LISA will be the first mission of its kind, producing a unique data set with the potential to provide insight into many areas of astrophysics, fundamental physics, and cosmology. Realizing LISA's science potential will require advance work to better understand LISA's science targets and the specific ways in which LISA data can be used to understand them. To this end, NASA has offered the LISA Preparatory Science (LPS) Program to support US-based researchers to conduct research activities related to LISA.
Future solicitations for the LISA Preparatory Science program will be announced on NASA NSPIRES.
Abstracts for LPS Awards in prior rounds can be found below.
The LISA observatory will produce a rich data set that contains signals from millions of overlapping sources immersed in complex, time varying instrument noise with numerous gaps and disturbances. Separating the signals from each other and from the noise is a challenging problem that has to be solved for the mission to achieve its science goals. We have developed the first global fit analysis pipeline that is capable of simultaneously extracting the signals of tens of thousands of galactic binaries and dozens of massive black hole binaries, while also modeling the instrument noise. The effectiveness of this prototype global fit algorithm has been demonstrated on simulated data as part of the LISA Data Challenge effort. In addition, we have developed wavelet based time-frequency analysis techniques that dramatically speed up the analysis and can handle non-stationary noise and gaps in the data. These techniques have been used to produce an analysis that can efficiently characterize stellar origin black holes. The current global fit pipeline works in the frequency domain, and does not account for gaps in the data or fluctuations in the instrument noise level or the time variation from the unresolved galactic binary signals. Nor does it include signals from stellar origin black holes or extreme mass ratio inspirals. The goal of this proposal is to bring all these elements together to produce a global fit analysis that can detect and characterize galactic binaries, massive black hole binaries, extreme mass ratio inspirals and stellar origin black holes in a fast wavelet domain analysis that also handles gaps and time varying noise sources. The key technical elements are trans dimensional Bayesian inference and wavelet compression of the likelihood function.
Ultracompact binaries are systems containing degenerate stars with orbital periods less than one hour. Millions of ultracompact binaries are predicted to exist within the Galaxy emitting gravitational waves (GW) at mHz frequencies. Double white dwarfs will be the most numerous LISA sources known. Tens of thousands of ultracompact binaries will be individually resolved by LISA, while the rest blend together to form a confusion-limited foreground that is the dominant source of noise in the LISA band between 0.4 mHz and 2 mHz. The catalog of resolvable binaries, as well as the spectral shape of the confusion noise, will contain a trove of new information about the formation and evolution of compact binaries in the galaxy, and the complicated physical interactions between binaries including mass transfer, common envelop interaction, tides, and the internal physics of compact stellar remnants.
LISA poses novel data analysis challenges due to the overwhelmingly large number of astrophysical sources and long duration (months to years) of detectable signals, resulting in non- negligible overlaps in time and frequency between sources. Robust detection and characterization of the numerous gravitational wave sources in LISA data can not be done sequentially, but rather through a simultaneous global fit of a data model containing the full suite of astrophysical and instrumental features present in the data. However, previous analyses on the detectability of ultracompact binaries have focused on individual sources in isolation, or relatively simple models of foreground confusion.
Here we propose to perform, for the first time, an analysis that combines real EM priors from observed ultracompact binaries with a proper treatment of parameter correlations and noise from overlapping sources in the LISA global fit analysis. We start with a detailed analysis of the observed ultracompact binary population to characterize the expected GW detection of these binaries. We will then perform a LISA global fit analysis containing combined populations from ultracompact binaries, a population of massive black hole mergers, and instrument noise model.
We propose to address two scientific goals in this proposal:
1. Characterize the population of known ultracompact binary white dwarfs with existing EM observations. We propose to perform a detailed analysis of the photometric, spectroscopic, and astrometric data available on each ultracompact binary white dwarf system to constrain the physical properties of each binary, including the radii and masses of the primary and secondary stars, the inclination, and the distance.
2. Study the LISA global fit analysis on the ultracompact binary white dwarf population to constrain the detectability of each system. We will use the simulated data from the second round of the LISA Data Challenges, which includes Gaussian detector noise, a simulated population of Milky Way ultracompact binaries, and a population of merging massive black hole binaries. We will incorporate precise EM priors and demonstrate LISA observations of known ultracompact binaries with or without prior EM information. The results have implications for the number of LISA verification binaries, detectable binaries, and the confusion noise in the global fit.
Direct-collapse black holes (DCBHs) are an important component of the massive black hole population of the early universe, and their formation and early mergers will be prominent in the data stream of the Laser Interferometer Space Antenna (LISA). However, the population and binary properties of these early black holes are poorly understood, with masses, mass ratios, spins, and orbital eccentricities strongly dependent on the details of their formation, and the properties of the remaining exterior material (baryonic and non-baryonic), which may be substantial to the point of merger.
We propose to simulate the formation, collapse, and/or merger of such DCBH regions in order to extract the resulting gravitational-wave signals, and to assess the ability of LISA to detect and identify them.
Our approach for this work will involve cosmological simulation data produced with the Enzo code. We will identify regions of likely DCBH formation in this data, and use Newtonian methods to follow these regions to the point of strong-field collapse. We will then employ fully dynamical general relativistic hydrodynamics (GRHD) to simulate the collapse of these DCBHs, and the merger of any resulting DCBH binaries, using standard techniques to extract the resulting gravitational radiation, and assess its observational potential for LISA.
This work will help prepare for the overall analysis and interpretation of the LISA data stream by investigating a less-understood LISA source class, and help deepen our understanding of the early gravitational Universe.
