Session T14: Low-Frequency GW Sources

Effects of dynamical tides on gravitational wave signals from eccentric double white dwarf systems

The tidal response of compact stars in a binary system is a useful probe of the stellar interior. While the equilibrium part of the tide dominates the matter response for systems with a large orbital separation, the dynamical tide starts to become important for small separation as the orbital period gets closer to the matter response time. In this talk, I will discuss the effect of dynamical tides on the gravitational wave signals from eccentric binary white dwarf systems. These systems are one of the target sources of the proposed space-based gravitational wave detector (LISA). We show numerically that for orbits with high eccentricities and small separations, the dynamical tide can cause a chaotic growth of the normal mode amplitudes and cause the orbit to evolve in a random manner, leading to a chaotic waveform. Meanwhile, for systems with lower eccentricities and larger separations, the dynamical tide causes a perturbative change in the orbital motion, increasing the rate of periastron precession compared to an orbit with only equilibrium tide. Including such an effect is crucial in determining the tidal properties of white dwarfs.   

Continuous Gravitational Waves from White Dwarf Mountains

Continuous gravitational waves (CGWs) from non-axisymmetrical deformations on neutron stars (NSs), better known as mountains, can reveal much about the crust of a NS, the interior of a NS, and the fundamental symmetries of nature. Recently, Gittins. et. al. [1] indicated that the maximum mountain that a NS crust can support is an order of magnitude smaller than the previous calculation made by Ushomirsky et. al. [2]. This discrepancy could have a serious impact on the detectability of CGWs from NS mountains. In this project, we present the calculation of the maximum mountain that solid white dwarfs (WDs) can support. The purpose of this calculation is two-fold: (1) it is easier to handle than the calculation of a maximum mountain of a realistic NS, which will enable us to explore deeper how and why the various maximum mountain calculations differ, (2) the equation of state and structure of WDs are better known than those of NSs, so the calculation has much less uncertainty. We present results for the maximum ellipticity of a solid WD and discuss the observability of CGWs from a rotating WD with LISA.

[1]: G. Ushomirsky, C. Cutler, and L. Bildsten, Deformations of accreting neutron star crusts and gravitational wave emission, Mon. Not. R. Astron. Soc. 319, 902 (2000), astro-ph/0001136.

[2]: F. Gittins, N. Andersson,  and D.I. Jones, Modelling neutron star mountains, Mon. Not. R.  Astron. Soc. 500(4) (2020)   

Big Bang Nucleosynthesis Limits and Relic Gravitational Wave Detection Prospects

Big bang nucleosynthesis (BBN) places upper limits on the relativistic energy density in the early universe, which places bounds on the strength of primordial magnetic fields and/or turbulent motions in the early universe and their resulting relic gravitational wave (GW) signals. Previous studies assumed that velocity and magnetic fields are “frozen-in” to the primordial plasma and that the ratio between the turbulent source energy density and thermal energy density remain unchanged during the radiation-dominated epoch. We revisit the BBN limits and properly account for the decaying nature of turbulent sources from their generation until BBN. We find that allowed values for the magnetic fields at the moment of generation are not constrained by order of microGauss as was claimed previously based on BBN bounds without accounting for decaying turbulence. This allows larger estimates for the initial magnetic field strength and stronger GW signals than were previously expected. We address the prospects of detecting these GW signals through space-based interferometers (for GWs generated around the electroweak scale) and by pulsar timing arrays and astrometric missions (for GWs generated around the quantum chromodynamics energy scale).   

Linking the Binary SMBH Population to PTA Data with a Self-Consistent Synthesis Framework

Pulsar timing arrays (PTAs) are galactic-scale gravitational wave observatories that are sensitive to the gravitational wave background (GWB) from the cosmic population of binary supermassive black holes (SMBH), which form following massive galaxy mergers. Due to a lack of direct observations, there remain many open questions about how to model the binary SMBH population that is producing the GWB detectable by PTAs. Currently, there are a wide variety of models commonly used to predict the GWB, including models based on cosmological simulations, semi-analytic models which can be built on top of observations of galaxies, and quasar-based models. Given the variety of assumptions that are included in each of these models, it is difficult to link the results derived from them individually into a single astrophysical inference. In this talk, I will present on-going work happening within the NANOGrav Astrophysics Working Group to create a self-consistent binary SMBH population synthesis framework that bridges these disparate modeling approaches. I will also show how we combine these new predictions for the GWB with a Gaussian Process emulator to extract a self-consistent astrophysical inference from NANOGrav's pulsar timing data.   

A quasar-based supermassive black hole binary population model: implications for the gravitational-wave background

The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual AGN systems will eventually form bound binary pairs. We present an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. We predict the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr dataset and compare to SMBHB population models which do not take the GWB as input. Finally, we show how our model can be used to place constraints on the population of SMBHBs with associated quasar activity.   

Forecasting constraints on anisotropy in the nanohertz gravitational wave background

Pulsar timing arrays are projected to make a detection of the nanohertz gravitational wave background (GWB) in the next few years. This background is expected to be produced by inspiraling supermassive black-hole binaries (SMBHBs) in the local universe. The angular power of the background produced by these SMBHBs will be anisotropic, with the brightest binaries standing out above the background. In this work, we present a method that makes use of the measured cross-correlations between pulsars in the array to place constraints on the anisotropy of the GWB. We then use realistic simulations of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) dataset to forecast the constraints that we will be able to place on the anisotropy of the GWB in the next few years.   

Disentangling Multiple Astrophysical Background Sources in PTA Datasets

With the strong evidence of a common-spectrum stochastic process in the NANOGrav Collaboration’s 12.5-yr dataset, it is crucial to assess the effects of the plethora of astrophysical sources that could contribute to the stochastic gravitational wave background (GWB). Using the same dataset creation and injection techniques as in Pol et al. (2021), we assess the separability of multiple GWBs by creating single and multiple source datasets. We investigate these injected sources using typical pulsar timing array (PTA) analysis techniques to assess recovery and separation of individual astrophysical backgrounds. We find that for a moderately strong underlying GWB on top of a GWB generated by primordial gravitational waves and supermassive black hole binaries, respectively (Ω_PGW/Ω_SMBHB = 0.5), PTAs begin accumulating evidence for the additional source after around 17 years of data. At 20 years of data, while the amplitude uncertainty region is still fairly large, we are able to constrain the spectral index of the weaker GWB to a fractional uncertainty of 40%. Using these methods and findings, we outline a basic protocol to search for multiple backgrounds in future PTA datasets.   

Construction of a Pulsar Interstellar Medium Array

Pulses from radio pulsars undergo dispersion as they pass through the ionized interstellar medium, delaying the pulses as a function of radio frequency and the dispersion measure (DM), the integrated line-of-sight electron density. DM time variability has been measured for many pulsars with both stochastic and systematic components; separating both types of variation is challenging and has led to biased physical interpretations for different lines of sight. We leverage the well-known statistical properties of autoregressive processes to model both types of variations. We invoke the physical mechanisms for turbulence and dynamics to drastically simplify the estimation of model parameters. The autoregressive approach can be extended to multiple DM timeseries to create a pulsar interstellar medium array (PISMA) with the goal of measuring correlated signatures in pulsar data due to the interstellar medium and solar wind, analogously to the observation of a pulsar timing array (PTA) with the goal of measuring correlated signatures due to gravitational waves. Using additional measurements from pulsar scintillation and scattering observations, we can globally estimate the electron-density wavenumber spectral index to test departures from the theoretical Kolmogorov model of turbulence.