contributions to the escape rate, ejection velocities, and escaper demographics. We show for the first time that three-body binary formation may dominate high-speed ejection from typical MWGCs, potentially explaining some of the hypervelocity stars in the MW. Due to their mass, black holes strongly catalyze this process, and their loss at the onset of observable core collapse, characterized by a steep central brightness profile, dramatically curtails three-body binary formation, despite the increased post-collapse density. We also demonstrate that even when born from a thermal eccentricity distribution, escaping binaries have significantly nonthermal eccentricities consistent with the roughly uniform distribution observed in the Galactic field.

astro-ph.GA↗

arXiv2023

Constraining the cosmic merger history of intermediate-mass black holes with gravitational wave detectors

Intermediate-mass black holes (IMBHs) have not been detected beyond any reasonable doubt through either dynamical or accretion signatures. Gravitational waves (GWs) represent an unparalleled opportunity to survey the sky and detect mergers of IMBHs, making it possible for the first time to constrain their formation, growth, and merger history across cosmic time. While the current network LIGO-Virgo-KAGRA is significantly limited in detecting mergers of IMBH binaries, the next generation of ground-based observatories and space-based missions promise to shed light on the IMBH population through the detection of several events per year. Here, we asses this possibility by determining the optimal network of next-generation of GW observatories to reconstruct the IMBH merger history across cosmic time. We show that Voyager, the Einstein Telescope, and Cosmic Explorer will be able to constrain the distribution of the primary masses of merging IMBHs up to $\sim 10^3\ M_\odot$ and with mass ratio $\gtrsim 0.1$, while LISA will complementary do so at higher mass and smaller mass ratios. Therefore, a network of next-generation ground-based and space-based observatories will potentially reconstruct the merger history of IMBHs. Moreover, IMBHs with masses $\lesssim 5\times 10^3\ M_\odot$ could be observed in multiband up to a redshift of $z\approx 4$, ushering in a new of era GW astronomy.

astro-ph.HE↗

arXiv2022

Millisecond Pulsars in Dense Star Clusters: Evolution, Scaling Relations, and the Galactic-Center Gamma-ray Excess

The number of millisecond pulsars (MSPs) observed in Milky Way globular clusters has increased explosively in recent years, but the underlying population is still uncertain due to observational biases. We use state-of-the-art $N$-body simulations to study the evolution of MSP populations in dense star clusters. These cluster models span a wide range in initial conditions, including different initial masses, metallicities, and virial radii, which nearly cover the full range of properties exhibited by the population of globular clusters in the Milky Way. We demonstrate how different initial cluster properties affect the number of MSPs, for which we provide scaling relations as a function of cluster age and mass. As an application, we use our formulae to estimate the number of MSPs delivered to the Galactic Center from inspiralling globular clusters to probe the origin of the Galactic-Center gamma-ray excess detected by Fermi. We predict about $400$ MSPs in the Galactic Center from disrupted globular clusters, which can potentially explain most of the observed gamma-ray excess.

astro-ph.HE↗

arXiv2022

Intermediate-mass Black Holes on the Run from Young Star Clusters

The existence of black holes (BHs) with masses in the range between stellar remnants and supermassive BHs has only recently become unambiguously established. GW190521, a gravitational wave signal detected by the LIGO/Virgo Collaboration, provides the first direct evidence for the existence of such intermediate-mass BHs (IMBHs). This event sparked and continues to fuel discussion on the possible formation channels for such massive BHs. As the detection revealed, IMBHs can form via binary mergers of BHs in the "upper mass gap" ($\approx40 -120\,M_{\odot}$). Alternatively, IMBHs may form via the collapse of a very massive star formed through stellar collisions and mergers in dense star clusters. In this study, we explore the formation of IMBHs with masses between $120$ and $500\,M_{\odot}$ in young, massive star clusters using state-of-the-art Cluster Monte Carlo ($\texttt{CMC}$) models. We examine the evolution of IMBHs throughout their dynamical lifetimes, ending with their ejection from the parent cluster due to gravitational radiation recoil from BH mergers, or dynamical recoil kicks from few-body scattering encounters. We find that $ \textit{all}$ of the IMBHs in our models are ejected from the host cluster within the first $\sim 500$ Myr, indicating a low retention probability of IMBHs in this mass range for globular clusters today. We estimate the peak IMBH merger rate to be $\mathcal{R} \approx 2 \, \rm{Gpc}^{-3}\,\rm{yr}^{-1}$ at redshift $z \approx 2$.

