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Quentin Baghi

Publications and source records attributed to Quentin Baghi.

At least 19 recordsLinked to original sources
arXiv2026

An explicit and differentiable Wilson-Daubechies-Meyer transform for gravitational-wave data analysis

The Wilson-Daubechies-Meyer (WDM) time-frequency transform has been widely used in gravitational-wave astronomy, yet a self-contained, mathematically explicit reference for practitioners remains lacking. This is especially true for those wishing to adopt the transform in modern Python and JAX inference workflows. We present wdm_transform, an open-source Python package implementing the WDM wavelet-packet time-frequency transform, and document its mathematical foundations, statistical properties, and practical implementation for gravitational-wave data analysis. The package supplies NumPy and JAX backends, both transforms (forward and inverse) validated to floating-point precision, with the JAX backend enabling GPU-accelerated transforms of million-point data streams in tens of milliseconds. As a worked example, we verify that the WDM-domain likelihood reproduces frequency-domain posteriors for a resolved LISA galactic binary under a shared stationary noise model, confirming numerical equivalence of the two representations in that controlled setting. This work paves the way for systematic optimisation of WDM tilings, a particularly promising direction for the non-stationary noise, stochastic backgrounds, and data gaps anticipated in future detectors, and for direct comparisons with alternative time-frequency representations needed to meet the challenges of future gravitational-wave data analysis.

gr-qc↗
arXiv2026

Sachs-Wolfe effect as a smoking gun for cosmological gravitational wave backgrounds

The Sachs-Wolfe (SW) effect, arising from large-scale structures in the universe, modifies the frequencies of gravitational waves (GWs) sourced by a cosmological background. We show that for backgrounds with $\Omega_{\rm GW}\gtrsim 10^{-10}$, this effect imprints anisotropies and spectral distortions that can be detectable with a network of space-based interferometers (such as LISA + Taiji) and, if not taken into account, may bias the estimate of the theoretical model of the GW background. The effect is particularly enhanced in the high-frequency end of the spectrum. The SW-induced anisotropies and spectral distortions present in a GW background sourced at primordial times will correlate with the SW signature present in the CMB. Any detection of a cross-correlation between the GW anisotropies and the CMB at large scales is therefore a smoking gun for confirming the primordial nature of the background.

gr-qc↗
arXiv2026

The Doppler effect of the Milky Way rotation on LISA

The galactic background of gravitational waves (GWs) is expected to be anisotropic due to the spatial distribution and kinematics of sources in the Milky Way. In this work, we model the stellar density and velocity profiles of the Galaxy and compute the resulting GW spectrum as a function of direction. We account for the Doppler shift induced by the peculiar velocities of stars and the observer's motion. Using a Fisher matrix formalism, we forecast the ability of future detectors (e.g., LISA) to distinguish between models that include or neglect these kinematic effects. We find that if one does not take into account the rotation of the galaxy, the inference of the parameters describing the galactic background can suffer observable biases.

astro-ph.GA↗
arXiv2026

LISA science ground segment conventions

This document sets out the conventions used for data simulations, waveforms, and analysis pipelines within the Distributed Data Processing Centre (DDPC) of the Laser Interferometer Space Antenna (LISA). It can also be considered a best practice guide for all publications related to the LISA mission. Topics covered include time-to-frequency transformations, gravitational-wave source parametrization, the instrumental response to gravitational waves, time-delay interferometry, and reference frame definitions.

astro-ph.IM↗
arXiv2026

Statistics of time and frequency-averaged spectra in gravitational-wave background searches

Time series analysis from gravitational-wave detectors often relies on the assumption that time chunks, or frequency bins, are uncorrelated. We discuss the validity of this approximation in the context of searches for stochastic gravitational-wave backgrounds. We examine the impact of averaging over time and frequency, a reduction technique commonly employed to minimize the computational expense of likelihood evaluations. We introduce an analytical tool based on Fisher information to quantify the error in parameter inference arising from ignoring these effects. Finally, we address the issue of locally stationary processes and optimal time chunking.

gr-qc↗
arXiv2026

Effect of noise characterization on the detection of mHz stochastic gravitational waves

Pulsar timing arrays' hint for a stochastic gravitational-wave background (SGWB) leverages the expectations of a future detection in the millihertz band, particularly with the LISA space mission. However, finding an SGWB with a single orbiting detector is challenging: It calls for cautious modelling of instrumental noise, which is also mainly stochastic. It was shown that agnostic noise reconstruction methods provide robustness in the detection process. We build on previous work to include more realistic instrumental simulations and additional degrees of freedom in the noise inference model and analyze the impact of LISA's sensitivity to SGWBs. Particularly, we model the two main types of noise sources with separate transfer functions and power spectral density spline fitting. We assess the detectability bounds and their dependence on the flexibility of the noise model and on the prior probability, allowing us to refine previously reported results.

