arXiv subjects

Giacomo Fragione

Publications and source records attributed to Giacomo Fragione.

At least 73 records · Page 4Linked to original sources

arXiv2020

GW190412 as a Third-Generation Black Hole Merger from a Super Star Cluster

We explore the possibility that GW190412, a binary black hole merger with a non-equal-mass ratio and significantly spinning primary, was formed through repeated black hole mergers in a dense super star cluster. Using a combination of semi-analytic prescriptions for the remnant spin and recoil kick of black hole mergers, we show that the mass ratio and spin of GW190412 are consistent with a binary black hole whose primary component has undergone two successive mergers from a population of $\sim 10M_{\odot}$ black holes in a high-metallicity environment. We then explore the production of GW190412-like analogs in the CMC Cluster Catalog, a grid of 148 $N$-body star cluster models, as well as a new model, behemoth, with nearly $10^7$ particles and initial conditions taken from a cosmological MHD simulation of galaxy formation. We show that the production of binaries with GW190412-like masses and spins is dominated by massive super star clusters with high metallicities and large central escape speeds. While many are observed in the local universe, our results suggest that a careful treatment of these massive clusters, many of which may have been disrupted before the present day, is necessary to characterize the production of unique gravitational-wave events produced through dynamics.

astro-ph.HE↗

arXiv2020

Gravitational-wave captures by intermediate-mass black holes in galactic nuclei

Intermediate-mass black holes (IMBHs) have not been detected beyond any reasonable doubt, despite their potential role as massive seeds for quasars and sources of tidal disruption events, ultra-luminous X-ray sources, dwarf galaxy feedback, and hypervelocity stars. Gravitational wave (GW) observations can help to find and confirm the existence of IMBHs. Current and upcoming detectors, such as LIGO, Virgo, KAGRA, LISA, ET, and DECIGO promise to identify the full range from stellar-mass to supermassive black holes (SMBHs). In this paper, we address the question of whether IMBHs can produce GWs in galactic nuclei. We consider the possibility that stellar black holes (SBHs) form bound systems and later coalesce with an IMBH through gravitational captures in the dense nucleus. We show that this mechanism is efficient for IMBH masses in the range $\sim 3\times 10^3$ M$_\odot$--$2\times 10^4$ M$_\odot$. We find that the typical distributions of peak frequencies and merger timescales depend mainly on the IMBH mass. In particular, the typical peak frequency is about $0.2\,$Hz, $0.1\,$Hz, $0.09\,$Hz, and $0.05\,$Hz for $M_{\rm IMBH}=5\times 10^3$ M$_\odot$, $8\times 10^3$ M$_\odot$, $1\times 10^4$ M$_\odot$, and $2\times 10^4$ M$_\odot$, respectively. Our results show that, at design sensitivity, both DECIGO and ET should be able to detect these IMBH--SBH mergers. Furthermore, most of the mergers will appear eccentric ($e \gtrsim 0.1$), providing an indication of their dynamical origin.

astro-ph.GA↗

arXiv2020

Merging Black Holes in the Low-mass and High-mass Gaps from 2+2 Quadruple Systems

The origin of the black hole (BH) binary mergers observed by LIGO-Virgo is still uncertain, as are the boundaries of the stellar BH mass function. Stellar evolution models predict a dearth of BHs both at masses $\gtrsim 50$ M$_\odot$ and $\lesssim 5$ M$_\odot$, thus leaving low- and high-mass gaps in the BH mass function. A natural way to form BHs of these masses is through mergers of neutron stars (NSs; for the low-mass gap) or lower-mass BHs (for the high-mass gap); the low- or high-mass-gap BH produced as a merger product can then be detected by LIGO-Virgo if it merges again with a new companion. We show that the evolution of a 2+2 quadruple system can naturally lead to BH mergers with component masses in the low- or high-mass gaps. In our scenario, the BH in the mass gap originates from the merger of two NSs, or two BHs, in one of the two binaries and the merger product is imparted a recoil kick (from anisotropic gravitational wave emission), which triggers its interaction with the other binary component of the quadruple system. The outcome of this three-body interaction is usually a new eccentric compact binary containing the BH in the mass gap, which can then merge again. The merger rate is $\sim 10^{-7} - 10^{-2}$ Gpc$^{-3}$ yr$^{-1}$ and $\sim 10^{-3} - 10^{-2}$ Gpc$^{-3}$ yr$^{-1}$ for BHs in the low-mass and high-mass gap, respectively. As the sensitivity of gravitational wave detectors improves, tighter constraints will soon be placed on the stellar BH mass function.

