Targeted search for the kinematic dipole of the gravitational-wave background

Chung, A. K. W.; Jenkins, A. C.; Romano, Joseph D.; Sakellariadou, Mairi

doi:10.1103/physrevd.106.082005

Cited by 12 publications

(5 citation statements)

References 64 publications

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“…A detection of pixel-scale anisotropy could provide the first indication of a single inspiraling SMBHB system which could then be subjected to targeted followup using both PTA GW and electromagnetic observatories. A detection of large scale anisotropy could indicate an over-density of SMBHB systems in a cluster environment, or might point to an extrinsic effect (Chung et al 2022), such as the kinematic dipole observed with the cosmic microwave background (Planck Collaboration et al 2020) and predicted to be detectable with a GWB (Bertacca et al 2020;Chung et al 2022;Cusin & Tasinato 2022;Valbusa Dall'Armi et al 2022). If we live in a Universe where both astrophysical (e.g., from SMBHBs) and cosmological (e.g., from cosmic strings) GWBs are present, we can leverage our knowledge of the different expected spatial (and spectral) distribution of these processes to disentangle them in the PTA datasets (Ungarelli & Vecchio 2001;Mandic et al 2012;Parida et al 2016;Biscoveanu et al 2020;Martinovic et al 2021;Suresh et al 2021;Kaiser et al 2022).…”

Section: Conclusion and Future Prospectsmentioning

confidence: 99%

The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background

Agazie

Anumarlapudi

Archibald

et al. 2023

ApJL

533

283

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We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings–Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of 1014, and this same model is favored over an uncorrelated common power-law spectrum model with Bayes factors of 200–1000, depending on spectral modeling choices. We have built a statistical background distribution for the latter Bayes factors using a method that removes interpulsar correlations from our data set, finding p = 10−3 (≈3σ) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of interpulsar correlations yields p = 5 × 10−5 to 1.9 × 10−4 (≈3.5σ–4σ). Assuming a fiducial f −2/3 characteristic strain spectrum, as appropriate for an ensemble of binary supermassive black hole inspirals, the strain amplitude is 2.4 − 0.6 + 0.7 × 10 − 15 (median + 90% credible interval) at a reference frequency of 1 yr−1. The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings–Downs correlations points to the gravitational-wave origin of this signal.

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Section: Conclusion and Future Prospectsmentioning

confidence: 99%

The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background

Agazie

Anumarlapudi

Archibald

et al. 2023

ApJL

533

283

View full text Add to dashboard Cite

show abstract

“…A detection of pixel-scale anisotropy could provide the first indication of a single inspiraling SMBHB system that could then be subjected to targeted follow-up using both PTA GW and electromagnetic observatories. A detection of large-scale anisotropy could indicate an overdensity of SMBHB systems in a cluster environment, or might point to an extrinsic effect (Chung et al 2022), such as the kinematic dipole observed with the cosmic microwave background (Planck Collaboration et al 2020) and predicted to be detectable with a GWB (Bertacca et al 2020;Chung et al 2022;Cusin & Tasinato 2022;Valbusa Dall'Armi et al 2022). If we live in a Universe where both astrophysical (e.g., from SMBHBs) and cosmological (e.g., from cosmic strings) GWBs are present, we can leverage our knowledge of the different expected spatial (and spectral) distribution of these processes to disentangle them in the PTA data sets (Ungarelli & Vecchio 2001;Mandic et al 2012;Parida et al 2016;Biscoveanu et al 2020;Martinovic et al 2021;Suresh et al 2021;Kaiser et al 2022).…”

Section: Conclusion and Future Prospectsmentioning

confidence: 99%

The NANOGrav 15 yr Data Set: Search for Anisotropy in the Gravitational-wave Background

Agazie,

Anumarlapudi,

Archibald

et al. 2023

ApJL

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The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has reported evidence for the presence of an isotropic nanohertz gravitational-wave background (GWB) in its 15 yr data set. However, if the GWB is produced by a population of inspiraling supermassive black hole binary (SMBHB) systems, then the background is predicted to be anisotropic, depending on the distribution of these systems in the local Universe and the statistical properties of the SMBHB population. In this work, we search for anisotropy in the GWB using multiple methods and bases to describe the distribution of the GWB power on the sky. We do not find significant evidence of anisotropy. By modeling the angular power distribution as a sum over spherical harmonics (where the coefficients are not bound to always generate positive power everywhere), we find that the Bayesian 95% upper limit on the level of dipole anisotropy is (C l=1/C l=0) < 27%. This is similar to the upper limit derived under the constraint of positive power everywhere, indicating that the dipole may be close to the data-informed regime. By contrast, the constraints on anisotropy at higher spherical-harmonic multipoles are strongly prior dominated. We also derive conservative estimates on the anisotropy expected from a random distribution of SMBHB systems using astrophysical simulations conditioned on the isotropic GWB inferred in the 15 yr data set and show that this data set has sufficient sensitivity to probe a large fraction of the predicted level of anisotropy. We end by highlighting the opportunities and challenges in searching for anisotropy in pulsar timing array data.

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“…But, as for the CMB, it is potentially well larger than intrinsic cosmological anisotropies (see, e.g., the studies [34][35][36][37][38][39][40]). Kinematic anisotropies is a topic of active research in the context of GW interferometers [23,[41][42][43][44][45]. It is then worth asking what information we can obtain from a possible future detection of kinematic anisotropies with PTA experiments.…”

Section: Introductionmentioning

confidence: 99%

Kinematic anisotropies and pulsar timing arrays

Tasinato

2023

Phys. Rev. D

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Doppler anisotropies, induced by our relative motion with respect to the source rest frame, are a guaranteed property of stochastic gravitational wave backgrounds of cosmological origin. If detected by future pulsar timing array measurements, they will provide interesting information on the physics sourcing gravitational waves, which is hard or even impossible to extract from measurements of the isotropic part of the background only. We analytically determine the pulsar response function to kinematic anisotropies, including possible effects due to parity violation, to features in the frequency dependence of the isotropic part of the spectrum, as well as to the presence of extra scalar and vector polarizations. For the first time, we show how the sensitivity to different effects crucially depends on the pulsar configuration with respect to the relative motion among frames. Correspondingly, we propose examples of strategies of detection, each aimed at exploiting future measurements of kinematic anisotropies for characterizing distinct features of the cosmological gravitational wave background.

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Targeted search for the kinematic dipole of the gravitational-wave background

Cited by 12 publications

References 64 publications

The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background

The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background

The NANOGrav 15 yr Data Set: Search for Anisotropy in the Gravitational-wave Background

Kinematic anisotropies and pulsar timing arrays

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