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Riccardo Della Monica

Publications and source records attributed to Riccardo Della Monica.

At least 19 recordsLinked to original sources

Timing Gravity with Pulsars in the Strong Field

We propose a novel approach for the timing of pulsars orbiting a supermassive black hole, which implements the fully relativistic calculations of the photon travel time into a robust timing model. We generate realistic mock catalogues of pulsar times-of-arrival for several putative pulsars on tight orbits around the Galactic Center supermassive black hole, Sagittarius A* (Sgr A*). Then, we perform a proof-of-concept sensitivity analysis to forecast the accuracy that future observational facilities, like the Squared Kilometer Array, will achieve in the characterization of the parameters of our timing model. Our analysis shows how the observation of pulsars at the Galactic Center will open an incredibly promising avenue for the characterization of the physical properties of Sgr A*, which can improve by at least three orders of magnitude the current constraints on the black hole's mass achieved with the S-stars and event-horizon scale observations.

astro-ph.HE↗

On relativistic observables in black bounce spacetimes

We investigate the phenomenology of black bounce spacetimes through a combined analytical and numerical study of relativistic observables associated with both time-like and null geodesics. Black bounce geometries provide a continuous interpolation between Schwarzschild black holes and traversable wormholes by introducing a regularization bounce parameter $α$, which removes the central singularity by replacing it with a finite-radius throat. Using the Hamilton--Jacobi and Lagrangian formalisms, we derive weak-field analytical expressions for the periastron advance of massive particles and the deflection angle of light, highlighting the leading corrections induced by the bounce parameter. These results are systematically compared with numerical integrations performed with the PyGRO code. We show that increasing $α$ enhances both the periastron precession and the light deflection with respect to the Schwarzschild case. Additionally, we analyze the relativistic redshift of bound orbits and identify cumulative temporal phase shifts associated with the modified geometry. For null geodesics, we also determine numerically the critical impact parameter separating scattered and captured (or transmitted through the throat) photon trajectories. Although this quantity remains identical to the Schwarzschild value in the regular black hole branch, it increases significantly in the traversable wormhole regime, providing a clear observational signature of the throat structure. Our results demonstrate that black bounce spacetimes can produce measurable deviations from standard black hole predictions, opening the possibility of constraining these geometries with current and future observations of strong gravitational fields.

gr-qc↗

Resonances as signatures of scalar clouds in eccentric extreme-mass-ratio inspirals

Ultralight scalars arise naturally in many extensions to the Standard Model and are compelling dark matter candidates. Around spinning black holes, dense scalar clouds could form through the conversion of rotational energy into particles via black hole superradiance. Extreme-mass-ratio inspirals (EMRIs) targeted by future space-based detectors will give us unparalleled access to the environments of massive black holes, allowing us to probe the presence of scalar clouds. We consider EMRIs around a Schwarzschild black hole and show that eccentricity induces a dense sequence of resonances in the scalar fluxes near the last stable orbit. These resonances arise only in a fully relativistic treatment, as they are intrinsically tied to the splitting between the azimuthal and radial orbital frequencies in the strong-field regime. By evolving the orbits adiabatically, we show that the resulting resonant transitions substantially enhance the exchange of energy and angular momentum between the EMRI and the scalar cloud, significantly amplifying the accumulated dephasing in the gravitational waveform relative to circular motion. Our results highlight the importance of eccentricity in shaping the observational signatures of EMRIs embedded in scalar clouds.

gr-qc↗

Relativistic effects in extreme-mass-ratio inspirals within scalar clouds: Eccentric and inclined orbits