astro-ph.HE↗

arXiv2022

Mergers of supermassive and intermediate-mass black holes in galactic nuclei from disruptions of star clusters

Gravitational waves (GWs) offer an unprecedented opportunity to survey the sky and detect mergers of compact objects. While intermediate-mass black holes (IMBHs) have not been detected beyond any reasonable doubt with either dynamical or accretion signatures, the GW landscape appears very promising. Mergers of an IMBH with a supermassive black hole (SMBH) will be primary sources for the planned space-based mission LISA and could be observed up to the distant Universe. SMBH-IMBH binaries can be formed as a result of the migration and merger of stellar clusters at the center of galaxies, where an SMBH lurks. We build for the first time a semi-analytical framework to model this scenario, and find that the the comoving merger rate of SMBH-IMBH binaries is $\sim 10^{-4}$ Gpc$^{-3}$ yr$^{-1}$ in the local Universe for a unity IMBH occupation fraction, scales linearly with it, and has a peak at $z\approx 0.5$-$2$. Our model predicts $\sim 0.1$ event yr$^{-1}$ within redshift $z\approx 3.5$ if $10\%$ of the inspiralled star clusters hosted an IMBH, while $\sim 1$ events yr$^{-1}$ for a unity occupation fraction. More than $90\%$ of these systems will be detectable with LISA with a signal-to-noise ratio larger than $10$, promising to potentially find a family of IMBHs.

astro-ph.HE↗

arXiv2022

On the Mass Ratio Distribution of Black Hole Mergers in Triple Systems

Observations have shown that the majority of massive stars, progenitors of black holes (BHs), have on average more than one stellar companion. In triple systems, wide inner binaries can be driven to a merger by the third body due to long-term secular interactions, most notably by the eccentric Lidov-Kozai effect. In this study, we explore the properties of BH mergers in triple systems and compare their population properties to those of binaries produced in isolation and assembled in dense star clusters. Using the same stellar physics and identical assumptions for the initial populations of binaries and triples, we show that stellar triples yield a significantly flatter mass ratio distribution from $q=1$ down to $q\sim0.3$ than either binary stars or dense stellar clusters, similar to the population properties inferred from the most recent catalog of gravitational-wave events, though we do not claim that all the observed events can be accounted for with triples. While hierarchical mergers in clusters can also produce asymmetric mass ratios, the unique spins of such mergers can be used to distinguished them from those produced from stellar triples. All three channels occupy distinct regions in total mass-mass ratio space, which may allow them to be disentangled as more BH mergers are detected by LIGO, Virgo, and KAGRA.

astro-ph.SR↗

arXiv2022

Implication of spin constraints by the Event Horizon Telescope on stellar orbits in the Galactic Center

The center of the Milky Way hosts the closest supermassive black hole, SgrA$^*$. Decades of near-infrared observations of our Galactic Center have shown the presence of a small population of stars (the so called S-star cluster) orbiting SgrA$^*$, which were recently reported to be arranged in two orthogonal disks. In this case, the timescale for Lense-Thirring precession of S-stars should be longer than their age, implying a low spin for SgrA$^*$. In contrast, the recent results by the Event Horizon Telescope favor a highly-spinning SgrA$^*$, which seem to suggest that the S-stars could not be arranged in disks. Alternatively, the spin of SgrA$^*$ must be small, suggesting that the models for its observed image are incomplete.

