Improved limits on a stochastic gravitational-wave background and its anisotropies from Advanced LIGO O1 and O2 runs

Abstract

We integrate the entire, publicly available, Advanced LIGO (ALIGO) data set to obtain maximum-likelihood constraint maps of the Stochastic Gravitational-Wave Background (SGWB). From these we derive limits on the energy density of the stochastic background $\Omega_{\rm GW}$, and its anisotropy. We find 95% confident limits $\Omega_{\rm GW} < 5.2\times 10^{-8}$ at $50$ Hz for a spectral index $\alpha=2/3$ consistent with a stochastic background due to inspiral events and $\Omega_{\rm GW} < 3.2\times 10^{-7}$ for a scale (frequency) invariant spectrum. We also report upper limits on the angular power spectra $C_\ell$ for three broadband integrations of the data. Finally we present an estimate where we integrate the data into ten separate spectral bins as a first attempt to carry out a model-independent estimate the SGWB and its anisotropies.