arXiv subjects

Giovanni Maria Tomaselli

Publications and source records attributed to Giovanni Maria Tomaselli.

18 recordsLinked to original sources

arXiv2026

Modeling Relativistic Tidal Disruptions of MESA Stars

Tidal disruption events (TDEs) occur when a star passes so close to a black hole that its self-gravity is overcome by the external tidal field. As the star passes, it initially deforms, then is ripped apart, and some of its material eventually falls back on bound orbits, forming an accretion disk around the black hole. A Newtonian model of TDEs, based on stellar perturbation theory of MESA stars, was recently introduced as an alternative to computationally intensive hydrodynamical simulations. In this work, we add relativistic corrections to the model, incorporating equatorial Kerr geodesics, relativistic tidal fields, and relativistic fallback times. Compared to the Newtonian case, we find that stars are disrupted earlier in their orbit, which gives them less time to accumulate physical deformations. Additionally, we find that the increased distance from the black hole at the time of disruption makes the fallback time longer. However, the black hole spin has a negligible impact on fallback time, except for orbits with exceptionally close pericenter. Our results allow for a more accurate calculation of fallback rates than the Newtonian model, while also remaining computationally cheap. The code is available on GitHub.

astro-ph.HE↗

arXiv2026

Is S301 the Captured Companion of the Hypervelocity Star S5-HVS1?

Stellar binary disruptions through the Hills mechanism produce two fossils: a hypervelocity star (HVS), and a star tightly bound to the supermassive black hole. Among known galactic HVSs, only S5-HVS1 has unambiguous galactic-centre origin. Its measured mass and velocity determine a relation between the mass and semi-major axis of its captured companion. GRAVITY has now discovered S301, whose orbit and photometrically inferred mass satisfy this relation, making it the only compelling candidate for the captured companion of S5-HVS1. We build a forward model for the Hills origin and compare it to the null hypothesis. This confirms that S301's orbit aligns much more closely with that of S5-HVS1's companion than a typical S-star. However, the catalog-level Bayes factor remains of order unity and dependent on the probabilities of survival and detection. S301 is thus a compelling candidate, but establishing its association with S5-HVS1 will require improved mass measurements, chemical comparisons and GRAVITY-calibrated selection functions.

astro-ph.GA↗

arXiv2026

Evolution of Binaries Under Stochastic Perturbations

We develop a general Fokker-Planck framework describing the dynamical evolution of Keplerian binaries subjected to stochastic perturbations. The formalism provides an algorithmic way to obtain the Fokker-Planck drift and diffusion coefficients of any set of orbital variables given the statistics of the perturbations. We apply the method to three physically distinct regimes: adiabatic tidal perturbations, white-noise tidal perturbations, and impulsive encounters with a third body of arbitrary density profile. In each regime we provide explicit drift and diffusion coefficients for all six orbital elements, derive the associated evolution timescale, and obtain analytic steady-state distribution functions. Our results extend previous treatments by including the evolution of the binary's orientation, retaining the complete tensor structure of tidal correlators, treating non-pointlike perturbers, and resolving the exact geometry of impulsive encounters. The latter correction leads to a steady-state eccentricity distribution that is slightly sub-thermal. We also show how these equations can be applied directly in several astrophysical scenarios, including binaries perturbed by dark matter subhaloes, ultralight dark matter, and the interstellar medium. This work delivers both a complete mathematical framework and a practical toolkit for stochastic binary evolution, providing ready-to-evaluate equations to be applied directly to binary population data.

astro-ph.GA↗

arXiv2026

Self-acceleration of Hardening Binaries

A Keplerian binary immersed in a bath of lighter particles hardens by ejecting them through gravitational slingshots. This process drives, for example, the evolution of supermassive black hole binaries following galaxy mergers, and has long been described with just two parameters: the hardening rate and the eccentricity growth rate. Here we show that the secular dynamics is substantially richer. Combining symmetry arguments with extensive three-body scattering experiments, we demonstrate that the medium exerts a net force on the binary's center of mass (CoM), induces apsidal precession, and rotates the orbital plane when the CoM velocity has an out-of-plane component. Remarkably, these deterministic effects persist even in a perfectly uniform and isotropic medium, as the binary's own asymmetry provides the propulsion. The interplay of self-acceleration, precession, and dynamical friction drives the CoM along an outward spiral. For supermassive black hole binaries, this displacement dominates over Brownian motion and approaches the radius of influence, suggesting they may be significantly offset from their host galaxies' centers. The displacement also enlarges the stellar loss cone, with direct implications for the final-parsec problem. We further show that the previously reported circularization of small-mass-ratio binaries is a numerical artifact of truncating long-lived encounters: all binaries undergo eccentricity growth. Our results enrich the standard picture of binary hardening and have implications in a variety of astrophysical contexts, including gravitational-wave source populations.