gr-qc↗
arXiv2024

Extraction of gravitational wave signals from LISA data in the presence of artifacts

The Laser Interferometer Space Antenna (LISA) mission is being developed by ESA with NASA participation. As it has recently passed the Mission Adoption milestone, models of the instruments and noise performance are becoming more detailed, and likewise prototype data analyses must as well. Assumptions such as Gaussianity, stationarity, and data continuity are unrealistic, and must be replaced with physically motivated data simulations, and data analysis methods adapted to accommodate such likely imperfections. To this end, the LISA Data Challenges have produced datasets featuring time-varying and unequal constellation armlength, and measurement artifacts including data interruptions and instrumental transients. In this work, we assess the impact of these data artifacts on the inference of Galactic Binary and Massive Black Hole properties. Our analysis shows that the treatment of noise transients and gaps is necessary for effective parameter estimation, as they substantially corrupt the analysis if unmitigated. We find that straightforward mitigation techniques can significantly if imperfectly suppress artifacts. For the Galactic Binaries, mitigation of glitches was essentially total, while mitigations of the data gaps increased parameter uncertainty by approximately 10%. For the Massive Black Hole binaries the particularly pernicious glitches resulted in a 30% uncertainty increase after mitigations, while the data gaps can increase parameter uncertainty by up to several times. Critically, this underlines the importance of early detection of transient gravitational waves to ensure they are protected from planned data interruptions.

gr-qc↗
arXiv2024

Exploring the no-hair theorem with LISA

In this study, we explore the possibility of testing the no-hair theorem with gravitational waves from massive black hole binaries in the frequency band of the Laser Interferometer Space Antenna (LISA). Based on its sensitivity, we consider LISA's ability to detect possible deviations from general relativity (GR) in the ringdown. Two approaches are considered: an agnostic quasi-normal mode (QNM) analysis, and a method explicitly targeting the deviations from GR for given QNMs. Both approaches allow us to find fractional deviations from general relativity as estimated parameters or by comparing the mass and spin estimated from different QNMs. However, depending on whether we rely on the prior knowledge of the source parameters from a pre-merger or inspiral-merger-ringdown (IMR) analysis, the estimated deviations may vary. Under some assumptions, the second approach targeting fractional deviations from GR allows us to recover the injected values with high accuracy and precision. We obtain $(5\%, 10\%)$ uncertainty on ($\delta \omega, \delta \tau)$ for the $(3,3,0)$ mode, and $(3\%, 17\%)$ for the $(4,4,0)$ mode. As each approach constrains different features, we conclude that combining both methods would be necessary to perform a better test. In this analysis, we also forecast the precision of the estimated deviation parameters for sources throughout the mass and distance ranges observable by LISA.

gr-qc↗
arXiv2024

Coronagraphic time-delay interferometry: characterization and updated geometric properties

The Laser Interferometer Space Antenna (LISA) will be a space-borne gravitational wave (GW) detector to be launched in the next decade. Central to LISA data analysis is time-delay interferometry (TDI), a numerical procedure which drastically reduces otherwise overwhelming laser frequency noise. LISA data analysis is usually performed on sets of TDI variables, e.g. Michelson variables $(X, Y, Z)$ or quasiorthogonal variables $(A, E, T)$. We investigate a less standard TDI variable denoted $\kappa$ which depends on time, or frequency, and two parameters $(\beta, \lambda)$. This so-called coronagraphic TDI variable has the singular property of canceling GW signal when $(\beta, \lambda)$ tend to the sky position of the GW source. Thanks to this property, coronagraphic TDI has the potential to be an efficient model-agnostic method for sky localization of GW sources with LISA. Those characteristics make it relevant for low-latency searches and a possible glitch veto. Although briefly discussed in the literature, coronagraphic TDI has only been tested on theoretical grounds. In this paper we validate the applicability of $\kappa$ to sky localization of typical LISA sources, namely Galactic binaries (GBs) and massive black hole binaries (MBHBs), when considering a simplified LISA instrument. The goal of this paper is to pave the way for applications of coronagraphic TDI to practical LISA data analysis problems.

