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

We integrate the entire, publicly available, Advanced LIGO dataset 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 ωGW and its anisotropy. We find 95% confident limits ωGW<5.2×10-8 at 50 Hz for a spectral index α=2/3 consistent with a stochastic background due to inspiral events and ωGW<3.2×10-7 for a scale (frequency) invariant spectrum. We also report upper limits on the angular power spectra C for three broadband integrations of the data. Finally, we present an estimate in which we integrate the data into ten separate spectral bins as a first attempt to carry out a model-independent estimate of the SGWB and its anisotropies.