Searching for Isotropic Stochastic Gravitational-Wave Background in the International Pulsar Timing Array Second Data Release (2109.00296v1)

Abstract: We search for isotropic stochastic gravitational-wave background (SGWB) in the International Pulsar Timing Array second data release. By modeling the SGWB as a power-law, we find very strong Bayesian evidence for a common-spectrum process, and further this process has scalar transverse (ST) correlations allowed in general metric theory of gravity as the Bayes factor in favor of the ST-correlated process versus the spatially uncorrelated common-spectrum process is $30\pm 2$. The median and the $90\%$ equal-tail amplitudes of ST mode are $\mathcal{A}{\mathrm{ST}}= 1.29^{+0.51}{-0.44} \times 10^{-15}$, or equivalently the energy density parameter per logarithm frequency is $\Omega_{\mathrm{GW}}^{\mathrm{ST}} = 2.31^{+2.19}_{-1.30} \times 10^{-9}$, at frequency of 1/year. However, we do not find any statistically significant evidence for the tensor transverse (TT) mode and then place the $95\%$ upper limits as $\mathcal{A}{\mathrm{TT}}< 3.95 \times 10^{-15}$, or equivalently $\Omega{\mathrm{GW}}^{\mathrm{TT}}< 2.16 \times 10^{-9}$, at frequency of 1/year.

• The paper demonstrates robust Bayesian evidence (Bayes factor ~30±2) for scalar transverse correlations over tensor modes in the stochastic gravitational-wave background.
• It estimates the scalar mode amplitude at approximately 1.29×10⁻¹⁵ and an energy density parameter of about 2.31×10⁻⁹ at 1/year.
• The study sets upper limits for tensor correlations, highlighting the need for further investigations into extended gravitational theories.

Searching for Isotropic Stochastic Gravitational-Wave Background in the International Pulsar Timing Array Second Data Release

This paper embarks on an exploration to identify an isotropic stochastic gravitational-wave background (SGWB) using data from the International Pulsar Timing Array (IPTA) second data release. The IPTA 2nd data release, composed of high-precision measurements from 65 millisecond pulsars, offers a unique opportunity to probe gravitational waves at nanohertz frequencies. By deploying a power-law model for the SGWB, the authors report significant findings concerning the presence of scalar transverse (ST) correlations, which align with predictions from general metric theories of gravity, as opposed to tensor transverse (TT) correlations typically expected under general relativity.

The analysis showcases robust Bayesian evidence for a common-spectrum process with a strong Bayes factor of 30±2 favoring ST correlations over spatially uncorrelated spectra. The ST mode amplitude median value is estimated at $$\mathcal{A}_{\mathrm{ST}} = 1.29^{+0.51}_{-0.44} \times 10^{-15}$$ with a corresponding energy density parameter per logarithm frequency of $$\Omega_{\mathrm{GW}}^{\mathrm{ST}} = 2.31^{+2.19}_{-1.30} \times 10^{-9}$$, at an observational frequency of 1/year. This finding corroborates previous evidence noted in similar investigations using data from the NANOGrav 12.5-year dataset.

Conversely, the paper did not find substantial evidence pointing to TT mode correlations. The lack of TT signal necessitated establishing upper limits for the TT amplitude, placed at $\A{\TT} < 3.95 \times 10^{-15}$, and for the energy density parameter, $\ogw^\TT < 2.16 \times 10^{-9}$ at 1/year.

The implications of these findings are twofold. Practically, they hint at the compelling prospect of alternative gravitational wave polarizations distinct from the quadrupolar modes anticipated by general relativity. Theoretically, the support for ST correlations invites contemplation and further investigation within the framework of broader gravitational theories potentially involving scalar fields or alternative extensions to general relativity.

This research inspires future studies to verify and refine these findings across expanded datasets, with improved sensitivity to distinct polarization modes. Such investigations could significantly enhance our understanding of both gravitational wave physics and the underlying geometry of spacetime, further integrating advancements in observational astrophysics with theoretical physics.