We study extreme-mass-ratio inspirals (EMRIs) evolving in a scalar cloud environment that may form through superradiant instabilities, using a fully relativistic perturbative framework that allows for eccentric and inclined orbits. EMRIs, consisting of a stellar-mass compact object inspiraling into a supermassive black hole, are key sources for space-based gravitational-wave detectors such as LISA. Previous relativistic studies of EMRIs in scalar clouds have been restricted to circular, equatorial motion. Here, instead, we focus on a Schwarzschild black hole background to incorporate eccentricity and orbital inclination. By computing the scalar energy and angular momentum scattered off to spatial infinity and absorbed at the event horizon, we show that orbital eccentricity can induce a dense spectrum of resonances near the last stable orbit, associated with strong relativistic apsidal precession. We further find that orbital inclination can significantly modify the orbital energy and angular momentum losses. In particular, we identify a critical inclination angle below which, at sufficiently small orbital radii, there is a net transfer of energy from the scalar cloud to the orbit. Moreover, for sufficiently large eccentricities, resonances associated with relativistic apsidal precession persist across the full range of inclinations, although their structure changes significantly between prograde and retrograde orbits. These results provide a foundation for future studies of EMRIs in scalar cloud environments on fully generic orbits around spinning black holes.

gr-qc↗

Pulsar timing in the Galactic Center

We propose a novel approach which implements the relativistic calculations of the photon travel time into a robust timing model for pulsars orbiting supermassive black holes. We demonstrate that timing models relying on the lowest-order (1PN) post-Newtonian approximation can produce significant discrepancies in strong-field configurations, affecting the predicted relativistic times of arrival at an Earth-based observatory. We also show how a misestimation of the pulsar parameters can lead to the appearance of phase-dependent residual, which hints at a tremendous constraining power of the binary and intrinsic parameters for timing observations of potential pulsars at the Galactic Center.

gr-qc↗

Testing the Spacetime Geometry of Sgr A* with the Relativistic Orbit of S2 star

In this work, we perform a relativistic test of the spacetime geometry of Sagittarius A* (Sgr A*) using the orbit of the S2 star. We consider a broad class of compact object models, including Schwarzschild, Reissner-Nordström, Bardeen, Hayward, and Simpson-Visser black holes, as well as the Janis-Newman-Winicour naked singularity spacetime. For each geometry, we integrate the timelike geodesic equations and consistently project the resulting trajectories onto astrometric and spectroscopic observables, incorporating Rømer time delay and relativistic redshift effects. The theoretical predictions are tested with current Very Large Telescope (VLT) observations of the S2 star, while simultaneously imposing constraints from the Event Horizon Telescope shadow size. We find that several spacetimes that are degenerate at the level of shadow imaging, most notably Schwarzschild, Reissner-Nordström, and Bardeen regular black hole geometries, remain statistically indistinguishable when tested against present S2 data. We further carry out a statistical model comparison based on the Akaike and Bayesian information criteria (AIC and BIC) to evaluate the relative performance of the alternative spacetime models. Our analysis also constrains the generalized charge like parameter $q/M$ in non-Schwarzschild spacetimes based on current S2 star observations, and identifies specific black hole and horizonless geometries that can be further tested with forthcoming high precision astrometric observations from the VLT and Keck telescopes.

gr-qc↗

Gravitational signatures beyond Newton: exploring hierarchical three-body dynamics

Hierarchical three-body systems offer a compelling framework to explore the subtle interplay between Newtonian and relativistic gravitational effects in astrophysical environments. In this work, we investigate post-Newtonian corrections to the periastron shift within such systems, focusing on the impact of orbital eccentricity. Modeling the secondary body's influence as a quadrupolar perturbation, we compare Newtonian, Schwarzschild, and post-Newtonian quadrupolar contributions to orbital precession. Our analysis demonstrates that Newtonian quadrupolar effects could be observable, for a long monitoring time, in the orbit of the S87 star around Sagittarius A* if an intermediate-mass black hole is present, under the assumptions of our model. Additionally, post-Newtonian quadrupolar corrections may influence the dynamics of small Solar System bodies in the presence of massive companions. Although the predicted effects are minute and require long monitoring periods to be measurable, our analysis clarifies how relativistic corrections enter the dynamics of the third body and outlines the conditions under which future observations could reveal them.