astro-ph.GA↗

arXiv2020

Electromagnetic transients and gravitational waves from white dwarf disruptions by stellar black holes in triple systems

Mergers of binaries comprised of compact objects can give rise to explosive transient events, heralding the birth of exotic objects which cannot be formed through single star evolution. Using a large number of direct N-body simulations, we explore the possibility that a white dwarf (WD) is dynamically driven to tidal disruption by a stellar-mass black hole (BH) as a consequence of the joint effects of gravitational wave (GW) emission and Lidov-Kozai oscillations imposed by the tidal field of a outer tertiary companion orbiting the inner BH-WD binary. We explore the sensitivity of our results to the distributions of natal kick velocities imparted to the BH and WD upon formation, adiabatic mass loss, semi-major axes and eccentricities of the triples, and stellar mass ratios. We find rates of WD-TDEs in the range $1.2\times 10^{-3}-1.4$ Gpc$^{-3}$ yr$^{-1}$ for $z\leq 0.1$, rarer than stellar TDEs in triples by a factor of $\sim 3$--$30$. The uncertainty in the TDE rates may be greatly reduced in the future using gravitational wave (GW) observations of Galactic binaries and triples with LISA. WD-TDEs may give rise to high energy X-ray or gamma-ray transients of duration similar to long gamma-ray bursts but lacking the signatures of a core-collapse supernova, while being accompanied by a supernova-like optical transient which lasts for only days. WD--BH and WD--NS binaries will also emit GWs in the LISA band before the TDE. The discovery and identification of triple-induced WD-TDE events by future time domain surveys and/or GWs could enable the study of the demographics of BHs in nearby galaxies.

astro-ph.HE↗

arXiv2020

Modeling Dense Star Clusters in the Milky Way and Beyond with the $\texttt{CMC}$ Cluster Catalog

We present a set of 148 independent $N$-body simulations of globular clusters (GCs) computed using the code $\texttt{CMC}$ ($\texttt{Cluster Monte Carlo}$). At an age of $\sim10-13\,$Gyr, the resulting models cover nearly the full range of cluster properties exhibited by the Milky Way GCs, including total mass, core and half-light radii, metallicity, and galactocentric distance. We use our models to investigate the role that stellar-mass black holes play in the process of core collapse. Furthermore, we study how dynamical interactions affect the formation and evolution of several important types of sources in GCs, including low-mass X-ray binaries, millisecond pulsars, blue stragglers, cataclysmic variables, Type Ia supernovae, calcium-rich transients, and merging compact binaries. While our focus here is on old, low-metallicity GCs, our $\texttt{CMC}$ simulations follow the evolution of clusters over a Hubble time, and they include a wide range of metallicities (up to solar), so that our results can also be used to study younger and higher-metallicity star clusters.

astro-ph.HE↗

arXiv2020

A new method to constrain the origins of dark matter-free galaxies and their unusual globular clusters

We present a novel method to constrain the past collisional evolution of observed globular cluster (GC) systems, in particular their mass functions. We apply our method to a pair of galaxies hypothesized to have recently undergone an episode of violent relaxation due to a strong galaxy-galaxy interaction, namely NGC 1052-DF2 and NGC 1052-DF4. We begin by exploring the observational evidence for a collisional origin for these two recently discovered ultra-diffuse galaxies observed in the NGC 1052 group, posited in the literature to be dark matter (DM)-free. We compute the timescales for infall to the central nucleus due to dynamical friction (DF) for the GCs in these galaxies, using the shortest of these times to constrain how long ago a galaxy-galaxy interaction could have occurred. We go on to quantify the initial GC numbers and densities needed for significant collisional evolution to occur within the allotted times, and show that, if the hypothesis of a previous galaxy-galaxy interaction is correct, a paucity of low-mass GCs should be revealed by deeper observational surveys. If any are found, they should be more spatially extended than the currently observed GC population. Finally, we apply our method to these galaxies, in order to illustrate its efficacy in constraining their dynamical evolution. Our results motivate more complete observations of the GC luminosity functions in these galaxies, in addition to future studies aimed at combining the method presented here with a suite of numerical simulations in order to further constrain the origins of the curious GC populations in these (and other) galaxies.