The space-based LISA gravitational wave detector is set for launch and operation in the 2030s. LISA will offer an unprecedented window onto black holes, detecting gravitational waves from black hole binaries at mass scales which are inaccessible to ground-based detectors. Binaries including supermassive black holes are especially promising targets, and these can be detected at cosmologically large distances, probing the history of the Universe in new ways. Detection of these binaries will also allow for stringent tests of relativity and reveal the mechanisms for the formation and growth of massive black holes. This proposal prepares for these detections and enables the accompanying science by providing theoretical predictions for gravitational waves from binary black holes across the wide range of mass ratios LISA can detect.
Detecting and interpreting gravitational waves from such binaries presents unprecedented challenges. To fully realize LISA’s scientific goals, signal models (waveforms) must be long, accurate, and available for binaries whose components have comparable masses as well as those where the mass ratios are nearly 1:1,000,000. Current methods fail to capture systems with largely disparate mass ratios, and methods in development to address the most extreme mass ratio inspirals (EMRIs) will be challenged by hypothetical systems including intermediate-mass black holes with about a thousand solar masses. Even in the case of comparable mass binaries where waveforms are fairly mature and in use in data analysis, work remains to be done to understand the extent of modeling uncertainties arising from the numerical errors associated with numerical simulations of binary black holes, and to create accuracy targets for LISA science.
We propose to advance the state of the art in modeling gravitational waves for the most challenging case, intermediate mass-ratios inspirals (IMRIs) with mass ratios from 1:100 to 1:1000. For this we will carry out numerical simulations of binary black holes at high mass ratios (at and above 1:10) and assess their accuracy. We will continue a long-term effort to develop EMRI models using the self-force approach. Self-force methods have recently succeeded at achieving the accuracy needed to model EMRIs in the simplest case, non-spinning binaries in circular orbits, and our work will push the development of self-force methods when the primary is spinning. We will bring these two research directions together, using theoretical predictions and sequences of numerical simulations to extract higher-order self-force predictions. Recent studies on a number of fronts indicate that a small number of orders in the self-force approximation may be sufficient to model even comparable mass systems. If this is true, our extracted self-force predictions will provide models for systems across a whole range of mass ratios, including the most challenging case of IMRIs, which can be combined with NR merger to help build understanding of a inspiral-merger-ringdown model of these cases.
Several of the scientific objectives for LISA depend on studies like those proposed here.
LISA will "trace the origin, growth and merger history of massive black holes across cosmic ages" and "to explore the fundamental nature of gravity and black holes" to name just two objectives from the LISA Mission Proposal.
The research we propose will directly contribute to these objectives and that of the overall mission of NASA to probe the Cosmos by providing the theoretical foundations necessary to interpret gravitational wave detections in terms of their astrophysical origins, providing gravitational waves of binary black holes across a scale of mass ratios.
This program aims at searching for extragalactic LISA sources and precursors, and at providing constraints on their rates using observations from galaxy surveys and ground based gravitational wave detectors. The first objective consists in identifying a sample of candidate massive black hole binaries (MBHBs) and short-period Galactic compact object binaries from Dark Energy Spectroscopic Instrument (DESI) spectra. DESI has already accumulated over 20 million spectra, creating the largest spectroscopic catalog ever made within less than a year of operations. While the main objectives of the DESI experiment are related to measuring cosmological parameters, its observations represent an unprecedented opportunity to search for rare objects such as LISA sources. First, the search will focus on existing candidate MBHBs identified through imaging variability, and it will aim at identifying Doppler shifts over time in the emission lines. Some of these sources are currently being monitored by DESI. Later, the search will be extended to all DESI galaxies, to look for new black hole (BH) binaries or dual Active Galactic Nuclei (AGN) candidates, by identifying Doppler shifts of broad lines with respect to other lines or close pairs of spectra. A combination of state-of-the-art tools for analyses of spectra available in DESI, and algorithms to be developed as part of this project, will be used to identify the aforementioned line features. Given that the selection function of DESI galaxies and quasars is well known for the cosmology analyses, from the sample of MBHB and dual AGN candidates it will be possible to derive rate estimates for LISA BH binaries and their coalescence. This will be achieved with the aid of theoretical modeling and simulations for the sources identified.
The second goal of this project is to combine population-level multi-messenger observational constraints from LIGO/Virgo/KAGRA on massive stellar mass BH (as potential central BH of ultra-dwarf galaxies), and on intermediate mass black holes (IMBH), with stellar mass functions and merger rates from galaxy surveys, to extend our understanding and rate expectations of LISA sources down to lower masses than what can be achieved with the DESI observations, and beyond the galaxies that are active.
This project will couple the observational expertise in the team with a set of state-of-the art multi scale hydrodynamical simulations of galaxy formation which include different implementations for BH growth and associated feedback. This project will reach beyond the current state and perform studies of the predictions for MBHB and IMBH merger rates based on the observational DESI and LVK IMBH findings, respectively. The significant strength of this approach is that the proposed simulations of BHs and galaxies in an evolving universe already provide a mock multi-messenger tool that can be directly combined with GW and electromagnetic observations to unfold the cosmic history of BH mass assembly and will aid in future searches for LISA.
This proposal is to provide both observational catalogs of candidate MBHBs and their host galaxies and tools that will enable the LISA community to simulate a variety of astrophysical investigations with mock GW source catalogs of MBHB and their EM counterparts and host galaxies, at several stages of the evolution of the Universe. Even if no LISA sources are identified, but only precursors, as expected, updated rates for LISA sources across the spectrum of BH masses will provide more stringent predictions for LISA than previously possible.
