The direct detection of gravitational waves (GWs) by LIGO and Virgo has opened a new observational window on the strong-field regime of gravity, almost unexplored so far. This regime forges the arena in which binaries composed by black holes (BHs) and neutron stars (NSs) evolve, acting as physics laboratories to test the rich phenomenology exhibited by fundamental fields and particles. Indeed, GWs emitted by coalescing systems carry specific signatures which can be used to trace back the structure of the binary components, and to distinguish among different compact sources. This is particularly relevant as it has been recently proposed that BHs may not represent the (only) endpoint of stellar evolution, and that other Exotic Compact Objects (ECOs) may populate the Universe and be potentially detectable by ground and space borne detectors, as the LISA satellite to be launched in 2034. Among the variety of astrophysical sources targeted by LISA, Extreme Mass Ratio Inspirals (EMRIs), i.e. binary systems in which a stellar-mass compact object orbits around a supermassive body, represent unique binaries to test fundamental physics. During the inspiral, the small source follows thousands of cycles before the plunge and the emitted GWs, which are continuously observable, allow to build a detailed map of the binary's spacetime. Most of the studies so far assumes that the EMRI central object is a Kerr BH. In this project we propose to make a step further and to study the GW emission by systems in which the massive object is a Boson Star (BS), a model of ECO which has received considerable attention in literature. The main goal of our proposal is to provide a consistent description of EMRI beyond the standard BH scenario, modelling their orbital evolution and GW-emission. We aim to provide ready-to-use waveforms that can be used, together with the large amount of data expected for LISA, to test the nature of compact objects and potentially unveil the existence of new physics.
Gravitational waves harbor the unique potential to characterize astrophysical events which may not emit electromagnetic radiation, or hardly interact with it.
Most of the analysis carried out so far on the possibility to detect boson stars with GWs have focused on equal mass binaries and on the signatures left on the inspiral phase of the coalescence due to the response of each body to the gravitational field of the companion. This includes the study of tidal effects, and of generic deformations of the multipolar structure of the binary components [Phys. Rev. D, 96, 024002 (2017)]. In this regard, M. Vaglio and collaborators have recently demonstrated how combining multiple effects from the inspiral signal can lead to strong constraints on the mass and the coupling of the BS scalar field [Phys.Rev. D 102, 083002 (2020)]. Study in the context of asymmetric binaries with large mass ratios has received less attention so far. Works on BS perturbations have mostly focused on the calculations of the stellar modes of oscillations. Few efforts have been devoted to investigate the GW emission from Extreme Mass Ratio Inspirals in which the primary is a boson star. For secondaries on circular orbits, the only known example considers perturbations around non-rotating spherically symmetric spacetimes, which represent a crude approximation of realistic scenarios. Indeed, conservation of angular momentum leads BSs to acquire a non-vanishing spin as a result of the formation process or of the interaction with other astrophysical objects. The spin is also crucial to determine the multipolar structure of the BS spacetime which then determines the response to external perturbations and encodes specific information that allow to distinguish from the case of spinning black holes.
Moreover, in realistic EMRIs astrophysical scenarios, the secondary is expected to follow inclined and eccentric orbits around the supermassive object. Given the complexity of the problem, we will start developing the formalism for equatorial circular trajectories, adding the eccentricity afterwards, and finally focusing on non-equatorial orbits.
Therefore, we expect that the research project we propose will add a significant physical contribution and provide new insights on our understanding of the evolution of these systems.