(An)isotropy measurement with gravitational wave observations

Abstract

We constrain the distribution of merging compact binaries across the celestial sphere using the GWTC-3 catalog from the LIGO-Virgo-KAGRA Collaborations' (LVK) third observing run. With 63 confident detections from O3, we constrain the relative variability (standard deviation) of the rate density across the sky to be $\ensuremath{\lesssim}16%$ at 90% confidence assuming the logarithm of the rate density is described by a Gaussian random field with correlation length $\ensuremath{\ge}10\ifmmode^\circ\else\textdegree\fi{}$. This tightens to $\ensuremath{\lesssim}3.5%$ when the correlation length is $\ensuremath{\ge}20\ifmmode^\circ\else\textdegree\fi{}$. While the new data provides the tightest constraints on anisotropies available to date, we do not find overwhelming evidence in favor of isotropy, either. A simple counting experiment favors an isotropic distribution by a factor of ${\mathcal{B}}_{\mathrm{ani}}^{\mathrm{iso}}=3.7$, which is nonetheless an improvement of more than a factor of two compared to analogous analyses based on only the LVK's first and second observing runs.