(An)isotropy measurement with gravitational wave observations
R. C. Essick, Will M. Farr, M. Fishbach, D. E. Holz, E. Katsavounidis
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.