Session G05: LISA Data Analysis

Quasimonochromatic LISA Sources in the Frequency Domain

Among the binary sources of interest for LISA some are quasimonochromatic, in the sense that the change in the gravitational wave frequency △f is on the order of or smaller than 1 yr^-1 during the observation time. For example, most Galactic double white dwarfs fall into this category. For these sources, we revisit the stationary phase approximation (SPA) commonly used in Fisher matrix calculations in the frequency domain and show how it is modified by the Doppler shift induced by LISA's motion and by the LISA pattern functions. We compare our results with previous work in the time domain and discuss the transition from the quasimonochromatic case to the conventional SPA which applies in the opposite limit, i.e. △f is on the order of or higher than 1 yr^-1.   

Identifying loud gravitational-wave bursts from galactic black hole binaries with LISA

Stellar-mass black hole binaries (BHBs) in galactic nuclei are gravitationally perturbed by the central supermassive black hole (SMBH) of the host galaxy, potentially inducing binary eccentricity oscillations detectable by the Laser Interferometer Space Antenna (LISA), a planned space-based gravitational-wave (GW) detector. These highly eccentric binaries emit GWs during each pericenter passage, producing a train of short GW bursts. In this work, we consider the scenario in which multiple BHBs orbit our galaxy's SMBH, Sgr A*, and subsequently undergo eccentricity oscillations. We simulate the emitted GWs in the time domain and add the resulting signal to synthetic LISA data. We analyze the bursts in time-frequency space using a sine-Gaussian wavelet decomposition, which yields the timing and signal-to-noise ratio (SNR) of each burst event. We further demonstrate how individual binaries can be distinguished based on their orbital periods by searching for periodicities in the event times. This timing information can then be passed to a wavelet reconstruction algorithm for the purposes of fitting and subtracting these sources from LISA data.   

Searches for Gravitational Waves in LISA Data

When the Laser Interferometer Space Antenna, LISA, launches in the mid-2030s, it will enable the first multi-band gravitational-wave searches and trigger advance searches for multi-messenger counterparts beginning weeks or longer before merger. Compact binary sources must be extracted from LISA data at relatively faint signal-to-noise to perform these searches. While past work has considered parameter estimation for individual gravitational-wave sources in LISA, the actual searches for faint sources have thus far proven difficult, especially for stellar-origin black hole binaries. In this talk, I will present new results in enabling the search phase of LISA data analysis.   

Discovering neutron stars with LISA via measurements of orbital eccentricity in Galactic binaries

LISA will detect 10⁴ Galactic binaries, the majority being double white dwarfs. However, approximately 1-5% of these systems will contain neutron stars (NSs) which, if they can be correctly identified, will provide new opportunities for studying binary evolution pathways as well as being promising targets for multi-messenger observations. Eccentricity, expected from NS natal kicks, will be a key identifying signature for binaries containing a NS. Eccentric binaries radiate at widely-spaced frequency harmonics that must first be identified as originating from a single source and then analysed coherently. In this work we use a multi-harmonic heterodyning approach to perform Bayesian parameter estimation on a range of simulated eccentric LISA signals. This is used to investigate LISA's ability to measure orbital eccentricity, to demonstrate how eccentricity and periastron precession help to break the mass degeneracy (allowing the individual component masses to be inferred), and to investigate the possibility of source misidentification when the individual harmonics of an eccentric binary masquerade as separate circular binaries. The broader implications of this for the ongoing design of the LISA global fit are also discussed.   

Incorporating distance information to a LISA Ultra-Compact Binary data pipeline

The Laser Interferometer Space Antenna (LISA) mission is expected to detect many Ultra-Compact Binaries (UCBs) in the Milky Way. In order to link these observations to existing models of star formation and evolution, the spatial distribution of UCBs is an important target for LISA data analysis. GBMCMC is a component of the Global Lisa Analysis Software Suite (GLASS) which identifies UCBs in simulated LISA data as part of a global fit pipeline. I will present modifications to GBMCMC's trans-dimensional Markov chain Monte Carlo algorithm. These modifications incorporate a Bayesian prior on the distance to UCBs from Earth using a simple model of the Milky Way.   

