Session H09: Astrophysics with LISA

Live

Supermassive black hole (SMBH) binaries are expected to be produced during galaxy formation, with many of these binaries surrounded by plenty of gas. I will discuss the coupled dynamics of a SMBH binary with a circumbinary gas disk, and the expected characteristics of electromagnetic (EM) emission from such a system. In particular, I will argue, based on analytic arguments and two-dimensional hydrodynamical simulations, that in the final stages of the merger of a SMBH, inside LISA's frequency band, it will remain bright, and that its emission will be highly time-variable. In particular, relativistic Doppler modulations, lensing effects, and hydrodynamical modulations of the accretion will inevitably imprint periodic variability in the EM light-curve, tracking the phase of the orbital motion, and serving as a template for the GW inspiral waveform. Advanced localization of the source by LISA weeks to months prior to merger will enable a measurement of this EM chirp by wide-field instruments in X-rays and possibly other wavelengths. A comparison of the phases of the GW and EM chirp signals will help break degeneracies between system parameters, and probe a fractional difference difference in the propagation speed of photons and gravitons as low as 1e-17.   

Live

Higher harmonics of the ringdown gravitational wave signal from massive black-hole binary mergers can be detected with a large signal-to-noise ratio (SNR) by LISA, while their inspiral contributes little to the SNR. These binaries are also more likely to have electromagnetic counterparts. Can we extract the binary parameters and localize the source using LISA observations of the ringdown only? In general, LISA inspiral sources are long-lived, and LISA’s motion around the Sun modulates the amplitude and phase of the signal, which in turn can be used to disentangle the source location and orientation. On the contrary, the ringdown is very short-lived, and hence we cannot use the modulation of the antenna pattern for localization. We show that (i) the mass ratio and inclination of a binary can be measured by carefully combining multiple ringdown harmonics, and (ii) we can constrain the sky location and luminosity distance by relying on the relative amplitudes and phases of various harmonics, as measured in different LISA channels.   

Live

LISA can detect higher harmonics of the ringdown gravitational-wave signal from massive black-hole binary mergers with large signal-to-noise ratio. The most massive black-hole binaries are more likely to have electromagnetic counterparts, and the inspiral will contribute little to their signal-to-noise ratio. Here we address the following question: can we extract the binary parameters and localize the source using LISA observations of the ringdown only? Modulations of the amplitude and phase due to LISA’s motion around the Sun can be used to disentangle the source location and orientation when we detect the long-lived inspiral signal, but they can not be used for ringdown-dominated signals, which are very short-lived. We show that (i) we can still measure the mass ratio and inclination of high-mass binaries by carefully combining multiple ringdown harmonics, and (ii) we can constrain the sky location and luminosity distance by relying on the relative amplitudes and phases of various harmonics, as measured in different LISA channels.   

Live

The primordial stochastic gravitational wave background (SGWB) carries first-hand messages of early-universe physics, possibly including effects from inflation, preheating, cosmic strings, electroweak symmetry breaking, and etc. However, the astrophysical foreground from compact binaries may mask the SGWB, introducing difficulties in detecting the signal and measuring it accurately. In this Letter, we propose a foreground cleaning method taking advantage of gravitational wave observations in other frequency bands. We apply this method to probing the SGWB with space-borne gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA). We find that the spectral density of the LISA-band astrophysical foreground can be predicted with percent-level accuracy assuming $10$-years' observations of third-generation GW detectors, e.g., Cosmic Explorer. After the foreground cleaning, LISA's sensitivity to the primordial SGWB will be substantially improved.   

Live

Well before the first detection of gravitational waves, numerical relativity and gravitational wave detectors were each desperately trying to improve to the point that they were prepared for such a detection. We are now at a time where we have detectors capable of observing gravitational waves and numerical relativity capable of creating theoretical models to understand the gravitational wave events. However, the detectors are continuing to improve and by 2034, the Laser Interferometer Space Antenna (LISA) is expected to fly, providing unprecedented sensitivity. With LISA expecting to see many overlapping, extremely loud signals, it is crucial that we provide waveforms accurate enough to subtract out the signals without contaminating other, quieter sources. While numerical relativity simulations have been accurate enough for current detectors, will this be the case for LISA and even third-generation ground based detectors?   

Live

Short-period galactic white dwarf binaries detectable by LISA are the only guaranteed persistent sources for multi-messenger gravitational-wave astronomy. Large-scale surveys in the 2020s present an opportunity to conduct preparatory science campaigns to maximize the science yield from future multi-messenger targets. The WFIRST microlensing survey will image seven fields in the galactic bulge approximately 40000 times each. Although the cadence is optimized for detecting exoplanets via microlensing, it is also capable of detecting eclipsing white dwarf binaries. I will present forecasts for the number of short-period binaries the WFIRST microlensing survey will discover and the implications for the design of electromagnetic surveys.   

Live

Now ground-based gravitational-wave detectors observe high frequency ($\sim 100~\mathrm{Hz}$) gravitational waves; in the 2030s the space-borne LISA will observe low frequency ($\sim 1~\mathrm{mHz}$) gravitational waves. Between the two, in the decihertz range, lies the opportunity for future discovery, and the potential to make mulitband gravitational-wave observations. A Decihertz Observatory is uniquely suited to the detection of intermediate-mass ($\sim10^2$--$10^4 M_\odot$) black holes; it would enable the detection of stellar-mass binaries days to years before they are observed by ground-based instruments, and it also serves as a new laboratory for fundamental physics, permitting unique tests of general relativity and the Standard Model. We review how a Decihertz Observatory will answer key questions about how black holes form and evolve across cosmic time, open new avenues for multimessenger astronomy, and advance our understanding of gravitation, particle physics and cosmology.   

Catching massive black hole binaries with LISA

The Laser Interferometer Space Antenna (LISA) will detect thousands of overlapping signals that are present in the data for months or years. We are developing a time-evolving global analysis of the LISA data, which will simultaneously detect and characterize all galactic binaries, back holes binaries, extreme mass ratio inspirals, and un-modeled sources while also modeling the detector noise. Here we discuss the techniques we are working on to tackle the specific challenges relating to the detection and characterization of multiple massive binary black holes.   

LISA observations of eccentric LIGO sources

Stellar-mass binary black holes are expected to have nearly circular orbits by the time they enter the LIGO band. Yet many years before merger, they will reside in the low-frequency band accessible to LISA, and may well retain significant eccentricity. We explore the statistical properties of such a population of sources, the expected signal-to-noise ratio of this population in LISA, and discuss the astrophysical implications of measuring non-zero eccentricity leading up to merger.