De LISA Pathfinder à LISA: Élaboration d’un simulateur dynamique pour la mission spatiale eLISA

The gravitational Universe and the rise of an entirely new astronomy using gravitational waves have been selected by ESA as the scientific theme for the future large space mission L3 planned for 2030 decade. In that context, eLISA (evolved Laser Interferometer Space Antenna) mission seems favored, and is besides preceded by the LISA Pathfinder mission, launched on December 2nd 2015, that aims to demonstrate the technology envisaged. The scientific outcome of this mission would be very wide, concerning at the same time fundamental physics, cosmology and astrophysics.The work described in this document allows to deliver a simulator reproducing the closed-loop dynamical behavior of a three-body system : the spacecraft and the two test-masses. The simulator is based on a Space State Model of the dynamical system, of the measurement and actuation systems and includes numerous modelled imperfections such as sensor noises and stray forces applied on the test masses (the inertial references of the system), hence allowing to study the impact of these imperfections on the detector performances. In particular, this simulation tool allows to perform preliminary estimates of a fundamental quantity, the residual acceleration, that will limit the sensitivity of eLISA to low frequency gravitational waves.L’Univers gravitationnel et l’émergence d’une nouvelle astronomie via les ondes gravitationnelles a été sélectionné par l’ESA en tant que thème scientifique de la future mission large L3 prévue pour l'horizon 2030. A ce titre, la mission spatiale eLISA (evolved Laser Interferometer Space Antenna) semble privilégiée, et précédée par ailleurs par une mission de démonstration technologique, LISA Pathfinder, lancée le 3 décembre 2015. L’impact scientifique d’une telle mission serait ainsi multiple et concernerait à la fois les domaines de la physique fondamentale, de la cosmologie et de l’astrophysique.L’objet de ce document est l’élaboration d’un simulateur reproduisant la dynamique en boucle fermée du système Satellite-Masses d’épreuve. Ce simulateur se base sur un modèle en espace d’états du système dynamique et des systèmes de mesure et d’actuation, et inclut également plusieurs modèles d’imperfection du dispositif, tels que le bruit des senseurs ou les forces parasites s’appliquant sur les masses d’épreuve (références d’inertie du système), permettant ainsi d’étudier l’impact de ces imperfections sur les performances du détecteur. En particulier, cet outil de simulation donne accès à des premières estimations d’une quantité d’intérêt majeur, le bruit d’accélération, qui limitera la sensibilité à basse fréquence du détecteur aux ondes gravitationnelles

Data series subtraction with unknown and unmodeled background noise

LISA Pathfinder (LPF), ESA's precursor mission to a gravitational wave observatory, will measure the degree to which two test-masses can be put into free-fall, aiming to demonstrate a residual relative acceleration with a power spectral density (PSD) below 30 fm/s$^2$/Hz$^{1/2}$ around 1 mHz. In LPF data analysis, the measured relative acceleration data series must be fit to other various measured time series data. This fitting is required in different experiments, from system identification of the test mass and satellite dynamics to the subtraction of noise contributions from measured known disturbances. In all cases, the background noise, described by the PSD of the fit residuals, is expected to be coloured, requiring that we perform such fits in the frequency domain. This PSD is unknown {\it a priori}, and a high accuracy estimate of this residual acceleration noise is an essential output of our analysis. In this paper we present a fitting method based on Bayesian parameter estimation with an unknown frequency-dependent background noise. The method uses noise marginalisation in connection with averaged Welch's periodograms to achieve unbiased parameter estimation, together with a consistent, non-parametric estimate of the residual PSD. Additionally, we find that the method is equivalent to some implementations of iteratively re-weighted least-squares fitting. We have tested the method both on simulated data of known PSD, and to analyze differential acceleration from several experiments with the LISA Pathfinder end-to-end mission simulator.Comment: To appear Phys. Rev. D90 August 201

Laser Interferometer Space Antenna

Following the selection of The Gravitational Universe by ESA, and the successful flight of LISA Pathfinder, the LISA Consortium now proposes a 4 year mission in response to ESA's call for missions for L3. The observatory will be based on three arms with six active laser links, between three identical spacecraft in a triangular formation separated by 2.5 million km. LISA is an all-sky monitor and will offer a wide view of a dynamic cosmos using Gravitational Waves as new and unique messengers to unveil The Gravitational Universe. It provides the closest ever view of the infant Universe at TeV energy scales, has known sources in the form of verification binaries in the Milky Way, and can probe the entire Universe, from its smallest scales near the horizons of black holes, all the way to cosmological scales. The LISA mission will scan the entire sky as it follows behind the Earth in its orbit, obtaining both polarisations of the Gravitational Waves simultaneously, and will measure source parameters with astrophysically relevant sensitivity in a band from below $10^{-4}\,$Hz to above $10^{-1}\,$Hz.Comment: Submitted to ESA on January 13th in response to the call for missions for the L3 slot in the Cosmic Vision Programm

MICROSCOPE mission: first results of a space test of the equivalence principle

According to the weak equivalence principle, all bodies should fall at the same rate in a gravitational field. The MICROSCOPE satellite, launched in April 2016, aims to test its validity at the 10−15 precision level, by measuring the force required to maintain two test masses (of titanium and platinum alloys) exactly in the same orbit. A nonvanishing result would correspond to a violation of the equivalence principle, or to the discovery of a new long-range force. Analysis of the first data gives δ(Ti,Pt)=[−1±9(stat)±9(syst)]×10−15 (1σ statistical uncertainty) for the titanium-platinum Eötvös parameter characterizing the relative difference in their free-fall accelerations

Optimal design of calibration signals in space borne gravitational wave detectors

International audienceFuture space-borne gravitational wave detectors will require a precise definition of calibration signals to ensure the achievement of their design sensitivity. The careful design of the test signals plays a key role in the correct understanding and characterization of these instruments. In that sense, methods achieving optimal experiment designs must be considered as complementary to the parameter estimation methods being used to determine the parameters describing the system. The relevance of experiment design is particularly significant for the LISA Pathfinder mission, which will spend most of its operation time performing experiments to characterize key technologies for future space-borne gravitational wave observatories. Here we propose a framework to derive the optimal signals—in terms of minimum parameter uncertainty—to be injected into these instruments during the calibration phase. We compare our results with an alternative numerical algorithm which achieves an optimal input signal by iteratively improving an initial guess. We show agreement of both approaches when applied to the LISA Pathfinder case