arXiv subjects

Arianna Foschi

Publications and source records attributed to Arianna Foschi.

7 recordsLinked to original sources

arXiv2026

Effects of a disk structure on stellar motion at the Galactic Center

Stellar orbits are key for probing the environment of the supermassive black hole at the Galactic Center, Sagittarius A$^*$. So far, the mass around SgrA$^*$ has been assumed to be spherically distributed. However, the extended mass may instead be flattened, creating disk-like structures. We investigate the effects that a thin disk structure would have on stars at the Galactic Center, focusing on star S2 and S301 and the clockwise stellar disk. We derive analytically the acceleration exerted by a disk with power law density $\Sigma \propto r^{-\gamma}$. We use this acceleration to compute the osculating equations and the variations of the orbital elements, showing how the latter depend on the orientation of the disk with respect to the orbital plane. We find that the disk structure induces a secular shift in the semi-latus rectum, an extra in-plane precession and an out-of-plane precession. The former is neither present at the low-order post Newtonian description that we use for the black hole, nor when a spherical mass distribution is considered. The latter can be competitive with the Lense-Thirring precession induced by the spin of SgrA$^*$ on S301 motion, depending on the mass, the radial extent and the orientation of the disk. Since the Lense-Thirring precession is negligible in S2 motion, the out-of-plane precession can be used to place upper limits on the non luminous mass of disk-like structures at the Galactic Center. The limits might significantly differ from those obtained for spherical distributions and depend on the disk parameters. These results highlight the importance of constraining disk-like structures when using stellar orbits to probe the central black hole, in particular its spin. Once mass estimates are at hand, one can quantify the disk's effect on S301 motion and the resulting degeneracy with a future measurement of SgrA$^*$ spin.

astro-ph.GA↗

arXiv2026

Using VLTI/GRAVITY+ to determine the identity of a third planet candidate in the PDS 70 system

Detections of protoplanets are rare and protoplanetary disk features mischaracterized as planets are common. PDS 70 is one of only two stars known to host multiple confirmed protoplanets, PDS 70 b and c, and repeat detections of a third point-like source in the system suggest the presence of third inner planet. However, previous observations of this third source are insufficient to distinguish whether it is a planet or a concentrated dust clump in Keplerian motion. Our observations with VLTI/GRAVITY+ did not re-detect this point-like source, suggesting that it is, in fact, a dust clump and not a planet. These observations demonstrate how the angular resolving power of VLTI/GRAVITY+ can be used to distinguish between protoplanets and protoplanetary disk features.

astro-ph.EP↗

arXiv2024

Improving constraints on the extended mass distribution in the Galactic Center with stellar orbits

Studying the orbital motion of stars around Sagittarius A* in the Galactic Center provides a unique opportunity to probe the gravitational potential near the supermassive black hole at the heart of our Galaxy. Interferometric data obtained with the GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) since 2016 has allowed us to achieve unprecedented precision in tracking the orbits of these stars. GRAVITY data have been key to detecting the in-plane, prograde Schwarzschild precession of the orbit of the star S2, as predicted by General Relativity. By combining astrometric and spectroscopic data from multiple stars, including S2, S29, S38, and S55 - for which we have data around their time of pericenter passage with GRAVITY - we can now strengthen the significance of this detection to an approximately $10 \sigma$ confidence level. The prograde precession of S2's orbit provides valuable insights into the potential presence of an extended mass distribution surrounding Sagittarius A*, which could consist of a dynamically relaxed stellar cusp comprised of old stars and stellar remnants, along with a possible dark matter spike. Our analysis, based on two plausible density profiles - a power-law and a Plummer profile - constrains the enclosed mass within the orbit of S2 to be consistent with zero, establishing an upper limit of approximately $1200 \, M_\odot$ with a $1 \sigma$ confidence level. This significantly improves our constraints on the mass distribution in the Galactic Center. Our upper limit is very close to the expected value from numerical simulations for a stellar cusp in the Galactic Center, leaving little room for a significant enhancement of dark matter density near Sagittarius A*.

astro-ph.GA↗

arXiv2023

Fundamental Physics Opportunities with the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT), which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable.

astro-ph.HE↗

arXiv2021

Geodesic structure and quasinormal modes of a tidally perturbed spacetime

Tidal perturbations play an important role in the study of the dynamics in the classical two-body system. Understanding tidal effects in strong-field regions may allow one to use gravitational-wave or electromagnetic observations to locate or constraint the location of possible companions. Here, we investigate how timelike and null geodesics of a Schwarzschild black hole are affected in the presence of a companion. There is a panoply of new effects. In some limiting cases, we find analytical solutions for closed null or timelike geodesics. Our results show that light ring period as measured by a far-away observer can be eiter shorter or longer, depending on the location of the companion. We also show that there are closed lightlike trajectories which are elliptic (for equatorial companions), and that timelike particles are affected in a similar manner. Finally, we attempt at estimating the ringdown from tidally perturbed geometries. Our results indicate that there are two stages in the relaxation of such geometries, one associated with a prompt decay of waves around the deformed photonsphere, and a later relaxation of the global geometry. These results are consistent with previous, full numerical studies.

gr-qc↗

arXiv2021

The light ring and the appearance of matter accreted by black holes

The geometry of black hole spacetimes can be probed with exquisite precision in the gravitational-wave window, and possibly also in the optical regime. We study the accretion of bright spots -- objects which emit strongly in the optical or in gravitational waves -- by non-spinning black holes. The emission as the object falls down the hole is dominated by photons or gravitons orbiting the light ring, causing the total luminosity to decrease exponentially as ${\cal L}_o\sim e^{-t/(3\sqrt{3}\,M)}$. Late-time radiation is blueshifted, due to its having been emitted during the infall, trapped at the light ring and subsequently re-emitted. These universal properties are a clear signature of the existence of light rings in the spacetime, and not particularly sensitive to near horizon details.

gr-qc↗

arXiv2020

Collective scalarization or tachyonization: when averaging fails

Certain scalar-tensor theories of gravity provide negative-energy, tachyonic modes to a fundamental scalar inside matter, giving rise to non-perturbative phenomena around compact stars. Studies of this and other tachyonic instabilities always average over local matter properties. We use elementary, flat space models to understand possible collective effects and the accuracy of the averaging procedure. In particular, we consider bodies made of elementary constituents which do not, in isolation, scalarize because their compactness ${\cal C}$ is too small, ${\cal C}\lesssim {\cal C}_{\rm crit}$. We show that when the individual constituents have compactness smaller but close to the threshold, one is able to scalarize composite bodies through collective effects, and the compactness of the composite body can be made arbitrarily small. On the other hand, our results suggest that when the fundamental building blocks have very low compactness, then scalarization of the composite body requires a global compactness ${\cal C}_{\rm global}\gtrsim {\cal C}_{\rm crit}$. Thus, our results rule out scalarization of dilute bodies via collective effects.

gr-qc↗