This project aligns well with the NASA Physics and Cosmos program, as it aims at understanding the nature and evolution of black holes and in laying the grounds for multi-messenger observations with LISA. This project also aligns with the recommendations of the Astro 2020 Decadal Survey, specifically the New Messengers and New Physics theme.
We propose an investigation of the astrophysics of ultra-compact white dwarf (WD) binaries observable by LISA. These will be by far the most numerous of LISA gravitational wave sources and tens of thousands will be identified by LISA, together with a diffuse background of binaries that will dominate the LISA continuum spectrum at long periods. Ultra-compact WD binaries are unique in that they are also detectable by both ground-based and space-based optical telescopes. White dwarfs are complex objects with atmospheres, radii, magnetic fields, tides, and, in some cases, accretion processes. These complexities are primarily accessible for study by electromagnetic (EM) observations, and thus crucial for fully exploiting the LISA observations of these systems. Because WD binaries are persistent sources, unlike transient sources, observations prior to the launch of LISA will be highly beneficial to the interpretation of LISA data once LISA begins acquiring data.
We have two objectives:
(1) Breaking the “spectroscopic bottleneck” for identifying UCBs and their remnants by utilizing newly available massively-multiplexed spectroscopic surveys, in particular SDSS-V.
(2) Directly linking the growing number of EM-identified LISA-detectable UCBs and their remnants to population synthesis estimates that help define the ZTF survey selection. This will provide a rigorous, quantitative framework for determining the evolution and characteristics of the population of UCBs that LISA will observe.
Individual massive black hole binaries (MBHBs) will be some of the loudest sources of gravitational radiation in the millihertz band observable by the upcoming Laser Interferometer Space Antenna (LISA). While the detection of such systems with LISA will be groundbreaking, we stand to learn a great deal more via multi-messenger detections that fold in electromagnetic (EM) observations. LISA's on-the-fly parameter estimation will provide early warning of upcoming mergers by constraining the time-evolving sky localization, luminosity distance, chirp mass, and mass ratio, making MBHBs prime targets. However, there remain real challenges to coordinating EM telescopes to facilitate these discoveries, most notably related to the identification of the EM counterpart. If gas is present in the vicinity of the MBHB as it merges, the expected signals are periodic changes in the brightness during inspiral, transient flares near coalescence, or delayed afterglows. However, a variety of astrophysical systems, including non-binary active galactic nuclei, stellar-mass binaries (e.g., X-ray binaries and cataclysmic variables), supernovae, tidal disruption events, and fast radio bursts can produce similar signals with magnitudes and timescales comparable to MBHBs. Therefore, we propose a census of the variable sky in LISA's time-evolving error volume in order to calculate the probability of observing an EM signature consistent with a MBHB from an unrelated source. Our specific goals are to (1) use real multi-wavelength astronomical surveys to calculate the likelihood of finding persistent variables and transients in the LISA error volume, (2) characterize the multi-wavelength and time-domain differences from signals expected from MBHBs that can be used to rule out these interlopers, and (3) develop the framework for performing real-time assessment of persistent variable and transient sources with LISA’s contemporary EM facilities. Given that LISA will begin detecting MBHB mergers immediately, it is critical to lay this groundwork now so that we are prepared to mobilize EM telescopes for multi-messenger detections.
The scientific purpose of this proposal is to facilitate the discovery of massive black hole (BH) binaries and to understand the demography of the population of wider binaries that are precursors to LISA’s detections. Our team will use state-of-the-art, cross-validated hydrodynamics simulations to predict the time-domain electromagnetic (EM) signatures of orbiting BH binaries that interact and co-evolve with environmental gas, primarily circumbinary accretion disks. We will develop the Binary Evolution and Light-curve Library (BELL): a public, downloadable resource compiling our results on binary-disk interactions. BELL will provide data that will maximize the potential to discover massive BH binaries in large time-domain EM surveys such as by the Vera Rubin Observatory’s LSST, and by the Roman Space Telescope.
Massive BH binaries (MBHBs) are expected to form frequently in galactic nuclei as a result of the build-up of galaxies through multiple mergers. They play crucial roles in several areas of astrophysics, and are often surrounded by significant quantities of gas. Accurate and deep exploration of the interaction between binaries and their gas environments will allow us to answer fundamental questions, such as: (i) How long does a BHB take to coalesce? (ii) How efficiently are the individual BHs fueled during the coalescence process? (iii) What are the basic characteristics of this accretion and the emerging electromagnetic (EM) emission? (iv) What is the impact of the circumbinary gas on gravitational wave (GW) measurements?
Describing the co-evolution of a binary-disk system requires precise and robust numerical modeling. We will follow a multi-pronged, systematic approach, combining analytic calculations with two- and three-dimensional (magneto-)hydrodynamical simulations. This is a significant computational challenge that will require the development of novel numerical technology. Furthermore, understanding numerical subtleties involved in the prediction of gas-driven binary evolution, and obtaining numerical convergence with independent codes, will be necessary to deliver robust results that can be trusted and adopted by the broader astrophysics community and safely applied to observations. Our team members are in a unique position to undertake the required software development and the cross-validation tasks, being the primary authors and developers of the codes that will be used for the investigation, and leaders in current code comparison efforts within this community. We will utilize and cross-validate three independent codes, which variously implement moving mesh (quasi-Lagrangian), fixed mesh (Eulerian) finite volume, and discontinuous Galerkin (DG) methods for numerical gas dynamics.