astro-ph.GA↗

arXiv2026

Probing dense environments around Sgr A* with S-stars dynamics

The orbits of stars around Sgr A*, the Milky Way's supermassive black hole, provide a unique laboratory for testing its environment with unprecedented precision. In this work, we compute the apsidal precession induced by extended matter distributions through Lagrange's equations and compare it with the measured precession of S2, reproducing and extending GRAVITY's constraints. In particular, we push bounds on boson clouds to larger gravitational couplings $α$ and to the second-fastest superradiant mode. We also show that environments with mass of order $1\%$ of Sgr A* drive stellar orbits to decay by dynamical friction within a few Myr. The inner star cluster is however efficiently replenished, masking this effect observationally. We also show that orbital resonances from boson clouds have no impact on relevant timescales. While S2 currently provides the cleanest dataset, our framework is readily applicable to other stars that we identify as particularly promising, whose orbits will be measured with increasing accuracy, opening up new opportunities to probe the environment of Sgr A*.

astro-ph.GA↗

arXiv2025

Modeling Tidal Disruptions with Dynamical Tides

Tidal disruption events (TDEs) occur when stars pass close enough to supermassive black holes to be torn apart by tidal forces. Traditionally, these events are studied with computationally intensive hydrodynamical simulations. In this paper, we present a fast, physically motivated two-stage model for TDEs. In the first stage, we model the star's tidal deformation using linear stellar perturbation theory, treating the star as a collection of driven harmonic oscillators. When the tidal energy exceeds a fraction $γ$ of the star's gravitational binding energy (with $γ\sim \mathcal O(1)$), we transition to the second stage, where we model the disrupted material as free particles. The parameter $γ$ is determined with a one-time calibration to the critical impact parameter obtained in hydrodynamical simulations. This method enables fast computation of the energy distribution ${\rm d} M/{\rm d}E$ and fallback rate ${\rm d} M/{\rm d} T$, while offering physical insight into the disruption process. We apply our model to MESA-generated profiles of middle-age main-sequence stars. Our code is available on GitHub.

astro-ph.HE↗

arXiv2025

Smooth binary evolution from wide resonances in boson clouds

Ultralight scalars can form superradiant clouds around rotating black holes. These may alter the dynamics of compact binaries and the ensuing waveform through orbital resonances and cloud ionization. We re-examine resonances involving states with nonzero decay width, deriving an effective treatment for resonances that are wider than the binary's frequency chirp. We demonstrate the utility of this approach by calculating an upper bound for the cloud's mass surviving up to the latest stages of the inspiral. Next, we study the accumulation of resonances with high-energy bound states. When these infinitely many, increasingly weak resonances are properly taken into account, they smooth out the "sharp features" in the binary's evolution that had been attributed to the ionization of the cloud. We compare our Newtonian results with recent relativistic calculations, highlighting common features as well as discrepancies. Our conclusions emphasize the need to carefully incorporate resonances in boson cloud waveform modeling.

gr-qc↗

arXiv2025

Resonant history of gravitational atoms in black hole binaries

Rotating black holes can produce superradiant clouds of ultralight bosons. When the black hole is part of a binary system, its cloud can undergo resonances and ionization. These processes leave a distinct signature on the gravitational waveform that depends on the cloud's properties. To determine the state of the cloud by the time the system enters the band of future millihertz detectors, we study the chronological sequence of resonances encountered during the inspiral. For the first time, we consistently take into account the nonlinearities induced by the orbital backreaction and we allow the orbit to have generic eccentricity and inclination. We find that the resonance phenomenology exhibits striking new features. Resonances can "start" or "break" above critical thresholds of the parameters, which we compute analytically, and induce dramatic changes in eccentricity and inclination. Applying these results to realistic systems, we find two possible outcomes. If the binary and the cloud are sufficiently close to counter-rotating, the cloud survives in its original state until the system enters in band; otherwise, the cloud is destroyed during a resonance at large separations, but leaves an imprint on the eccentricity and inclination. In both scenarios, we characterize the observational signatures, with particular focus on future gravitational wave detectors.

