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 (PTAs) through excursions from, and breaks in, the expected <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>GW</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> power law of the GWB strain spectrum. To do this, we create a semianalytic SMBHB population model, fit to North American Nanohertz Observatory for Gravitational Waves (NANOGrav’s) 15 yr GWB amplitude, and with 1000 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 nHz, is below our GWB realizations with a p -value significance p = 0.05–0.06 (≈1.8 σ –1.9 σ ). The second, at 16 nHz, is above our GWB realizations with p = 0.04–0.15 (≈1.4 σ –2.1 σ ). We explore the properties of a loud SMBHB that could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by 3 orders of magnitude, from ∼10 6 to ∼10 3 , between 2 and 20 nHz. This causes a break in the strain spectrum as the stochasticity of the background breaks down at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>26</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>28</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi>nHz</mml:mi> </mml:math> , consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the 26 nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early Universe.

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Publication Details

Journal
The Astrophysical Journal
Published
2024-12-23
DOI
https://doi.org/10.3847/1538-4357/ad93d5
Citations
12
Primary Topic
Pulsars and Gravitational Waves Research
Type
article
Field-Weighted Citation Impact
3.98

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

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12 citations
The Astrophysical Journal
Pulsars and Gravitational Waves Research
3.98
article

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

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article en
12 citations

Abstract

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 (PTAs) through excursions from, and breaks in, the expected <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>GW</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> power law of the GWB strain spectrum. To do this, we create a semianalytic SMBHB population model, fit to North American Nanohertz Observatory for Gravitational Waves (NANOGrav’s) 15 yr GWB amplitude, and with 1000 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 nHz, is below our GWB realizations with a p -value significance p = 0.05–0.06 (≈1.8 σ –1.9 σ ). The second, at 16 nHz, is above our GWB realizations with p = 0.04–0.15 (≈1.4 σ –2.1 σ ). We explore the properties of a loud SMBHB that could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by 3 orders of magnitude, from ∼10 6 to ∼10 3 , between 2 and 20 nHz. This causes a break in the strain spectrum as the stochasticity of the background breaks down at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>26</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>28</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi>nHz</mml:mi> </mml:math> , consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the 26 nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early Universe.

The Astrophysical JournalVol. 978(1)
National Science Foundation, National Aeronautics and Space Administration, Research Corporation for Science Advancement, Deutsche Forschungsgemeinschaft, Jet Propulsion Laboratory, Science and Technology Facilities Council, Office of Naval Research, U.S. Naval Research Laboratory
Openalex Percentile: Top 6%
Pulsars and Gravitational Waves Research
3.98
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