gr-qc↗

Dwarf galaxies in non-local gravity

The nature of dark matter remains one of the most pressing open questions in modern cosmology. Despite extensive experimental efforts, no direct or indirect detection of dark matter particles has been confirmed. This has motivated alternative approaches, including modifications to the underlying theory of gravity. In this work, we investigate the implications of a specific non-local gravity (NLG) theory, which modifies General Relativity by introducing non-local effects that manifest as an effective dark matter component. We analyze the velocity dispersion profiles of eight classical dwarf spheroidal (dSph) galaxies - Carina, Draco, Fornax, Leo I, Leo II, Sculptor, Sextans, and Ursa Minor - to test the predictions of NLG. Using the Jeans equation, we model the kinematics of these galaxies and perform a Bayesian Markov Chain Monte Carlo analysis to constrain the parameters of the NLG kernel chosen for our analysis. Our results indicate that NLG might successfully reproduce the observed kinematics of dSph galaxies without requiring particle dark matter, providing constraints on the scale-dependent modifications to gravity that are compatible with previous studies in the literature. However, a parameter inconsistency remains in the cases of Fornax and Sextans galaxies that requires further attention.

astro-ph.CO↗

Detectability of gravitational atoms in black hole binaries with the Einstein Telescope

Rotating black holes can amplify ultralight bosonic fields through superradiance, forming macroscopic clouds known as gravitational atoms. When the cloud forms around one of the components of a binary system, it can undergo a series of distinctive interactions, comprising both secular effects, such as dynamical friction or accretion, and resonant behaviour. These processes are expected to leave a distinctive signature on the gravitational waveform emitted by the binary, whose detectability we investigate in this paper. To do so, we implement a numerical code that integrates these effects, computed within a Newtonian approximation, for intermediate-to-high mass-ratio binaries on circular equatorial orbits. Realistic waveforms incorporating these environmental influences are generated and analyzed using the Fisher matrix formalism to evaluate the detectability of bosonic clouds with current and next-generation ground-based gravitational wave observatories. Our results demonstrate the potential for gravitational wave astronomy to probe the existence and properties of ultralight bosons.

gr-qc↗

Time Delay of Pulsar Signals in Astrophysical Black Hole Spacetimes

In this paper, we investigate the fully relativistic time delay of pulsar signals propagating in the vicinity of a rotating black hole and its potential mimickers, including a deformed Kerr black hole and the Janis-Newman-Winicour naked singularity. We aim to compute and compare the pulsar time delays caused by different spacetime geometries to explore possible observational signatures that distinguish between black holes and their alternatives. We begin by solving the equations of motion for null geodesics in these background geometries. Subsequently, we address the emitter-observer problem to compute the time delay of pulsar signals in Kerr, deformed Kerr, and JNW spacetimes. A comparative analysis between Schwarzschild and Kerr black holes allows us to observe the effect of spin on propagation delay in pulsar timing. Further, we examine the impact of the deformation parameter in the deformed Kerr black hole and the influence of the scalar field on the rotating JNW spacetime. Our study considers both direct and indirect photons emitted by a source in the equatorial circular orbit. We find that the variations in the spin parameter show very small but detectable changes when we compare the time delay cases of a Kerr black hole with deformed Kerr and rotating JNW spacetimes. Our pulsar time delay results suggest a potential observable distinguishing feature of these astrophysical black hole geometries which could be useful for the forthcoming observational facilities such as Square Kilometer Array Observatory, Five-hundred-meter Aperture Spherical Telescope and Event Horizon Telescope.

gr-qc↗

PyGRO: a Python Integrator for General Relativistic Orbits

Advancement in recent years in the field of experimental gravitation has allowed to test the equivalence principle in regimes that were previously unexplored, allowing for unprecedented verifications of general relativity and also enabling tests of alternative theories of gravity. We introduce a new computational tool, PyGRO, with the aim of integrating numerically the geodesic equations for the trajectories of massive and massless test particles in any analytic four-dimensional space-time. The result is a modern, fast, open-source, highly customizable and user-friendly Python package to perform the numerical integration of the the geodesic equations. Combining symbolic and numerical calculations, PyGRO offers a variety of methods to obtain fully relativistic orbits with minimal intervention by the user, working in full generality with any user-given symbolic expression of the space-time metric tensor. We have tested PyGRO in an array of different scenarios, validating the methodology employed by successfully reproducing classical results from general relativity, which we report in this article.