astro-ph.GA↗

arXiv2020

Effective spin distribution of black hole mergers in triples

Many astrophysical scenarios have been proposed to explain the several black hole (BH) and neutron star binary mergers observed via gravitational waves (GWs) by the LIGO-Virgo collaboration. Contributions from various channels can be statistically disentangled by mass, spin, eccentricity and redshift distributions of merging binaries. In this paper, we investigate the signatures of BH-BH binary mergers induced by a third companion through the Lidov-Kozai mechanism in triple systems. We adopt different prescriptions for the supernovae natal kicks and consider different progenitor metallicities and initial orbital parameters. We show that the typical eccentricity in the LIGO band is $0.01$-$0.1$ and that the merger rate is in the range $0.008-9 \ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$, depending on the natal kick prescriptions and progenitor metallicity. Furthermore, we find that the typical distribution of effective projected spin is peaked at $χ_{\rm eff}\sim 0$ with significant tails. We show that the triple scenario could reproduce the distribution of $χ_{\rm eff}$. We find that the triple channel may be strongly constrained by the misalignment angle between the binary component spins in future detections with spin precession.

astro-ph.GA↗

arXiv2020

On the eccentricity evolution of massive black hole binaries in stellar backgrounds

We study the dynamical evolution of eccentric massive black hole binaries (MBHBs) interacting with unbound stars by means of an extensive set of three body scattering experiments. Compared to previous studies, we extend the investigation down to a MBHB mass ratio of $q=m_2/m_1=10^{-4}$, where $m_1$ and $m_2$ are the masses of the primary and secondary hole respectively. Contrary to a simple extrapolation from higher mass ratios, we find that for $q\lesssim 10^{-3}$ the eccentricity growth rate becomes negative, i.e., the binary {\it circularises} as it shrinks. This behaviour is due to the subset of interacting stars captured in metastable counter-rotating orbits; those stars tend to extract angular momentum from the binary, promoting eccentricity growth for $q>10^{-3}$, but tend to inject angular momentum into the binary driving it towards circularisation for $q<10^{-3}$. The physical origin of this behaviour requires a detailed study of the orbits of this subset of stars and is currently under investigation. Our findings might have important consequences for intermediate MBHs (IMBHs) inspiralling onto MBHs (e.g. a putative $10^3\rm M_{\odot}$ black hole inspiralling onto SgrA$^*$).

astro-ph.GA↗

arXiv2019

On the Rate of Neutron Star Binary Mergers from Globular Clusters

The first detection of gravitational waves from a neutron star - neutron star (NS-NS) merger, GW170817, and the increasing number of observations of short gamma-ray bursts (SGRBs) have greatly motivated studies of the origins of NS-NS and neutron star - black hole (NS-BH) binaries. We calculate the merger rates of NS-NS and NS-BH binaries from globular clusters (GCs) using realistic GC simulations with the \texttt{CMC} cluster catalog. We use a large sample of models with a range of initial numbers of stars, metallicities, virial radii and galactocentric distances, representative of the present-day Milky Way GCs, to quantify the inspiral times and volumetric merger rates as a function of redshift, both inside and ejected from clusters. We find that over the complete lifetime of most GCs, stellar BHs dominate the cluster cores and prevent the mass segregation of NSs, thereby reducing the dynamical interaction rates of NSs so that at most a few NS binary mergers are ever produced. We estimate the merger rate in the local universe to be $\sim\rm{0.02\,Gpc^{-3}\,yr^{-1}}$ for both NS-NS and NS-BH binaries, or a total of $\sim 0.04$~Gpc$^{-3}$~yr$^{-1}$ for both populations. These rates are about 5 orders of magnitude below the current empirical merger rate from LIGO/Virgo. We conclude that dynamical interactions in GCs do not play a significant role in enhancing the NS-NS and NS-BH merger rates.

