# Dr Fabio Antonini

## Academic and research departments

Department of Physics, Faculty of Engineering and Physical Sciences.### Research

### Research interests

Fabio is an STFC E. Rutherford fellow in the astrophysics group. Previously, he was a postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA), and a CIERA postdoctoral fellow at Northwestern University. Prior to that, he received a PhD in Astrophysical Sciences and Technologies at the Rochester Institute of Technology (with David Merritt), and a Master degree in astrophysics from the university of Rome La Sapienza. His main research interests include stellar dynamics near supermassive black holes and computational astrophysics, formation of gravitational wave sources, and dynamics of exoplanet systems.

### My publications

### Publications

merger by a third companion through the Lidov-Kozai mechanism. Using a population synthesis

approach, we model the creation of stellar-mass black hole triples across a range of different initial

conditions and stellar metallicities. We find that the production of triple-mediated mergers is enhanced

at low metallicities by a factor of ~ 100 due to the lower black hole natal kicks and reduced stellar mass

loss. These triples naturally yield heavy binary black holes with near-zero effective spins, consistent

with most of the mergers observed to date. This process produces a merger rate of between 2 and 25

Gpc

^{-3}yr

^{-1}in the local universe, suggesting that the Lidov-Kozai mechanism can potentially explain

all of the low-spin, heavy black hole mergers observed by Advanced LIGO/Virgo. Finally, we show

that triples admit a unique eccentricity and spin distribution that will allow this model to be tested

in the near future.

with near-zero effective spin, Monthly Notices of the Royal Astronomical Society: Letters 480 (1) pp. L58-L62 Oxford University Press

evidence of large black hole spins. However, because LIGO/Virgo best measures the

effective combination of the two spins along the orbital angular momentum (Xeff), it is

difficult to distinguish between binaries with slowly-spinning black holes and binaries

with spins lying in the orbital plane. Here, we study the spin dynamics for binaries with

a distant black hole companion. For spins initially aligned with the orbital angular

momentum of the binary, we find that Xeff \freezes" near zero as the orbit decays

through the emission of gravitational waves. Through a population study, we show

that this process predominantly leads to merging black hole binaries with near-zero

Xeff. We conclude that if the detected black hole binaries were formed in triples, then

this would explain their low _eff without the need to invoke near-zero spins or initially

large spin-orbit angles.

the Lidov-Kozai (LK) mechanism, which can drive orbits to high eccentricities and trigger strong interactions

within the binary such as the emission of gravitational waves (GWs), and mergers of compact objects. These

events are potential sources for GW detectors such as Advanced LIGO and VIRGO. The LK mechanism is

only effective if the binary is highly inclined with respect to its orbit around the MBH (within a few degrees of

90æ), implying low rates. However, close to an MBH, torques from the stellar cluster give rise to the process of

vector resonant relaxation (VRR). VRR can bring a low-inclination binary into an ?active? LK regime in which

high eccentricities and strong interactions are triggered in the binary. Here, we study the coupled LK-VRR

dynamics, with implications for LIGO and VIRGO GW sources. We carry out Monte Carlo simulations and

find that the merger fraction enhancement due to LK-VRR dynamics is up to a factor of end of assumed MBH masses (M" = 104M), and decreases sharply with increasing M". We find that, even

in our most optimistic scenario, the baseline BH-BH merger rate is small, and the enhancement by LK-VRR

coupling is not large enough to increase the rate to well above the LIGO/VIRGO lower limit, 12Gpc

^{?3}yr

^{?1}.

For the Galactic Center, the LK-VRR-enhanced rate is M" = 104M, the rate barely reaches 12Gpc

^{?3}yr

^{?1}.

^{?1}in the core and density s105M

_{`}pc

^{?3}`, repeated mergers lead to the formation of BHs in the mass range 100?10

^{5}M

_{`}, populating any upper mass gap created by pair-instability supernovae. This result is independent of cluster metallicity and the initial BH spin distribution. We show that about 10 per cent of the present-day nuclear star clusters meet these extreme conditions, and estimate that BH binary mergers with total mass s100M

_{`}should be produced in these systems at a maximum rate H0.05Gpc

^{?3}yr

^{?1}`, corresponding to one detectable event every few years with Advanced LIGO/Virgo at design sensitivity.

^{?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.

source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection

rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three

orders of magnitude. To understand this variation, we simulate the merger of equal-mass,

eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We

model the formation and hardening of a central BHB using the Fast Multiple Method as a

force solver, which features a O¹Nº scaling with the number N of particles and obtains results

equivalent to direct-summation simulations. At N ý 5ý105, typical for contemporary studies,

the eccentricity of the BHBs can vary significantly for different random realisations of the

same initial condition, resulting in a substantial variation of the merger timescale. This scatter

owes to the stochasticity of stellar encounters with the BHB and decreases with increasing N.

We estimate that N ý 107 within the stellar half-light radius suffices to reduce the scatter in

the merger timescale to ý 10%. Our results suggest that at least some of the uncertainty in

low-frequency GW rates owes to insufficient numerical resolution.