gr-qc↗

arXiv2025

Scattering of wave dark matter by supermassive black holes

Recent simulations of wave dark matter around black hole binaries revealed the formation of a universal density profile that co-rotates with the binary. We derive this profile from first principles, interpreting it as the steady state of a scattering process. We find that the scattering becomes particularly efficient when the ratio of the binary separation to the dark matter's de Broglie wavelength assumes certain discrete values, which can be interpreted as bound state resonances. After estimating the amount of dark matter that undergoes this type of scattering off supermassive black hole binaries at galactic centers, we demonstrate that the process can induce an observable modification of the slope of the Pulsar Timing Array spectrum. This opens up a new possibility to gain insights on the nature of dark matter from observations of low-frequency gravitational waves.

gr-qc↗

arXiv2024

Gravitational Atoms and Black Hole Binaries

Several models of physics beyond the Standard Model predict the existence of new ultralight bosons. This thesis investigates a way to discover such particles through observations of gravitational waves from binary black holes. This is possible through black hole superradiance, which spontaneously creates a "boson cloud" around a rapidly spinning black hole. The system is also known as a gravitational atom, due to its similarities with the hydrogen atom. The thesis focuses on a scenario where a gravitational atom is orbited by a binary companion. The goal is to characterize the dynamics of the system and identify the signatures left by the boson cloud on the gravitational waves emitted by the binary. The predictions can be tested with current and future interferometers, such as LISA, LIGO, DECIGO, Einstein Telescope and TianQin. First, I demonstrate that the cloud catalyzes the binary formation by increasing the dynamical capture cross section. I then introduce and study the ionization of the cloud, wherein the perturbation from the binary unbinds the bosons, analogous to the photoelectric effect in atomic physics. After that, I examine the accretion of the cloud on the companion black hole. To achieve realistic and complete results, I proceed to extend the treatment of ionization, as well as of the orbital resonances discussed in earlier works, to orbits with generic inclination and eccentricity. This allows to study the entire history of the system, from formation to merger. The most distinctive observational signatures of the cloud are found to be the orbital energy lost through ionization and the preference for specific inclinations and eccentricities induced by the orbital resonances.

gr-qc↗

arXiv2024

Dynamical Friction in Gravitational Atoms

Due to superradiant instabilities, clouds of ultralight bosons can spontaneously grow around rotating black holes, creating so-called "gravitational atoms". In this work, we study their dynamical effects on binary systems. We first focus on open orbits, showing that the presence of a cloud can increase the cross section for the dynamical capture of a compact object by more than an order of magnitude. We then consider closed orbits and demonstrate that the backreaction of the cloud's ionization on the orbital motion should be identified as dynamical friction. Finally, we study for the first time eccentric and inclined orbits. We find that, while ionization quickly circularizes the binary, it barely affects the inclination angle. These results enable a more realistic description of the dynamics of gravitational atoms in binaries and pave the way for dedicated searches with future gravitational wave detectors.

gr-qc↗

arXiv2024

Legacy of boson clouds on black hole binaries

Superradiant clouds of ultralight bosons can leave an imprint on the gravitational waveform of black hole binaries through "ionization" and "resonances." We study the sequence of resonances as the binary evolves, and show that there are only two possible outcomes, each with a distinct imprint on the waveform. If the cloud and the binary are nearly counter-rotating, then the cloud survives in its original state until it enters the sensitivity band of future gravitational wave detectors, such as LISA. In all other cases, resonances destroy the cloud, while driving the binary to co-rotate with it and its eccentricity close to a fixed point. This opens up the possibility of inferring the existence of a new boson from the statistical analysis of a population of black hole binaries.

gr-qc↗

arXiv2023

Quantum Field Theory on compact stars near the Buchdahl limit

Very compact stars seem to be forbidden in General Relativity. While Buchdahl's theorem sets an upper bound on compactness, further no-go results rely on the existence of two light rings, the inner of which has been associated to gravitational instabilities. However, little is known about the role of quantum fields in these strong gravity regimes. Here, we consider the particularly simple model of a constant density star and we work in the probe approximation where the backreaction is ignored. We show that the trapping of modes inside the star leads the renormalized stress tensor of Conformal Field Theories to diverge faster than the classical source in the Buchdahl limit. This leads to the violation of the Null Energy Condition around the inner light ring. The backreaction of quantum fields in this regime therefore cannot be ignored. This happens as the star's surface approaches the Buchdahl radius $9GM/4$ rather than the Schwarzschild radius. The results are independent of the details of the interactions, but contain an ambiguity associated to the renormalization scheme.