astro-ph.HE↗

arXiv2019

Black hole-neutron star mergers from triples II: the role of metallicity and spin-orbit misalignment

Observations of black hole-neutron star (BH-NS) mergers via gravitational waves (GWs) are of great interest for their electromagnetic counterparts, such as short gamma-ray bursts, and could provide crucial information on the nature of BHs and the NS crust and magnetosphere. While no event has been confirmed, a recent possible detection of a BH-NS merger event by the LIGO-Virgo collaboration has attracted a lot of attention to these sources. In this second paper of the series, we follow-up our study of the dynamical evolution of triples comprised of an inner BH-NS binary. In particular, we examine how the progenitor metallicity affects the characteristics of the BH-NS mergers in triples. We determine the distributions of masses, orbital parameters and merger times, as a function of the progenitor metallicity and initial triple orbital distributions, and show that the typical eccentricity in the LIGO band is $\sim 10^{-2}-10^{-1}$. We derive a merger rate range of $Γ_\mathrm{BH-NS}=1.9\times 10^{-4}-22 \ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$, consistent the LIGO-Virgo upper limit. Finally, we study the expected spin-orbit misalignments of merging BH-NS binaries from this channel, and find that typically the effective spin distribution is peaked at $χ_{\rm eff}\sim 0$ with significant tails.

astro-ph.GA↗

arXiv2019

Supernovae in massive binaries and compact object mergers near supermassive black holes

Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are among the densest systems in the Universe, harbouring millions of stars and compact objects (COs). Within a few parsecs from the SMBH, stars can form binaries. In this paper, we model the supernova (SN) process of massive binaries that are born in proximity of the SMBH and that produce CO binaries. These binaries can later merge via emission of gravitational waves as a consequence of the Lidov-Kozai mechanism. We study the dynamical evolution of these systems by means of high-precision $N$-body simulations, including post-Newtonian (PN) terms up to 2.5PN order. We adopt different prescriptions for the natal velocity kicks imparted during the SN processes and find that larger kicks lead to more compact binaries that merge closer to the SMBH. We also conclude that most of the mergers enter the LIGO band with very high eccentricities. Finally, we compute a merger rate of $0.05$--$0.07\ \mathrm{Gpc}^{-3} \mathrm{yr}^{-1}$, $0.04$--$2\times 10^{-3}\ \mathrm{Gpc}^{-3} \mathrm{yr}^{-1}$, $9.6\times 10^{-6}$--$2.7\times 10^{-3}\ \mathrm{Gpc}^{-3} \mathrm{yr}^{-1}$ for BH-BH, BH-NS, NS-NS, respectively, smaller than the actual LIGO-Virgo observed rate.

astro-ph.HE↗

arXiv2019

Tidal disruption events, main-sequence extreme-mass ratio inspirals and binary star disruptions in galactic nuclei

The Galactic Center has been under intense scrutiny in the recent years thanks to the unprecedented missions aiming at measuring the gas and star dynamics near the supermassive black hole (SMBH) and at finding gravitational wave (GW) signatures of inspiralling stellar black holes. In the crowded environment of galactic nuclei, the two-body interactions alter the distribution of stars on long timescales, making them drift in energy and angular momentum. We present a simplified analytical treatment of the scattering processes in galactic stellar nuclei, assuming all stars have the same mass. We have discussed how the interplay between two-body relaxation and gravitational wave emission modifies the slope of the inner stellar cusp within the SMBH sphere of influence, and calculated the rates of tidal disruption events (TDEs) and main-sequence extreme-mass ratio inspirals (MS-EMRIs) of stars that are tidally disrupted by the SMBH. We find that typically the ratio of the TDE and MS-EMRI rates is the square of the ratio of the tidal and Schwarzschild radii. For our Galaxy, this implies that the rate of MS-EMRIs is just about a percent of the TDE rate. We then consider the role of stars injected on highly eccentric orbits in the vicinity of the SMBH due to Hills binary disruption mechanism, and show that the MS-EMRI rate can almost approach the TDE rate if the binary fraction at the SMBH influence radius is close to unity. Finally, we discuss that physical stellar collisions affect a large area of phase space.