Towards an Early Warning Search for LISA Massive Black Holes

The space based laser interferometer LISA, expected to be operational in the next decades, will be able to probe the millihertz frequency band. This will make it sensitive to a vast array of compact object mergers, including the massive black holes or MBHs. These black holes, straddling the intermediate and supermassive types of black holes, have masses extending above a minimum of 1000 solar mass. They are expected to be observable within the LISA band for several weeks to months before they merge. This makes them excellent candidates for low latency, pre-merger observations. Also, some mergers of MBHs are expected to have electromagnetic counterparts due to the presence of gas or disks. Pre-merger alerts with sky location information from LISA data analysis sent out to the astronomy community, would enable early detections of such mergers in multiple electromagnetic bands. Such multimessenger observations stand to further our knowledge of astrophysics, including that of black hole formation and evolution. In my talk we explore the possibility of sending out such pre-merger alerts, using a simulated LISA training data set. We also include some updates and preliminary test results using such data.   

The Gravitational Wave Peep and Its Implication for LISA Signal Confusion Noise

Scattering events around the center of massive galaxies will occasionally toss a stellar-mass compact object into an orbit around the massive black hole (MBH) at the center, beginning an extreme mass ratio inspiral (EMRI). The early stages of such a highly eccentric orbit are not likely to produce detectable gravitational waves, as the source will only be in a suitable frequency band briefly when it is close to periapsis during each long-period orbit. This repeated burst of emission, firmly in the millihertz band, is the gravitational wave peep. While a single peep is not likely to be detectable, if we consider an ensemble of such subthreshold sources, spread across the universe, together they may produce an unresolvable background noise that could obscure sources otherwise detectable by LISA. Previous studies of EMRI signal confusion noise focused either on parabolic orbits near the massive black hole or events close to merger. We seek to improve this characterization by implementing numerical kludge waveforms that can calculate highly eccentric orbits. Our focus is on orbits at the point of capture that are less likely to be detectable on their own but will otherwise contribute to the background. We then use estimations of EMRI capture parameters along with tracking the MBH population from a redshift of z=0 to z=3 using the Illustris Project. This information is combined with an estimate of the number of EMRI mergers per unit volume to obtain the number of events contributing to the signal confusion noise.   

A weakly-parametric approach to stochastic background inference in LISA

Detecting stochastic gravitational wave backgrounds (SGWBs) with The Laser Interferometer Space Antenna (LISA) is one of the mission's scientific objectives.

Disentangling SGWBs of astrophysical and cosmological origin is a challenging task, further complicated by the noise level uncertainties.

In this study, we present a Bayesian methodology for inferring SGWBs, drawing inspiration from Gaussian stochastic processes.

We assess the effectiveness of this approach for signals with unknown spectral shapes by systematically exploring the model hyperparameters—a preliminary step towards a more efficient transdimensional exploration.

To validate our method, we apply it to a representative astrophysical scenario: the inference of the astrophysical background of Extreme Mass Ratio Inspirals. Our findings indicate that the algorithm is capable of recovering the injected signal even with uninformative priors, simultaneously providing an estimate of the noise level.

Resolving Structure in the Stochastic Gravitational-Wave Power Spectrum with LISA Using Non-parametric Methods

The Laser Interferometer Space Antenna (LISA) is an upcoming space-based detector designed to study gravitational waves (GWs) in the millihertz frequency range. A particularly numerous source of GWs at these frequencies are Galactic compact binaries, especially binaries with one or two white dwarf stars. While a small percentage of such binaries will be individually resolved by LISA, the GWs from most of them will form a stochastic foreground that will be loud and detectable. In this study, we develop Bayesian non-parametric methods based on autoregressive (AR) processes to map the power spectrum of the GW foreground from Galactic Double White Dwarfs (DWDs) and Cataclysmic Variables (CVs). We validate the non-parametric AR model through a series of simulations and analyses, including the recovery of GW foregrounds from simulated DWD and CV populations. We hierarchically inferred the parameters of the AR model using a Bayesian inference pipeline for LISA (BLIP). The flexibility of the AR process will allow us to probe complex structures in the spectrum that deviate from simple power law models. Non-parametric methods will be crucial for studying the stochastic foreground from a combination of source categories and extracting population properties of Galactic compact objects.