gr-qc↗
arXiv2024

LISA Definition Study Report

The Laser Interferometer Space Antenna (LISA) is the first scientific endeavour to detect and study gravitational waves from space. LISA will survey the sky for Gravitational Waves in the 0.1 mHz to 1 Hz frequency band which will enable the study of a vast number of objects ranging from Galactic binaries and stellar mass black holes in the Milky Way, to distant massive black-hole mergers and the expansion of the Universe. This definition study report, or Red Book, presents a summary of the very large body of work that has been undertaken on the LISA mission over the LISA definition phase.

astro-ph.CO↗
arXiv2023

Astrophysics with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultracompact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

gr-qc↗
arXiv2023

Uncovering gravitational-wave backgrounds from noises of unknown shape with LISA

Detecting stochastic background radiation of cosmological origin is an exciting possibility for current and future gravitational-wave (GW) detectors. However, distinguishing it from other stochastic processes, such as instrumental noise and astrophysical backgrounds, is challenging. It is even more delicate for the space-based GW observatory LISA since it cannot correlate its observations with other detectors, unlike today's terrestrial network. Nonetheless, with multiple measurements across the constellation and high accuracy in the noise level, detection is still possible. In the context of GW background detection, previous studies have assumed that instrumental noise has a known, possibly parameterized, spectral shape. To make our analysis robust against imperfect knowledge of the instrumental noise, we challenge this crucial assumption and assume that the single-link interferometric noises have an arbitrary and unknown spectrum. We investigate possible ways of separating instrumental and GW contributions by using realistic LISA data simulations with time-varying arms and second-generation time-delay interferometry. By fitting a generic spline model to the interferometer noise and a power-law template to the signal, we can detect GW stochastic backgrounds up to energy density levels comparable with fixed-shape models. We also demonstrate that we can probe a region of the GW background parameter space that today's detectors cannot access.

gr-qc↗
arXiv2023

Gravitational waves from compact binaries to the fourth post-Newtonian order

The precise knowledge of the gravitational phase evolution of compact binaries is crucial to the data analysis for gravitational waves. Until recently, it was known analytically (for non-spinning systems) up to the 3.5 post-Newtonian (PN) order, i.e. up to the $(v/c)^7$ correction beyond the leading order quadrupole formula. Using a PN-multipolar-post-Minkowskian algorithm, we have pushed the accuracy to the next 4PN level. This derivation involved challenging technical issues, due to the appearance of non-physical divergences, which have to be properly regularized, as well as effects of non-linear multipole interactions.

gr-qc↗
arXiv2023

Fully data-driven time-delay interferometry with time-varying delays

Raw space-based gravitational-wave data like LISA's phase measurements are dominated by laser frequency noise. The standard technique to make this data usable for science is time-delay interferometry (TDI), which cancels laser noise terms by forming suitable combinations of delayed measurements. We recently introduced the basic concepts of an alternative approach which, unlike TDI, does not rely on independent knowledge of temporal correlations in the dominant noise. Instead, our automated Principal Component Interferometry (aPCI) processing only assumes that one can produce some linear combinations of the temporally nearby regularly spaced phase measurements, which cancel the laser noise. Then we let the data reveal those combinations. Our previous work relies on the simplifying additional assumption that the filters which lead to the laser-noise-free data streams are time-independent. In LISA, however, these filters will vary as the constellation armlengths evolve. Here, we discuss a generalization of the basic aPCI concept compatible with data dominated by a still unmodeled but slowly varying noise covariance. Despite its independence on any model, aPCI successfully mitigates laser frequency noise below the other noises' level, and its sensitivity to gravitational waves is the same as the state-of-the-art second-generation TDI, up to a 2\% error.

