The astrophysics of nanohertz gravitational waves

Abstract

Pulsar timing array (PTA) collaborations in North America, Australia, and Europe, have been exploiting the exquisite timing precision of millisecond pulsars over decades of observations to search for correlated timing deviations induced by gravitational waves (GWs). PTAs are sensitive to the frequency band ranging just below 1 nanohertz to a few tens of microhertz. The discovery space of this band is potentially rich with populations of inspiraling supermassive black-holes binaries, decaying cosmic string networks, relic postinflation GWs, and even non-GW imprints of axionic dark matter.This article aims to provide an understanding of the exciting open science questions in cosmology, galaxy evolution, and fundamental physics that will be addressed by the detection and study of GWs through PTAs. The focus of the article is on providing an understanding of the mechanisms by which PTAs can address specific questions in these fields, and to outline some of the subtleties and difficulties in each case. The material included is weighted most heavily towards the questions which we expect will be answered in the near-term with PTAs; however, we have made efforts to include most currently anticipated applications of nanohertz GWs.PACS numbers: § Note, a more precise remark is that the Earth and pulsar terms are not correlated as long as two pulsars are separated by many gravitational wavelengths, that is to say that f L 1, where f is the GW frequency and L is distance to the pulsar. This assumption is called the short-wavelength approximation (e. g. Mingarelli & Mingarelli 2018).

Pulsar timing array signals induced by black hole binaries in relativistic eccentric orbits

“…We begin by comparing the computational cost of our semi-analytic approach against numerically solving Eqs. (17) to obtain the orbital evolution. Excluding the one-time cost of computing the look-up table for e(τ ), the execution time t exec taken to compute the state of the orbit (n, e t , γ, l) at a given set of TOAs should depend on the number of TOAs (N TOA ) as well as their total observation span/integration span (specified by some t 0 and t 1 ).…”

“…This behavior is expected, since a numerical ODE solver is required to compute the right hand side of Eqs. (17) at many points between the TOAs where the solutions are required while evolving the binary over time. In contrast, our semi-analytic approach only computes the solutions at the required TOAs.…”

“…The International Pulsar Timing Array (IPTA) consortium combines data and resources to achieve more quickly the first detection of nanohertz GWs [13][14][15]. * Email: s.abhimanyu@tifr.res.in There are a number of promising astrophysical and cosmological GW sources in the nanohertz frequency window [16,17]. We expect the first detected signal to be the ensemble of GWs from many SMBH binaries, producing a stochastic GW background (SGWB).…”

“…These considerations ensure that the prescription is general relativistically accurate and computationally efficient. It turns out that the PN description is quite appropriate to model such PTA signals as the SMBH binaries are expected to merge at orbital frequencies outside the PTA frequency window [17]. Additionally, we provide a fully-analytic prescription to compute PTA signals induced by isolated SMBH binaries inspiraling along moderately eccentric orbits.…”

Joint search for isolated sources and an unresolved confusion background in pulsar timing array data

“…Pulsar Timing Arrays (PTAs) are designed to detect gravitational waves (GWs) in the nanohertz regime by looking for signatures of a GW signal in the pulse arrival times of regularly observed millisecond pulsars (see e.g. [1]). Currently there are three major PTA projects: the North American Nanohertz Observatory for Gravitational Waves (NANOGrav, [2]), the European Pulsar Timing Array (EPTA, [3]), and the Parkes Pulsar Timing Array (PPTA, [4]).…”