astro-ph.GA↗

arXiv2022

Merger rates of intermediate-mass black hole binaries in nuclear star clusters

Repeated mergers of stellar-mass black holes (BHs) in dense star clusters can produce intermediate-mass black holes (IMBHs). In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products, in spite of the significant recoil kicks due to anisotropic emission of gravitational radiation. These events can be detected in gravitational waves (GWs), which represent an unprecedented opportunity to reveal IMBHs. In this paper, we analyze the statistical results of a wide range of numerical simulations, which encompass different cluster metallicities, initial BH seed masses, and initial BH spins, and we compute the merger rate of IMBH binaries. We find that merger rates are in the range $0.01$-$10$\,Gpc$^{-3}$\,yr$^{-1}$ depending on IMBH masses. We also compute the number of multi-band detections in ground-based and space-based observatories. Our model predicts that a few merger events per year should be detectable with LISA, DECIGO, ET, and LIGO for IMBHs with masses $\lesssim 1000\msun$, and a few tens of merger events per year with DECIGO, ET, and LIGO only.

astro-ph.HE↗

arXiv2022

Compact Object Modeling in the Globular Cluster 47 Tucanae

The globular cluster 47~Tucanae (47~Tuc) is one of the most massive star clusters in the Milky Way and is exceptionally rich in exotic stellar populations. For several decades it has been a favorite target of observers, and yet it is computationally very challenging to model because of its large number of stars ($N\gtrsim 10^6$) and high density. Here we present detailed and self-consistent 47~Tuc models computed with the \texttt{Cluster Monte Carlo} code (\texttt{CMC}). The models include all relevant dynamical interactions coupled to stellar and binary evolution, and reproduce various observations, including the surface brightness and velocity dispersion profiles, pulsar accelerations, and numbers of compact objects. We show that the present properties of 47~Tuc are best reproduced by adopting an initial stellar mass function that is both bottom-heavy and top-light relative to standard assumptions \citep[as in, e.g.,][]{Kroupa2001}, and an initial Elson profile \citep{Elson1987} that is overfilling the cluster's tidal radius. We include new prescriptions in \texttt{CMC} for the formation of binaries through giant star collisions and tidal captures, and we show that these mechanisms play a crucial role in the formation of neutron star binaries and millisecond pulsars in 47~Tuc; our best-fit model contains $\sim 50$ millisecond pulsars, $70\%$ of which are formed through giant collisions and tidal captures. Our models also suggest that 47~Tuc presently contains up to $\sim 200$ stellar-mass black holes, $\sim 5$ binary black holes, $\sim 15$ low-mass X-ray binaries, and $\sim 300$ cataclysmic variables.

astro-ph.HE↗

arXiv2022

Hunting intermediate-mass black holes with LISA binary radial velocity measurements

Despite their potential role as massive seeds for quasars, in dwarf galaxy feedback, and in tidal disruption events, the observational evidence for intermediate-mass black holes (IMBHs) is scarce. LISA may observe stellar-mass black hole binaries orbiting Galactic IMBHs, and reveal the presence of the IMBH by measuring the Doppler shift in the gravitational waveform induced by the binary's radial velocity. We estimate the number of detectable Doppler shift events from the Milky Way globular clusters (assuming they host IMBHs) and we find that it decreases with the IMBH mass. A few Galactic globular clusters (including M22 and $ω$ Centauri) may produce at least one event detectable by LISA. Even in more pessimistic scenarios, one could still expect $\sim$ 1 event overall in the Milky Way. We also estimate the number of Doppler shift events for IMBHs wandering in the Milky Way as a result of the disruption of their parent clusters. If there is at least one binary black hole orbiting around each wandering IMBH, LISA may detect up to a few tens of Doppler shift events from this elusive IMBH population. Under more pessimistic assumptions, LISA may still detect $\sim 1$ wandering IMBH that would hardly be observable otherwise.

astro-ph.HE↗

arXiv2022

Intermediate-mass black holes born via repeated mergers are unlikely thermodynamically stable