astro-ph.GA↗

arXiv2019

Hypervelocity stars from star clusters hosting Intermediate-Mass Black Holes

Hypervelocity stars (HVSs) represent a unique population of stars in the Galaxy reflecting properties of the whole Galactic potential. Determining their origin is of fundamental importance to constrain the shape and mass of the dark halo. The leading scenario for the ejection of HVSs is an encounter with the supermassive black hole in the Galactic Centre. However, new proper motions from the \textit{Gaia} mission indicate that only the fastest HVSs can be traced back to the Galactic centre and the remaining stars originate in the disc or halo. In this paper, we study HVSs generated by encounters of stellar binaries with an intermediate-mass black hole (IMBH) in the core of a star cluster. For the first time, we model the effect of the cluster orbit in the Galactic potential on the observable properties of the ejected population. HVSs generated by this mechanism do not travel on radial orbits consistent with a Galactic centre origin, but rather point back to their parent cluster, thus providing observational evidence for the presence of an IMBH. We also model the ejection of high-velocity stars from the Galactic population of globular clusters, assuming that they all contain an IMBH, including the effects of the cluster's orbit and propagation of the star in the Galactic potential up to detection. We find that high-velocity stars ejected by IMBHs have distinctive distributions in velocity, Galactocentric distance and Galactic latitude, which can be used to distinguish them from runaway stars and stars ejected from the Galactic Centre.

astro-ph.GA↗

arXiv2019

Tidal disruption events onto stellar black holes in triples

Stars passing too close to a black hole can produce tidal disruption events (TDEs), when the tidal force across the star exceeds the gravitational force that binds it. TDEs have usually been discussed in relation to massive black holes that reside in the centres of galaxies or lurk in star clusters. In this paper, we investigate the possibility that triple stars hosting a stellar black hole (SBH) may be sources of TDEs. We start from a triple system made up of three main sequence (MS) stars and model the supernova (SN) kick event that led to the production of an inner binary comprised of a SBH. We evolve these triples in isolation with a high precision $N$-body code and study their TDEs as a result of Kozai-Lidov oscillations. We explore a variety of distributions of natal kicks imparted during the SN event, various maximum initial separations for the triples, and different distributions of eccentricities. We show that the main parameter that governs the properties of the SBH-MS binaries which produce a TDE in triples is the mean velocity of the natal kick distribution. Smaller $σ$'s lead to larger inner and outer semi-major axes of the systems that undergo a TDE, smaller SBH masses, and longer timescales. We find that the fraction of systems that produce a TDE is roughly independent of the initial conditions, while estimate a TDE rate of $7.3\times 10^{-5}-4.1 \ \mathrm{yr}^{-1}$, depending on the prescriptions adopted for the SBH natal kicks. This rate is almost comparable to the expected TDE rate for massive black holes.

astro-ph.GA↗

arXiv2019

Eccentric binary black hole mergers in globular clusters hosting intermediate-mass black holes

Globular clusters (GCs) may harbour intermediate-mass black holes (IMBHs) at their centres. In these dynamically active environments stellar-mass black holes (SBHs) sink to the center soon after formation, due to dynamical friction and start interacting among themselves and with the central IMBH. Likely, some of the SBHs will form bound systems with the IMBH. A fraction of those will be triple systems composed of binary SBHs and the IMBH acting as a third distant perturber. If the SBH binary orbit is sufficiently inclined it can develop Lidov-Kozai (LK) oscillations, which can drive the system to high eccentricities and eventually to a merger due to gravitational wave (GW) emission on short timescales. In this work, we focus on the dynamics of the IMBH-SBH-SBH triples and illustrate that these systems can be possible sources of GWs. A distinctive signature of this scenario is that a considerable fraction of these mergers are highly eccentric when entering the LIGO band ($10$ Hz). Assuming that $\sim 20\%$ of GCs host IMBHs and a GC density in the range $n_{_{\rm GC}}=0.32$-$2.31\,\mathrm{Mpc}^{-3}$, we have estimated a rate $Γ=0.06$-$0.46\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$ of these events. This suggests that dynamically-driven binary SBH mergers in this scenario could contribute to the merger events observed by LIGO/VIRGO. Full $N$-body simulations of GCs harbouring IMBHs are highly desirable to give a more precise constrain on this scenario.