The BELL will enable the identification of massive BH binary candidates with time-domain observations by supplying tables and fitting formulas of the light curves of accreting BH binaries. It will also supply predictions of gas-driven orbital evolution of binary BH systems, enabling unique predictions of the gravitational wave emission from the same systems. As part of this proposal, we will apply the BELL to interpret observations of the stochastic GW background (GWB) and of any individual SMBHBs discovered by Pulsar Timing Arrays (PTAs). Our results will be directly applicable to massive BH binaries discovered by LISA, as well as to several large forthcoming time-domain EM surveys.
To accomplish the above ambitious goals, we have assembled a five-member team with unparalleled combined expertise in theoretical analysis and modeling, numerical simulations, and GW astrophysics and interpretation. Our team has a history of close collaboration, and will form a diverse and highly interactive network that will effectively support and train junior scientists and will serve as a resource for the LISA mission.
Collisions between massive black holes are extreme events. At merger and for a brief instant, a single massive black hole binary outshines with its gravitational wave emission the entire electromagnetic output of the observable Universe. The gravitational wave signal encodes the properties of the binaries, the two black hole masses and spins, which are key to understand their origin and their environment, and contains clues about the strong-field dynamics of gravity. LISA will detect gravitational waves from massive black hole mergers to cosmological distances with extremely high signal to noise ratio, allowing us to study black holes at an unprecedented level of precision.
However, to maximize the science returns of LISA, extremely accurate theoretical predictions of the gravitational wave signatures of these events are needed. This projects aims to provide high-precision theoretical models for LISA using a combination of innovative numerical relativity and high-performance computing techniques. It will combine the open source Athena++ octree adaptive mesh refinement with task based parallelism, the Kokkos performance portability framework, and novel high-order spectral-like finite differencing methods to perform binary black hole simulations at unprecedented scale and precision.
This project will deliver a catalog of binary black hole waveforms spanning a wide range of mass ratios, spins, and eccentricities. These waveforms will be made available to the LISA mock data challenge team and will be used as a basis for the development, calibration, and validation of data analysis techniques for LISA.
Gravitational-wave astronomy is the study of astrophysical phenomena through their gravitational radiation. The field is barely out of its infancy, having been born from the first direct detection of gravitational waves by the twin LIGO detectors in 2015. Tens of merging stellar-mass binaries have been observed since then, with hundreds or even thousands more to follow over the next decade. But while LIGO and other ground-based detectors continue searching for these high-frequency signals, near-future space interferometers such as LISA will observe a far richer population of sources in the lower-frequency gravitational-wave sky. Some of the most important sources for LISA will be the capture orbits of stellar-origin compact objects into the massive black holes that reside in galactic nuclei. These are known as extreme-mass-ratio inspirals (EMRIs), and the observation of even a single such source has the potential to significantly advance our understanding of massive black holes, their stellar environments, or even the true theory of gravitation itself.
Unlocking this scientific potential will be challenging, as EMRIs are the only LISA sources (and indeed the only astrophysical sources across the gravitational spectrum) that combine strong-field complexity with long observation periods. Their orbits will generally exhibit high eccentricity and extreme versions of relativistic precession, while their signals will contain hundreds of thousands of cycles and can persist over the entire mission duration. The result is a profoundly difficult inverse problem, with several key aspects that remain unsolved. These are hindered by theoretical and computational challenges both in the forward modeling of the predicted EMRI waveform, and in the recovery of an inverse solution for the presence and properties of actual EMRI signals in LISA data. Standard modeling and measuring techniques that have been applied to ground-based observing, or proposed for other classes of LISA source, cannot be employed for EMRIs without additional considerations that are specific to their nature.
This research proposal addresses the EMRI inverse problem on both its fronts, by focusing on topics at and around the interface between source modeling and data analysis. Specifically, it describes: i) the construction of accurate, efficient and extensive next-generation waveform models for EMRI signals; ii) studies to explore and understand the largely uncharted signal space of LISA-observable EMRIs, where modeling and analysis interact; and iii) the development of statistical techniques and strategies for finding and characterizing EMRI signals in eventual LISA data. The work outlined in this proposal is crucial in enabling the LISA mission to achieve its targeted science yield - not just in terms of the EMRI observations themselves and the science objectives that they contribute directly to, but also to ensure that EMRIs can be properly disentangled from the multitude of other sources in the LISA catalog.
Intermediate-mass black holes (IMBHs) are among the most puzzling objects in modern astrophysics. They play a crucial role in cosmology and galaxy formation, but very little is known about their origin and evolution. Observational evidence for IMBHs is still lacking because standard detection techniques, which use stellar and gas dynamics or X-ray and radio emission, suffer from various limitations, which have led to a handful of (often controversial) IMBH candidates. Recently, LIGO/Virgo has confirmed the first IMBH ever, from the detection of GW190521, demonstrating that gravitational waves (GWs) are uniquely well suited to measuring the properties of IMBHs.
LISA offers a unique opportunity to discover IMBHs out to large redshifts, since the inspiral and merger of IMBH binaries is a loud source of low-frequency GWs. Moreover, the intermediate mass-ratio inspiral (IMRI) of a stellar compact remnant into an IMBH is a potential target for multi-band detection, since LISA measurements will alert astronomers of an incoming merger detectable within the next few years by LIGO/Virgo/Kagra, Einstein Telescope, and Cosmic Explorer. The next decade may bring hundreds of GW events, promising a spectacular range of new science that touches on nearly every astronomical subfield, from stellar evolution to cosmology.
