The NANOGrav 15 yr Data Set: Looking for Signs of Discreteness in the Gravitational-wave Background

Abstract

The cosmic merger history of supermassive black hole binaries (SMBHBs) is expected to produce a low-frequency gravitational wave background (GWB). Here we investigate how signs of the discrete nature of this GWB can manifest in pulsar timing arrays through excursions from, and breaks in, the expected $f_{\mathrm{GW}}^{-2/3}$ power-law of the GWB strain spectrum. To do this, we create a semi-analytic SMBHB population model, fit to NANOGrav's 15 yr GWB amplitude, and with 1,000 realizations we study the populations' characteristic strain and residual spectra. Comparing our models to the NANOGrav 15 yr spectrum, we find two interesting excursions from the power-law. The first, at $2\mathrm{nHz}$, is below our GWB realizations with $p$-value significance $p=0.05$ to $0.06$ ($\approx 1.8\sigma -1.9\sigma$). The second, at $16\mathrm{nHz}$, is above our GWB realizations with $p=0.04$ to $0.15$ ($\approx 1.4\sigma -2.1\sigma$). We explore the properties of a loud SMBHB which could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by three orders of magnitude, from $\sim 10^6$ to $\sim 10^3$, between $2\mathrm{nHz}$ and $20\mathrm{nHz}$. This causes a break in the strain spectrum as the stochasticity of the background breaks down at $26_{-19}^{+28}\mathrm{nHz}$, consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the $26\mathrm{nHz}$ break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early universe.

13 pages, 8 figures, 2 appendices, in press at ApJ

References in corpus (30)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background
• Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array
• Searching for the nano-Hertz stochastic gravitational wave background with the Chinese Pulsar Timing Array Data Release I
• The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals
• The NANOGrav 15-year Data Set: Search for Signals from New Physics
• The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background
• The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays
• The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational Wave Background
• The NANOGrav 15-year Data Set: Observations and Timing of 68 Millisecond Pulsars
• Expected properties of the first gravitational wave signal detected with pulsar timing arrays
• Viscous Hydrodynamics Simulations of Circumbinary Accretion Discs: Variability, Quasi-Steady State, and Angular Momentum Transfer
• The NANOGrav 15-year Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries
• The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget
• Circumbinary Accretion from Finite and Infinite Disks
• Mapping gravitational-wave backgrounds using methods from CMB analysis: Application to pulsar timing arrays
• The MeerKAT Pulsar Timing Array: First Data Release
• From Bright Binaries To Bumpy Backgrounds: Mapping Realistic Gravitational Wave Skies With Pulsar-Timing Arrays
• The Need For Speed: Rapid Refitting Techniques for Bayesian Spectral Characterization of the Gravitational Wave Background Using PTAs
• Constraining the Solution to the Last Parsec Problem with Pulsar Timing
• Searching for continuous Gravitational Waves in the second data release of the International Pulsar Timing Array
• Multi-band light curves from eccentric accreting supermassive black hole binaries
• Noise analysis in the European Pulsar Timing Array data release 2 and its implications on the gravitational-wave background search
• Forecasting pulsar timing array sensitivity to anisotropy in the stochastic gravitational wave background
• A quasar-based supermassive black hole binary population model: implications for the gravitational-wave background
• The NANOGrav 12.5-year Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries
• Multimessenger pulsar timing array constraints on supermassive black hole binaries traced by periodic light curves
• Electromagnetic signatures from accreting massive black hole binaries in time domain photometric surveys
• Spectral Variance in a Stochastic Gravitational-Wave Background From a Binary Population

Cited by in corpus (5)

• Distinctive GWBs from eccentric inspiraling SMBH binaries with a DM spike
• Sub-GeV dark matter and nano-Hertz gravitational waves from a classically conformal dark sector
• Signals of merging supermassive primordial black holes in pulsar timing arrays
• Insights into Supermassive Black Hole Mergers from the Gravitational Wave Background
• Finite Populations & Finite Time: The Non-Gaussianity of a Gravitational Wave Background