The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background

Abstract

We search for an isotropic stochastic gravitational-wave background (GWB) in the 12.5 yr pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power law, with common amplitude and spectral slope across pulsars. Under our fiducial model, the Bayesian posterior of the amplitude for an f −2/3 power-law spectrum, expressed as the characteristic GW strain, has median 1.92 × 10−15 and 5%–95% quantiles of 1.37–2.67 × 10−15 at a reference frequency of f yr = 1 yr − 1 ; the Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds 10,000. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature.

Parameter estimation of a two-component neutron star model with spin wandering

“…In many papers, e.g. Arzoumanian et al (2020), it is common to take the timing residual power spectral density to be…”

“…Precision pulsar timing experiments in both the X-ray and radio bands continue to yield ground-breaking measurements and discoveries. Most recently, this includes tight constraints on the mass and radius of neutron stars using the Neutron-star Interior Composition Explorer (Gendreau et al 2016;Riley et al 2019;Raaijmakers et al 2019;Miller et al 2019;Bogdanov et al 2019), as well as evidence for a common noise process in the NANOGrav 12.5 year data set that is consistent with the shape of a stochastic gravitational-wave background (Arzoumanian et al 2020). One common feature revealed by pulsar timing experiments is deviations from the long-term secular rotation and spin-down of the star known as 'spin-wandering.'…”

“…One common feature revealed by pulsar timing experiments is deviations from the long-term secular rotation and spin-down of the star known as 'spin-wandering.' When testing general relativity or seeking to detect a stochastic gravitational-wave background, spin wandering is treated as a nuisance, which needs to be characterized accurately and then subtracted in a statistical sense (Groth 1975;Cordes 1980;Arzoumanian et al 1994;Shannon & Cordes 2010;Price et al 2012; ★ E-mail: pat.meyers@unimelb.edu.au Namkham et al 2019;Parthasarathy et al 2019;Lower et al 2020;Parthasarathy et al 2020;Goncharov et al 2020). In this paper, we use 'spin wandering' to refer to the achromatic fluctuations in pulse times-of-arrival (TOAs) that are intrinsic to the pulsar, specifically the rotation of its crust and corotating magnetosphere.…”

Loop decay in Abelian-Higgs string networks

“…In this model, the most stringent constraints are provided by the generation of a background of gravitational waves (GWs) by slowly decaying loops of cosmic string, currently Gμ ≲ 10 −10 [9-13] (with μ the string tension and G Newton's constant). Indeed, the recent observations of an excess in the timing residuals of millisecond pulsars [14] could be accounted for by GWs from Nambu-Goto strings with tension saturating the bound [15,16].…”

To Observe, or Not to Observe, Quantum-Coherent Dark Matter in the Milky Way, That is a Question

“…Therefore, photons which pass through such oscillating gravitational fields will likewise suffer shifts in arrival times. As a result, this effect can be searched for using radio telescope networks for pulsar timing arrays such as NANOGrav (Arzoumanian et al, 2020), which can indirectly infer the presence of oscillating scalar fields through a measurement of coherent shifts in the pulse arrival times of a sample of precisely spinning millisecond pulsars (Khmelnitsky and Rubakov, 2014). find that the effect on pulsar timing is comparable to the corresponding effect of a monochromatic gravitational wave with characteristic (Porayko et al, 2018).…”