Black hole (BH) thermodynamics is one of the most fascinating aspects of BH physics. While its direct proof is challenging to observe for astrophysical BHs, gravitational waves (GWs) have recently offered a new indirect powerful method to probe that it is operating in the Universe. In this paper, we discuss the thermodynamic stability of IMBHs that are born through repeated mergers of stellar-mass BHs via GW emission in dense star clusters. We show that $\lesssim 20\%$ of the IMBHs born via repeated mergers are thermodynamically unstable for a non-charged BH, while $\lesssim 40\%$ are thermodynamically unstable for a moderately-charged BH. Only in the case of unlikely high charges, $\gtrsim 60\%$ of the IMBHs are thermodynamically stable. With the present and upcoming GW missions, such as LIGO/Virgo/KAGRA, LISA, DECIGO, and ET that promise to detect tens, or even hundreds, of IMBH mergers via GW emission in the next decade, IMBH thermodynamics may soon be tested.

astro-ph.HE↗

arXiv2022

Gravitational Microlensing Rates in Milky Way Globular Clusters

Many recent observational and theoretical studies suggest that globular clusters (GCs) host compact object populations large enough to play dominant roles in their overall dynamical evolution. Yet direct detection, particularly of black holes and neutron stars, remains rare and limited to special cases, such as when these objects reside in close binaries with bright companions. Here we examine the potential of microlensing detections to further constrain these dark populations. Based on state-of-the-art GC models from the CMC Cluster Catalog, we estimate the microlensing event rates for black holes, neutron stars, white dwarfs, and, for comparison, also for M dwarfs in Milky Way GCs, as well as the effects of different initial conditions on these rates. Among compact objects, we find that white dwarfs dominate the microlensing rates, simply because they largely dominate by numbers. We show that microlensing detections are in general more likely in GCs with higher initial densities, especially in clusters that undergo core collapse. We also estimate microlensing rates in the specific cases of M22 and 47 Tuc using our best-fitting models for these GCs. Because their positions on the sky lie near the rich stellar backgrounds of the Galactic bulge and the Small Magellanic Cloud, respectively, these clusters are among the Galactic GCs best-suited for dedicated microlensing surveys. The upcoming 10-year survey with the Rubin Observatory may be ideal for detecting lensing events in GCs.

astro-ph.GA↗

arXiv2022

Searching for Ultra-light Bosons and Constraining Black Hole Spin Distributions with Stellar Tidal Disruption Events

Stars that pass close to the supermassive black holes located in the center of galaxies can be violently disrupted by tidal forces, leading to flares that are observed as bright transient events in sky surveys. The rate for these events to occur depends on the black hole spins, which in turn can be affected by ultra-light bosons due to superradiance. We perform a detailed analysis of these effects and show that searches for stellar tidal disruptions have a significant potential to uncover the existence of ultra-light bosons. In particular, we find that upcoming stellar tidal disruption rate measurements by the Vera Rubin Observatory's Legacy Survey of Space and Time can be used to either discover or rule out bosons with masses ranging from $10^{-20}$ to $10^{-18}$ eV. Our analysis also indicates that these measurements may be used to constrain a variety of supermassive black hole spin distributions and determine if close-to maximal spins are preferred.

hep-ph↗

arXiv2022

Repeated mergers, mass-gap black holes, and formation of intermediate-mass black holes in nuclear star clusters

Current theoretical models predict a mass gap with a dearth of stellar black holes (BHs) between roughly $50\,M_\odot$ and $100\,M_\odot$, while, above the range accessible through massive star evolution, intermediate-mass BHs (IMBHs) still remain elusive. Repeated mergers of binary BHs, detectable via gravitational wave emission with the current LIGO/Virgo/Kagra interferometers and future detectors such as LISA or the Einstein Telescope, can form both mass-gap BHs and IMBHs. Here we explore the possibility that mass-gap BHs and IMBHs are born as a result of successive BH mergers in dense star clusters. In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products after they receive significant recoil kicks due to anisotropic emission of gravitational radiation. We show that a massive stellar BH seed can easily grow to $\sim 10^3 - 10^4\,M_\odot$ as a result of repeated mergers with other smaller BHs. We find that lowering the cluster metallicity leads to larger final BH masses. We also show that the growing BH spin tends to decrease in magnitude with the number of mergers, so that a negative correlation exists between final mass and spin of the resulting IMBHs. Assumptions about the birth spins of stellar BHs affect our results significantly, with low birth spins leading to the production of a larger population of massive BHs.