With so little known about IMBHs from previous observations, our ability to prepare for this new science relies almost entirely on theoretical modeling and predictions. This proposal focuses on understanding the formation and GW signatures of IMBHs in dense stellar environments, where they are most likely to be produced and to form GW-emitting binaries through dynamical interactions with other compact objects. We will constrain the distribution and properties of all GW sources containing IMBHs throughout the Universe, quantify the rate of mergers, and estimate the fraction of multi-band events. Using an extensive library of star cluster models, we will build a mock catalog of IMBH sources detectable by LISA, covering all source types (e.g., IMRIs vs IMBH-IMBH binaries), their physical parameters (masses, spins, orbital eccentricity), and the host environment (star cluster properties, host galaxy type). The final goal of our proposal will be to provide LISA Data Challenges with an extensive catalog of IMBH sources. This catalog will be an important guide for LISA data analysis and instrumental design strategies, and it will play a key role in extracting as much IMBH science as possible from LISA observations.
Ultracompact binaries are a class of binary systems with orbital periods below 60 minutes (detached or semi-detached), consisting of a white dwarf, neutron star and in rare cases a black hole primary and a helium star, white dwarf or neutron star secondary. The study of these systems are important to our understanding of such diverse areas as supernova Ia progenitors, binary evolution and they are predicted to be the most abundant and strongest Galactic gravitational wave sources in the LISA band allowing for precise multi-messenger studies, if we find and study them over the next years.
Systems with orbital periods <30min will be the strongest Galactic LISA sources and will be detected by LISA within weeks to months. Currently, we know only of about a dozen of these so-called verification binaries although hundreds to thousands are theoretically predicted to be detectable in our Galaxy. The current number is strongly biased towards the Northern Hemisphere with no double white dwarf verification system known in the Southern Hemisphere. These verification binaries are crucial in facilitating the functional tests of the instrument and maximize LISAs scientific output but a representative population is required across both hemispheres. Ultracompact binaries show up in light curves with variations on timescales of the orbital period, e.g. due to eclipses or tidal deformation of the components. Therefore, photometric time-domain surveys are well suited to identify ultracompact binaries in a homogeneous way. BlackGEM is the next generation of optical surveys for high-cadence time domain astronomy in the Southern hemisphere before LSST becomes available. We will combine BlackGEM with other large-scale surveys, such as Gaia and eROSITA, to discover and fully characterize an unbiased sample of ultracompact binaries. This dataset will be compared to predictions from binary population studies to predict the importance of different formation channels on a significant statistical sample. The expected large sample will challenge common envelope and binary evolution theories, e.g. predicted vs. observed orbital period and component mass distributions which is the largest uncertainty in predictions for the number of detectable Galactic binaries in the LISA band. We expect to find hundreds of Galactic LISA foreground sources, tens of verification binaries, for the first time in the Southern hemisphere including the Galactic Bulge region, and will be able to predict the gravitational wave strength detectable by LISA for a subsample with accurate masses, periods and distances.
The proposal includes detection, characterization and population studies of the detected systems. We will also calculate the expected gravitational wave strength and perform multi-messenger predictions where we combine results from the electromagnetic studies with simulated gravitational wave data for our sample of verification binaries. Our team includes observers as well as binary evolution theorists and gravitational wave analysis experts.
Massive black hole binaries (MBHBs) with masses of 0.1-10 million Solar masses in low-redshift (z<4) galaxies are among the loudest sources of gravitational radiation at milli-Hz frequencies that will be observable by the Laser Interferometer Space Antenna (LISA) up to weeks before their coalescence. While the detection of such systems with LISA will be groundbreaking, we can learn a great deal more from them if we can also detect their electromagnetic (EM) counterparts. To help identify the counterpart, early warning from LISA on-the-fly parameter estimation will yield time-evolving, critical constraints on sky localization, luminosity distance, chirp mass, and mass ratio. But developing strategies to pick out the EM counterpart from all the candidates in the 3-D error volume of the GW source requires a detailed inventory of this volume and a systematic evaluation of the credentials of the galaxies within it.
With the above considerations in mind, we propose to lay the groundwork for future searches for EM counterparts of LISA MBHB mergers by conducting a census of sources in the time-evolving error volume for the current LISA design. Our approach is accretion and obscuration agnostic. We will include the host galaxies of MBHBs since it is not certain whether accretion occurs, and even if it does, we may not be able to observe it because >75% of accretion is obscured at the relevant luminosities. Our specific goals are to (1) use large-area optical and infrared astronomical surveys like the Sloan Digital Sky Survey to determine the number of real galaxies in the LISA error volume, build a modular framework for querying these and upcoming databases for an on-the-fly census, and develop a prioritization scheme for ranking galaxies in the volume. (2) Conduct a census of the LISA error volume from cosmological simulations to find the likelihood of identifying the EM counterpart depending on MBHB and galaxy properties and free of observational limits. (3) Test the performance of methods for detecting the expected EM signatures before and after coalescence, and develop new diagnostics for identifying flares based on their time-dependent impact on gas in the host galaxy. The outcome of this work will be a framework for determining the detectability of LISA EM counterparts that can incorporate constraints from future facilities like Athena, and the development of practical strategies for EM counterpart identification.
Our group has discovered over a dozen new compact binaries with periods less than 1 hour, including two eclipsing binary systems with periods less than 10 minutes. Most of these compact binary systems will be detectable by LISA. Detailed analysis of these systems, including light curve modeling and spectroscopic analysis, have shown a rich variety of astrophysical characteristics depending on the white dwarf components of the systems (i.e. He, C/O, or hybrid WDs), their temperatures, and their orbital separations and periods. Because several of the newly discovered systems are eclipsing, we have been able to constrain the expected gravitational wave polarization signatures of the systems, a critical aspect of LISA observations.