gr-qc↗

Measuring the boson mass of fuzzy dark matter with stellar proper motions

Fuzzy Dark Matter (FDM) is among the most suitable candidates to replace WIMPs and to resolve the puzzling mystery of dark matter. A galactic dark matter halo made of these ultralight bosonic particles leads to the formation of a solitonic core surrounded by quantum interference patterns that, on average, reproduce a Navarro-Frenk-White-like mass density profile in the outskirts of the halo. The structure of such a core is determined once the boson mass and the total mass of the halo are set. We investigated the capability of future astrometric Theia-like missions to detect the properties of such a halo within the FDM model, namely the boson mass and the core radius. To this aim, we built mock catalogs containing three-dimensional positions and velocities of stars within a target dwarf galaxy. We exploited these catalogs using a Markov Chain Monte Carlo algorithm and found that measuring the proper motion of at least 2000 stars within the target galaxy, with uncertainty $σ_v \leq 3$ km/s on the velocity components, will constrain the boson mass and the core radius with 3\% accuracy. Furthermore, the transition between the solitonic core and the outermost NFW-like density profile could be detected with an uncertainty of 7\%. Such results would not only help to confirm the existence of FDM, but they would also be useful for alleviating the current tension between galactic and cosmological estimations of the boson mass, or demonstrating the need for multiple particles with a broad mass spectrum as naturally arise String Axiverse.

astro-ph.GA↗

Future prospects for measuring 1PPN parameters using observations of S2 and S62 at the Galactic Center

The Parameterized Post-Newtonian (PPN) formalism offers an agnostic framework for evaluating theories of gravity that extend beyond General Relativity. Departures from General Relativity are represented by a set of dimensionless parameters that, at the first order in the expansion, reduce to $β$ and $γ$, which describe deviations in spatial curvature and non-linear superposition effects of gravity, respectively. We exploit future observations of stars at the Galactic Center, orbiting the supermassive black hole Sagittarius A*, to forecast the ability to constrain the first-order PPN parameters $γ$ and $β$. We have generated a mock catalog of astrometric and spectroscopic data for S2, based on the Schwarzschild metric, simulating observations over multiple orbital periods with the GRAVITY and SINFONI instruments. Our analysis includes the effects of relativistic orbital precession and line-of-sight (LOS) velocity gravitational redshift. Since future data for S2 can provide constraints only on a linear combination of the PPN parameters $β$ and $γ$ we also analyzed the impact of future observations of the gravitational lensing for stars that pass closer in the sky to Sgr A*, like the known star S62, which can potentially provide tight constraints on the parameter $γ$, that alone regulates the amplitude of the astrometric deviations due to lensing. When combining lensing observations for S62, and the precise orbital tracking of S2, one obtains independent constraints on both $γ$ (with a potential precision as good as $\sim 1\%$) and $β$ (with a corresponding precision of $\sim 2\%$), providing a precision test of General Relativity and its extensions.

astro-ph.GA↗

Constraints on metric-affine gravity black holes from the stellar motion at the Galactic Center

We consider a static, spherically symmetric space-time with an electric field arising from a quadratic metric-affine extension of General Relativity. Such a space-time is free of singularities in the centre of the black holes, while at large distances it quickly boils down to the usual Reissner-Nordström solution. We probe this space-time metric, which is uniquely characterized by two length scales, $r_q$ and $\ell$, using the astrometric and spectroscopic measurements of the orbital motion of the S2 star around the Galactic Center. Our analysis constrains $r_q$ to be below $2.7M$ for values $\ell<120 AU$, strongly favouring a central object that resembles a Schwarzschild black hole.

gr-qc↗

Bounding the mass of ultralight bosonic Dark Matter particles with the motion of the S2 star around Sgr A*