astro-ph.GA↗

arXiv2021

Black hole-neutron star mergers are unlikely multi-messenger sources

The promise by the LIGO/Virgo/Kagra (LVK) collaboration to detect black hole--neutron star (BH--NS) mergers via gravitational wave (GW) emission has recently been fulfilled with the detection of GW200105 and GW200115. Mergers of BH--NS binaries are particularly exciting for their multi-messenger potential, since the GW detection can be followed by an electromagnetic (EM) counterpart (kilonova, gamma-ray burst, afterglow) that can reveal important information on the equation of state (EOS) of NSs and the nature of the BH spin. We carry out a statistical study of the binary stars that evolve to form a BH--NS binary and compute the rate of merger events that can be followed by an EM counterpart. We find that $\gtrsim 50\%$ of the mergers can lead to an EM counterpart only in the case BHs are born highly spinning ($χ_{\rm BH}\gtrsim 0.7$), while this fraction does not exceed about $30\%$ for stiff NS EOSs and a few percent for soft NS EOSs for low-spinning BHs ($χ_{\rm BH}\lesssim 0.2$), suggesting that a high rate of EM counterparts of BH--NS would provide support for high natal BH spins. However, the possibilities that BHs are born with near-maximal spins and that NS internal structure is described by a stiff EOS are disfavored by current LVK constraints. Considering that these values only represent an upper limit to observe an EM counterpart due to current observational limitations, as in brightness sensitivity and sky localization, BH--NS mergers are unlikely multi-messenger sources.

astro-ph.HE↗

arXiv2021

Modeling Dense Star Clusters in the Milky Way and Beyond with the Cluster Monte Carlo Code

We describe the public release of the Cluster Monte Carlo Code (CMC) a parallel, star-by-star $N$-body code for modeling dense star clusters. CMC treats collisional stellar dynamics using Hénon's method, where the cumulative effect of many two-body encounters is statistically reproduced as a single effective encounter between nearest-neighbor particles on a relaxation timescale. The star-by-star approach allows for the inclusion of additional physics, including strong gravitational three- and four-body encounters, two-body tidal and gravitational-wave captures, mass loss in arbitrary galactic tidal fields, and stellar evolution for both single and binary stars. The public release of CMC is pinned directly to the COSMIC population synthesis code, allowing dynamical star cluster simulations and population synthesis studies to be performed using identical assumptions about the stellar physics and initial conditions. As a demonstration, we present two examples of star cluster modeling: first, we perform the largest ($N = 10^8$) star-by-star $N$-body simulation of a Plummer sphere evolving to core collapse, reproducing the expected self-similar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution (such as GW190521, a recently reported merger with component masses in the pulsational pair-instability mass gap).

astro-ph.GA↗

arXiv2021

Impact of natal kicks on merger rates and spin-orbit misalignments of black hole -- neutron star mergers

The long wait for the detection of merging black hole -- neutron star (BH--NS) binaries is finally over with the announcement by the LIGO/Virgo/Kagra collaboration of GW200105 and GW200115. Remarkably, the primary of GW200115 has a negative spin projection onto the orbital angular momentum, with about $90\%$ probability. Merging BH--NS binaries are expected to form mainly through the evolution of massive binary stars in the field, since their dynamical formation in dense star clusters is strongly suppressed by mass segregation. In this paper, we carry out a systematic statistical study of the binary stars that evolve to form a BH--NS binary, considering different metallicities and taking into account the uncertainties on the natal kick distributions for BHs and NSs and on the common envelope phase of binary evolution. Under the assumption that the initial stellar spins are aligned with the binary angular momentum, we show that both large natal kicks ($\gtrsim 150$ km s$^{-1}$) and high efficiencies for common envelope ejection are required to simultaneously explain the inferred high merger rates and the large spin-orbit misalignment of GW200115.

astro-ph.HE↗