We propose to investigate in detail the complementary aspects of GW and optical observations, using our sample of newly discovered objects as a guide. In particular, we will consider the needed optical photometric and spectroscopic observations in the LISA era required to extract optimal astrophysical information from LISA-detectable compact binary systems. Specifically, we will evaluate the complementarity afforded by the Rubin/LSST observatory and the very large telescopes (ELT, GMT, TMT) expected to be available in the LISA era.
In 2015, the terrestrial LIGO gravitational-wave detectors observed the first gravitational-wave signal generated in a merger of two black holes about 1.3 billion light-years away. This event has marked the beginning of the new field of gravitational-wave astronomy. Gravitational-wave detectors have since started to routinely observe objects and events in the universe that are not accessible to the traditional electromagnetic observations. Furthermore, in 2016-2017 the successful LISA Pathfinder mission demonstrated some of the key technologies needed for developing a space-borne gravitational-wave detector. Coming on the heels of these remarkable breakthroughs, the European Space Agency (ESA) recently selected the Laser Interferometer Space Antenna (LISA) as the third large-class mission in ESA's Science Programme. LISA is expected to be launched in 2034, and with a significant contribution from NASA, it will be the first space-borne gravitational-wave observatory.
This proposal focuses on the stochastic gravitational-wave background (SGWB), which is one of the science targets of the LISA mission. The SGWB is expected to arise as a superposition (sum) of many incoherent sources of gravitational waves. It could be of astrophysical origin, for example due to contributions of numerous binary systems in the Milky Way or throughout the universe. It could also be of cosmological origin, generated by very energetic processes in the early universe. Detection of a cosmological SGWB would provide unique information about the fundamental physical laws that apply at very high energy scales, inaccessible to standard laboratory experiments. Detection of an astrophysical SGWB would provide unique information about properties and evolution of structure we observe today in the universe, including objects such as neutron stars and black holes.
This proposal will develop a new method for measuring the properties of the SGWB using LISA data, drawing from similar methods developed in the context of terrestrial gravitational-wave detectors and of pulsar timing experiments. The new method, known as the phase-coherent mapping approach, will enable direct estimates of the frequency, directionality, and polarization content of the SGWB. It is also compatible with the global-solution approach to LISA data analysis, which advocates doing a global fit to all of the gravitational-wave signal and noise components in the data.
Furthermore, a statistical framework will be developed to perform the SGWB model selection and parameter estimation for LISA. By leveraging differences in directional, polarization, and frequency structure in different SGWB models, this framework will enable identification of contributions from different SGWB sources, hence separating the cosmological SGWB from the astrophysical foregrounds. The framework will therefore combine the development of SGWB models and SGWB search techniques to fully exploit the science potential of SGWB searches with LISA.
The LISA observatory will allow us to study the milli-Hertz band of the gravitational wave spectrum, which is thought to contain many millions of sources. The challenge of extracting thousands of individual signals in the presence of fluctuating instrument noise, glitches and data drop-outs remains an unsolved problem. The success of the mission depends on solving this problem, which is very different, and far more challenging, than anything that has been encountered with the ground-based LIGO-Virgo gravitational wave detectors. We propose to develop a flexible, trans-dimensional analysis algorithm that dynamically determines the number of resolvable sources and their physical parameters, while simultaneously modeling the instrument noise and accounting for gaps in the data. We plan to build on our experience using stochastic analysis techniques to extract tens of thousands of galactic binary signals and hundreds of binary black hole signals from simulated LISA data sets. We also propose to develop techniques for detecting and characterize un-anticipated and un-modeled sources of gravitational waves. The signal extraction will be done in concert with advanced noise modeling techniques to develop a prototype global solution to the LISA science analysis problem.
The ultimate expression of strong gravity in the Universe is a black hole, a region of spacetime from which nothing, not even light, can escape. Hidden from our direct view by their very nature, black holes are only "seen" by their interaction with a light- producing environment. Two black holes merging together light up the cosmos in gravitational waves, while remaining completely dark in electromagnetic light. On September 14, 2015, such a cataclysmic collision was observed for the first time when LIGO detected the gravitational waves emitted by two black holes merging, called GW150914. Gravitational waves, or ripples in spacetime, are the only means to detect black holes directly. LISA will offer unprecedented views of black holes, detecting gravitational waves from black hole binaries across multiple mass scales and deep into the history of the Universe inaccessible by ground based detectors like LIGO. This proposal prepares for such detections by providing theoretical predictions of black holes at multiple mass scales. The central thesis of the proposal is the determination of the signature of black hole binaries of unequal mass ratio in which one of the black holes is a hundred or more times the mass of its companion. These sources are expected to be relatively common and current methods fail to capture this likely LISA source.
The theoretical foundation of black hole mergers and their generation of gravitational waves is Einstein's theory of general relativity. There are several methods to produce accurate gravitational waveforms for the coalescence of black hole binaries. This work will require knowledge of these approaches and new ones. Numerical relativity provides a means to solve Einstein's equations during the nonlinear regime when the two black holes merge. While advances in numerical relativity over the last 13 years have been monumental, as of yet numerical relativity cannot solve systems of black holes with mass ratios more disparate than about one to one hundred. For systems with vastly different black-hole masses, called extreme mass ratio inspirals, perturbation theory provides solutions. Unfortunately, perturbation theory is best suited for mass ratios in the thousands or more rather than hundreds. When the black holes are effectively point masses moving at a fraction of the speed of light, an approximation to Einstein's theory known as post-Newtonian provides the answer. All of these approaches will be important in the binary black hole landscape of LISA. Black-hole binaries of moderate mass ratios in the hundreds will require pushing the boundaries of numerical relativity and perturbation theory in order to provide reasonable solutions for which neither approach can do alone.