gr-qc↗

arXiv2023

Black hole superradiance with (dark) matter accretion

Studies of black hole superradiance often focus on the growth of a cloud in isolation, accompanied by the spin-down of the black hole. In this paper, we consider the additional effect of the accretion of matter and angular momentum from the environment. We show that, in many cases, the black hole evolves by drifting along the superradiance threshold, in which case the evolution of its parameters can be described analytically or semi-analytically. We quantify the conditions under which accretion can serve as a mechanism to increase the cloud-to-black hole mass ratio, beyond the standard maximum of about 10%. This occurs by a process we call over-superradiance, whereby accretion effectively feeds the superradiance cloud, by way of the black hole. We give two explicit examples: accretion from a vortex expected in wave dark matter and accretion from a baryonic disk. In the former case, we estimate the accretion rate by using an analytical fit to the asymptotic behavior of the confluent Heun function. Level transition, whereby one cloud level grows while the other shrinks, can be understood in a similar way.

gr-qc↗

arXiv2022

Disks, spikes, and clouds: distinguishing environmental effects on BBH gravitational waveforms

Future gravitational wave interferometers such as LISA, Taiji, DECIGO, and TianQin, will enable precision studies of the environment surrounding black holes. In this paper, we study intermediate and extreme mass ratio binary black hole inspirals, and consider three possible environments surrounding the primary black hole: accretion disks, dark matter spikes, and clouds of ultra-light scalar fields, also known as gravitational atoms. We present a Bayesian analysis of the detectability and measurability of these three environments. Focusing for concreteness on the case of a detection with LISA, we show that the characteristic imprint they leave on the gravitational waveform would allow us to identify the environment that generated the signal, and to accurately reconstruct its model parameters.

gr-qc↗

arXiv2022

Ionization of Gravitational Atoms

Superradiant instabilities may create clouds of ultralight bosons around rotating black holes, forming so-called "gravitational atoms." It was recently shown that the presence of a binary companion can induce resonant transitions between bound states of these clouds, whose backreaction on the binary's orbit leads to characteristic signatures in the emitted gravitational waves. In this work, we show that the interaction with the companion can also trigger transitions from bound to unbound states of the cloud -- a process that we refer to as "ionization" in analogy with the photoelectric effect in atomic physics. The orbital energy lost in the process overwhelms the losses due to gravitational wave emission and contains sharp features carrying information about the energy spectrum of the cloud. Moreover, we also show that if the companion is a black hole, then the part of the cloud impinging on the event horizon will be absorbed. This "accretion" leads to a significant increase of the companion's mass, which alters the dynamical evolution and ensuing waveform of the binary. We argue that a combined treatment of resonances, ionization, and accretion is crucial to discover and characterize gravitational atoms with upcoming gravitational wave detectors.

gr-qc↗

arXiv2022

Sharp Signals of Boson Clouds in Black Hole Binary Inspirals

Gravitational waves (GWs) are an exciting new probe of physics beyond the standard models of gravity and particle physics. One interesting possibility is provided by the so-called "gravitational atom," wherein a superradiant instability spontaneously forms a cloud of ultralight bosons around a rotating black hole. The presence of these boson clouds affects the dynamics of black hole binary inspirals and their associated GW signals. In this Letter, we show that the binary companion can induce transitions between bound and unbound states of the cloud, effectively "ionizing" it, analogous to the photoelectric effect in atomic physics. The orbital energy lost in this process can overwhelm the losses due to GW emission, so that ionization drives the inspiral rather than merely perturbing it. We show that the ionization power contains sharp features that lead to distinctive "kinks" in the evolution of the emitted GW frequency. These discontinuities are a unique signature of the boson cloud and observing them would not only constitute a detection of the ultralight boson itself, but also provide direct information about its mass and the state of the cloud.

gr-qc↗

arXiv2021

Lyman-alpha radiation pressure: an analytical exploration

We study radiation pressure due to Lyman alpha line photons, obtaining and exploring analytical expressions for the force-multiplier, $M_F(N_H, Z) = F_α/(L_α/c)$, as a function of gas column density, $N_H$, and metallicity, $Z$, for both dust-free and dusty media, employing a WKB approach for the latter case. Solutions for frequency offset emission to emulate non-static media moving with a bulk velocity $v$, have also been obtained. We find that, in static media, Ly$α$ pressure dominates over both photoionization and dust-mediated UV radiation pressure in a very wide parameter range ($16 < \log N_H < 23$; $-4 < \log[Z/Z_\odot] < 0$). For example, it overwhelms the other two forces by 10 (300) times in standard (low-$Z$) star-forming clouds. Thus, in agreement with previous studies, we conclude that Ly$α$ pressure plays a dominant role in the initial acceleration of the gas around luminous sources, and must be implemented in galaxy formation, evolution and outflow models and simulations.

astro-ph.GA↗