astro-ph.GA↗

arXiv2019

Black hole and neutron star mergers in Galactic Nuclei: the role of triples

Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are thought to contain large numbers of black holes (BHs) and neutron stars (NSs), a fraction of which form binaries and could merge by Kozai-Lidov oscillations (KL). Triple compact objects are likely to be present, given what is known about the multiplicity of massive stars, whose life ends either as a NS or a BH. In this paper, we present a new possible scenario for merging BHs and NSs in galactic nuclei. We study the evolution of a triple black hole (BH) or neutron star (NS) system orbiting an SMBH in a galactic nucleus by means of direct high-precision $N$-body simulations, including Post-Newtonian terms. We find that the four-body dynamical interactions can increase the KL angle window for mergers compared to the binary case and make BH and NS binaries merge on shorter timescales. We show that the merger fraction can be up to $\sim 5$--$8$ times higher for triples than for binaries. Therefore, even if the triple fraction is only $\sim 10\%$--$20\%$ of the binary fraction, they could contribute to the merger events observed by LIGO/VIRGO in comparable numbers.

astro-ph.GA↗

arXiv2019

Massive binary star mergers in galactic nuclei: implications for blue stragglers, binary S-stars and gravitational waves

Galactic nuclei are often found to contain young stellar populations and, in most cases, a central supermassive black hole (SMBH). Most known massive stars are found in binaries or higher-multiplicity systems, and in a galactic nucleus the gravitational interaction with the SMBH can affect their long-term evolution. In this paper, we study the orbital evolution of stellar binaries near SMBHs using high precision $N$-body simulations, and including tidal forces and Post-Newtonian corrections to the motion. We focus on the Lidov-Kozai (LK) effect induced by the SMBH on massive star binaries. We investigate how the properties of the merging binaries change with varying the SMBH mass, the slope of the initial mass function, the distributions of the binary orbital parameters and the efficiency in energy dissipation in dissipative tides. We find that the fraction of merging massive binary stars is in the range $\sim 4\%$--$15\%$ regardless of the details of the initial distributions of masses and orbital elements. For a Milky Way-like nucleus, we find a typical rate of binary mergers $Γ\approx 1.4\times 10^{-7}$ yr$^{-1}$. The merger products of massive binaries can be rejuvenated blue-straggler stars, more massive than each of their original progenitors, and G2-like objects. Binary systems that survive the LK cycles can be source of X-rays and gravitational waves, observable with present and upcoming instruments.

astro-ph.GA↗

arXiv2019

Black hole and neutron star mergers in galactic nuclei

Nuclear star clusters surrounding supermassive black holes (SMBHs) in galactic nuclei contain large numbers of stars, black holes (BHs) and neutron stars (NSs), a fraction of which are likely to form binaries. These binaries were suggested to form a triple system with the SMBH, which acts as a perturber and may enhance BH and NS mergers via the Lidov-Kozai mechanism. We follow-up previous studies, but for the first time perform an extensive statistical study of BH-BH, NS-NS and BH-NS binary mergers by means of direct high-precision regularized $N$-body simulations, including Post-Newtonian (PN) terms up to order PN2.5. We consider different SMBH masses, slopes for the BH mass function, binary semi-major axis and eccentricity distributions, and different spatial distributions for the binaries. We find that the merger rates are a decreasing function of the SMBH mass and are in the ranges $\sim 0.17$-$0.52 \ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$, $\sim 0.06$-$0.10 \ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$ and $\sim 0.04$-$0.16 \ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$ for BH-BH, BH-NS and NS-NS binaries, respectively. However, the rate estimate from this channel remains highly uncertain and depends on the specific assumptions regarding the star-formation history in galactic nuclei and the supply rate of compact objects. We find that $\sim 10\%$--$20\%$ of the mergers enter the LIGO band with eccentricities $\gtrsim 0.1$. We also compare our results to the secular approximation, and show that $N$-body simulations generally predict a larger number of mergers. Finally, these events can also be observable via their electromagnetic counterparts, thus making these compact object mergers especially valuable for cosmological and astrophysical purposes.

astro-ph.GA↗