Several of the driving scientific objectives for LISA will depend on successful studies like that proposed. LISA is to "trace the origin, growth and merger history of massive black holes across cosmic ages" and "to explore the fundamental nature of gravity and black holes" to name just two objectives from the LISA Mission Proposal. Our work will contribute to this goal and that of the overall mission of NASA to probe the Cosmos by providing the theoretical foundations necessary to interpret gravitational wave detections in terms of their astrophysical origins, providing a map of the gravitational waves of binary black holes of unequal-mass.
There are several products this proposal will produce for LISA science if funded. The main product will be waveforms of black-hole mergers with mass ratios in the hundreds. These waveforms will be provided to the LISA Mock Data Challenges to stress test the community's ability to detect and interpret gravitational waves from these systems. The proposed work will also produce papers and numerical algorithms that provide guidance and insight into gravitational waves from moderate mass ratio black hole binaries and their interpretation.
The analysis of LISA data will be significantly different than data from ground-based detectors, owing to the fact that there will be thousands of overlapping signals from multiple source classes constantly in the data. This poses a variety of interesting problems to develop strategies to identify, isolate, and extract signals and parameters from individual sources. Astrophysical inference and interpretation of signals from single and whole populations of sources will require targeted, quantitative models for observational selection biases. Additionally, multi-messenger observations of LISA sources, before, during, and after the LISA mission will play critical roles in interpreting and understanding the LISA source catalogs.
Observational bias has many origins, including selection effects derived from the fundamental astrophysical parameters of a source, uncharacterized physical effects that systematically alter data relative to naive expectations, the response of the instrument to different sources at different frequencies and sky locations, and on the methods used to identify and extract source parameters from the data.
In our proposal we will consider the problem of identifying and quantifying astrophysical information contained in LISA data for the population of ultra-compact binaries. We will produce a set of public tools that the astrophysical community can use to simulate data catalogs for inference and interpretation studies of compact binaries in LISA analysis, and foundational data for simulating populations of ultra-compact binaries for studies in LISA data analysis, stellar evolution, and gravitational wave source simulation. We are principally interested in how astrophysical information imprints in LISA observations compact stellar mass binaries, and what biases are introduced in catalogs derived from different approaches to LISA analysis of the compact binaries. We will include non- standard sources, e.g., eccentric binary black holes and interacting double white dwarfs with inverse gravitational-wave chirps, which hold special promise for multi-wavelength and multi-messenger studies and can be used to leverage maximal information.
The Laser Interferometer Space Antenna (LISA) aims to observe gravitational waves from a cornucopia of low-frequency astrophysical sources including extragalactic black holes with masses ranging from tens to tens of millions of solar masses, thousands of binary star systems in our own Milky Way galaxy and cosmological stochastic background sources and exotic physics in the early universe. To achieve this aim, LISA will employ a pm-precision interferometric measurement of six test masses in three spacecraft, disturbed by accelerations of order fm/s2 and separated by 2.5 million km. Unlike almost any astrophysics mission that has preceded it, the LISA instrument and spacecraft are inextricably linked in determining the quality of the final scientific product. The scientific performance of the mission depends on the interplay between almost every element of the payload and platform. For example, the optical metrology system (OMS) is used not only as the primary distance measurement between test masses but also for spacecraft attitude control. The need to maintain the interferometric reference test masses in pure free fall drives the spacecraft dynamics through the on-board micro-propulsion system and the quality of free-fall depends critically on the stability of the platform.
Studying the performance of the instrument and generating realistic synthetic data with which to train data processing algorithms will require detailed simulations capable of reproducing the full complexity of the LISA instrument. The work described in this proposal will contribute to this effort in three areas: simulating the drag-free and attitude control system (DFACS) of LISA that governs the dynamics of the test masses and spacecraft; simulating the relevant light path for the entire optical chain of each laser link, including modeling the interferometric length and alignment sensing; and developing tools to predict laser heterodyne frequencies based on orbit data and plan necessary switches or tunings of the offset-frequencies in the phase-locked loops.
We propose to develop simulation tools describing the LISA DFACS, incorporating non-trivial test mass acceleration and read-out noise, the motion of the moving optical sub-assemblies (MOSAs) and the interaction of spacecraft and test mass dynamics through the control laws and actuators.
We will develop a simulation of the LISA optical metrology system based on a Hermite-Gauss mode expansion and scatter matrices capable of modeling tilt-to-length coupling, as well as the effects of other imperfections in the optical path such as telescope wavefront errors, mode mismatches between interfered beams, and beam spot mis-centering on quadrant photodetectors.
We propose to develop a tool that will rapidly devise suitable frequency plans to keep the heterodyne beat signals within a workable range based on inputs of orbital data. The integration of this tool with the OMS and DFACS simulation tools will allow us to transition from a solely frequency-limit-based figure of merit to one that is driven by the actual impact of the heterodyne frequency on the end-to-end noise.
To provide maximum benefit to the LISA mission, the products from each of these tasks will be built for compatibility with an end-to-end performance simulator architecture. The LISANODE infrastructure is being developed within the LISA Consortium to facilitate the implementation of, and communication between, distinct but not isolated simulations of the LISA subsystems. The distinct simulations will be developed as modules called Nodes within the simulation environment.