gr-qc↗
arXiv2023

On the detectability of higher harmonics with LISA

Supermassive black hole binaries (SMBHBs) are expected to be detected by the future space-based gravitational-wave detector LISA with a large signal-to-noise ratio (SNR). This prospect enhances the possibility of differentiating higher harmonics in the inspiral-merger-ringdown (IMR) waveform. In this study, we test the ability of LISA to identify the presence of different modes in the IMR waveform from a SMBHB. We analyze the contribution of each mode to the total SNR for different sources. Higher modes, in particular the mode $(3, 3)$ and $(4, 4)$, can dominate the signal observed through the LISA detector for SMBHB of the order of $10^8 M_\odot$. With Bayesian analysis, we can discriminate models with different harmonics. While spherical harmonics are often considered orthogonal, we observe it is not the case in the merger-ringdown phase observed by LISA. Omitting harmonics not only diminishes the SNR but can also lead to biased parameter estimates. We analyze the bias for each model in a source example and quantify the threshold SNR where we can expect the parameter bias to be comparable to the statistical error. By computing the waveform model error with the Fisher approximation and comparing it with the posterior distribution from our sampler results, we can evaluate the veracity of the analytical bias, which converges with the sampler results as more harmonics are introduced. To conclude, SMBHB events with SNR of a few hundred, as expected in LISA, are required to use templates with at least modes $(2, 2)$, $(2, 1)$, $(3, 3)$, $(3, 2)$, $(4, 4)$, $(4, 3)$ to estimate all intrinsic parameters correctly. Our work highlights the importance of higher modes to describe the gravitational waveform of events detected by LISA.

gr-qc↗
arXiv2022

Result of the MICROSCOPE Weak Equivalence Principle test

The space mission MICROSCOPE dedicated to the test of the Equivalence Principle (EP) operated from April 25, 2016 until the deactivation of the satellite on October 16, 2018. In this analysis we compare the free-fall accelerations ($a_{\rm A}$ and $a_{\rm B}$) of two test masses in terms of the Eötvös parameter $η({\rm{A, B}}) = 2 \frac{a_{\rm A}- a_{\rm B}}{a_{\rm A}+ a_{\rm B}}$. No EP violation has been detected for two test masses, made from platinum and titanium alloys, in a sequence of 19 segments lasting from 13 to 198 hours down to the limit of the statistical error which is smaller than $10^{-14}$ for $ η({\rm{Ti, Pt}})$. Accumulating data from all segments leads to $η({\rm{Ti, Pt}}) =[-1.5\pm{}2.3{\rm (stat)}\pm{}1.5{\rm (syst)}] \times{}10^{-15}$ showing no EP violation at the level of $2.7\times{}10^{-15}$ if we combine stochastic and systematic errors quadratically. This represents an improvement of almost two orders of magnitude with respect to the previous best such test performed by the Eöt-Wash group. The reliability of this limit has been verified by comparing the free falls of two test masses of the same composition (platinum) leading to a null Eötvös parameter with a statistical uncertainty of $1.1\times{}10^{-15}$.

gr-qc↗
arXiv2022

MICROSCOPE mission: final results of the test of the Equivalence Principle

The MICROSCOPE mission was designed to test the Weak Equivalence Principle (WEP), stating the equality between the inertial and the gravitational masses, with a precision of $10^{-15}$ in terms of the Eötvös ratio $η$. Its experimental test consisted of comparing the accelerations undergone by two collocated test masses of different compositions as they orbited the Earth, by measuring the electrostatic forces required to keep them in equilibrium. This was done with ultra-sensitive differential electrostatic accelerometers onboard a drag-free satellite. The mission lasted two and a half years, cumulating five-months-worth of science free-fall data, two thirds with a pair of test masses of different compositions -- Titanium and Platinum alloys -- and the last third with a reference pair of test masses of the same composition -- Platinum. We summarize the data analysis, with an emphasis on the characterization of the systematic uncertainties due to thermal instabilities and on the correction of short-lived events which could mimic a WEP violation signal. We found no violation of the WEP, with the Eötvös parameter of the Titanium and Platinum pair constrained to $η({\rm Ti, Pt})~=~ [-1.5 \pm 2.3~{\rm (stat)} \pm 1.5~{\rm (syst)}]~\times 10^{-15}$ at $1σ$ in statistical errors.

gr-qc↗
arXiv2022

A statistical inference approach to time-delay interferometry for gravitational-wave detection

The future space-based gravitational wave observatory LISA will consist of a constellation of three spacecraft in a triangular constellation, connected by laser interferometers with 2.5 million-kilometer arms. Among other challenges, the success of the mission strongly depends on the quality of the cancellation of laser frequency noise, whose power lies eight orders of magnitude above the gravitational signal. The standard technique to perform noise removal is time-delay interferometry (TDI). TDI constructs linear combinations of delayed phasemeter measurements tailored to cancel laser noise terms. Previous work has demonstrated the relationship between TDI and principal component analysis (PCA). We build on this idea to develop an extension of TDI based on a model likelihood that directly depends on the phasemeter measurements. Assuming stationary Gaussian noise, we decompose the measurement covariance using PCA in the frequency domain. We obtain a comprehensive and compact framework that we call PCI for "principal component interferometry," and show that it provides an optimal description of the LISA data analysis problem.

gr-qc↗