Dark matter is undoubtedly one of the fundamental, albeit unknown, components of the standard cosmological model. The failure to detect WIMPs, the most promising candidate particle for cold dark matter, actually opens the way for the exploration of viable alternatives, of which ultralight bosonic particles with masses $\sim 10^{-21}$ eV represent one of the most encouraging. Numerical simulations have shown that such particles form solitonic cores in the innermost parts of virialized galactic halos that are supported by internal quantum pressure on characteristic $\sim$kpc de Broglie scales. In the Galaxy, this halo region can be probed by means of S-stars orbiting the supermassive black hole Sagittarius A* to unveil the presence of such a solitonic core and, ultimately, to bound the boson mass $m_ψ$. Employing a Monte Carlo Markov Chain algorithm, we compare the predicted orbital motion of S2 with publicly available data and set an upper bound $m_ψ\lesssim 3.2\times 10^{-19}$ eV on the boson mass, at 95 \% confidence level. When combined with other galactic and cosmological probes, our constraints help to reduce the allowed range of the bosonic mass to $(2.0 \lesssim m_ψ\lesssim 32.2)\times 10^{-20}$ eV, at the 95 \% confidence level, which opens the way to precision measurements of the mass of the ultralight bosonic dark matter.

gr-qc↗

Explaining the oblate morphology of dwarf spheroidals with Wave Dark Matter perturbations

We investigate whether the oblate, spheroidal morphology of common dwarf spheroidal galaxies (dSph) may result from the slow relaxation of stellar orbits within a halo of Wave Dark Matter ($ψ$DM) when starting from an initial disk of stars. Stellar orbits randomly walk over a Hubble time, perturbed by the pervasive "granular" interference pattern of $ψ$DM, that fully modulates the dark matter density on the de Broglie scale. Our simulations quantify the level of stellar disk thickening over the Hubble time, showing that distribution of stars is predicted to become an oblate spheroid of increasing radius, that plausibly accounts for the morphology of dSph galaxies. We predict a low level of residual rotation remains after a Hubble time at the 1-3 km/s level, depending on orientation, that compares with recent claims of rotation for some well studied local dSph galaxies. This steady internal dynamical evolution may be witnessed directly with JWST for well resolved dwarf galaxies, appearing more oblate with look back time and tending to small disks of young stars at high redshift.

astro-ph.GA↗

Testing space-time geometries and theories of gravity at the Galactic Center with pulsar's time delay

We developed a numerical methodology to compute the fully-relativistic propagation time of photons emitted by a pulsar in orbit around a massive compact object, like the supermassive black hole Sagittarius A* in the Galactic Center, whose gravitational field is described by a generic spherically symmetric space-time. Pulsars at the Galactic Center are usually regarded as the next major precision probe for theories of gravity, filling the current experimental gap between horizon-scale gravity tests and those at larger scales. We retain a completely general approach, which allows us to apply our code to the Schwarzschild space-time (by which we successfully validate our methodology) and to three different well-motivated alternatives to the standard black hole paradigm. The results of our calculations highlight departures spanning several orders of magnitudes in timing residuals, that are supposed to be detectable with future observing facilities like the Square Kilometer Array.

gr-qc↗

Constraining an Einstein-Maxwell-dilaton-axion black hole at the Galactic Center with the orbit of the S2 star

We derive new constraints on the dilaton parameter appearing in the spherically-symmetric black hole solution of Einstein-Maxwell-dilaton-axion gravity, by studying the geodesic motion of the S2 star in the Galactic Center. Einstein-Maxwell-dilaton-axion black holes represent a compelling alternative to the standard black hole paradigm in General Relativity. This theory emerges from the low energy effective action of the heterotic string theory and has been proven to predict peculiar observational features from the direct imaging of black hole shadows. At a fundamental level, Einstein-Maxwell-dilaton-axion includes additional electromagnetic, dilatonic and axionic fields coupled to the space-time metric. When considering charged non-rotating black hole solutions, the additional fields endow the metric with one extra parameter $b$, called dilaton parameter, that is theoretically bound to $0<b<M$. Using publicly available astrometric data for S2 we derive an upper bound on $b\lesssim 12M$ at 95% confidence level and we demonstrate that only including the measurement of the relativistic orbital precession for S2 is sufficient to reduce this bound to $b\lesssim 1.4M$ at the same confidence level. Additionally, using a mock data mimicking future observations of S2 with the GRAVITY interferometer, we show that improved astrometric precision can help further narrow down the allowed dilaton parameter range to $b\lesssim0.033M$ after monitoring the S2 orbit for one and a half period.

gr-qc↗