Ultra-compact binaries are one of the major classes of sources detectable by LISA. The number of such sources is estimated to be very large – more than 10,000 individually detectable sources, and an even larger number of sources contributing to (and dominating) the LISA background below about 2 mHz.
Much remains to be learned about the nature of ultra-compact binaries. To date, only a hand-full of LISA Verification Sources have been identified and the total number of known binaries with periods less than 60 minutes is only a few dozen.
The Zwicky Transient Facility (ZTF) is a major new large-area ground-based optical survey instrument, having just begun survey observations in May-June 2019. ZTF will likely be the most powerful near-term instrument for detection and characterization of ultra-compact binaries at least until LSST becomes available. Combined with Gaia source distant estimates, ZTF will significantly increase the number of known LISA verification binaries as well as detect hundreds of other compact binaries that will provide a deeper understanding of the astrophysics of the various binary channels that will contribute to the LISA GW signal. The proposed work involves observational detection and characterization of ZTF identified compact binaries. Our proposal team includes observers experienced in time-domain analysis as well as consultant theorists.
What are the properties of accretion flows in the vicinity of coalescing massive black hole binaries (MBHBs)? The answer to this question has direct implications for the feasibility of coincident detections of electromagnetic (EM) and gravitational wave (GW) signals from coalescences. Such detections are considered to be the next observational grand challenge that will provide a more complete understanding of evolution and growth of these massive objects.
In anticipation of future detections by the Laser Interferometer Space Antenna (LISA), we propose to investigate the coincident EM and GW signatures of MBHBs immersed in accretion flows. The proposed program of research centers on a suite of high-resolution simulations with the radiation-magnetohydrodynamic code Athena++ , which will follow binaries as they evolve through the LISA frequency band. This study will set the stage for the first simulations of accreting MBHBs with radiative transfer and will use them to predict the resulting EM spectra, light curves, and gravitational waveforms.
Various astrophysical sources have benefited from multi-wavelength and multi- messenger analyses which provided valuable information on the behavior of these sources. The proposed study aims to explore the combination of LISA with other observatories.
In particular, we suggest investigating two possible counterparts to LISA detections, (1) an electromagnetic precursor in the form of a supernova, and (2) LIGO gravitational wave (GW) detections. Both counterparts have the same underlying physical system; they are based on the astrophysical evolution of binaries in galactic nuclei.
Within the vicinity of a supermassive black hole (SMBH), the members of a stable binary have a tighter orbital configuration than the orbit of their mutual center of mass around the SMBH. The SMBH can induce collisions and mergers, between stars, and between compact objects, such as black holes and neutron stars. During the supernova that led to the formation of the stellar-compact object, the stars may undergo a supernova kick which can change the velocity magnitude and its direction. We recently showed, in a proof-of-concept study (Lu & Naoz 2018), that this could lead to one of the compact objects plunging into the nearby SMBH. This plunge forms an extreme mass ratio inspiral which will be detected by LISA. Thus, this detection will have both a LISA GW emission detection with an optical counterpart precursor. On the other hand, if the binary survived and formed a stellar-mass compact object binary, the proximity of the binary to the SMBH, yields strong acceleration and thus produces a large phase shift in the GW signal measurable by LISA (e.g., Meiron et al. 2017; Inayoshi et al. 2017). This LISA measurement will inform LIGO of an incoming merger. Depending on the merger rates (Hoang, Naoz, et al. 2018) can provide a powerful tool for synergy between LISA and LIGO observations.
Our proposal will be guided by the following questions: What is the rate of these sources? What is the probability of detecting these sources? Will LISA be able to distinguish these sources from other sources?
The planned investigation will focus on the interpretation and analysis of LISA's data from these sources. Therefore, the proposed plan will assess the capability of LISA to differentiate these sources from others. Thus, this proposed research is in accordance with NASA's goal to "Conduct astrophysics investigation that prepares for the analysis and interpretation of the LISA data, and to Evaluate the capability of LISA data for enabling astrophysics investigations."
The recent direct detections of gravitational waves (GWs) by LIGO have opened a new window on the Universe. The LIGO detections, and the success of the LISA Pathfinder mission, together have led to a resurgent interest in an evolved LISA mission, currently positioned as the ESA L3 mission, with NASA providing a 20% contribution, and with launch in the early 2030s. LISA will offer unparalleled science returns, including a view of supermassive black-hole mergers (SMBHs) to high redshifts, extreme-mass-ratio inspirals (EMRIs) of stellar-mass black holes into supermassive black holes, a census of thousands of compact binaries in the Galaxy, precision tests of general relativity and black-hole structure, and the possibility of detecting stochastic signals from the early Universe.
This proposal supports the early development of data-analysis and interpretation techniques that will maximize the future science yield of the Laser Interferometer Space Antenna (LISA), the planned low-frequency gravitational-wave (GW) observatory in space. We aim to refine numerical methods, map likelihood surfaces, and demonstrate prototype software to optimally determine the system parameters of LISA's premier GW sources (massive black-hole binaries or MBHBs, and extreme mass-ratio inspirals or EMRIs). We also plan to develop a broadly applicable technique for estimating astrophysical population parameters from an ensemble of detected and characterized systems, with special emphasis on the population of MBHBs. These contributions fit within the work-package framework outlined by the LISA Consortium Science Group, which aims to deliver a data-and science-analysis system that will meet all of the mission's key science objectives. Furthermore, the proposed work will be carried out in close collaboration and coordination with the new LISA Data Challenges (LDCs). Working within the LDCs will enable us to compare our solutions with the contributions of other LISA Consortium members, and to make them immediately available to ongoing studies of mission parameters and science requirements.