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

## TL;DR

This paper combines Advanced LIGO O1 and O2 data to set new constraints on the energy density and anisotropies of the stochastic gravitational-wave background, improving previous limits and exploring model-independent estimates.

## Contribution

It provides the first comprehensive maximum-likelihood constraint maps of the SGWB using the full publicly available LIGO data set, including anisotropy analysis and spectral binning.

## Key findings

- 95% confidence limit on $ ext{Ω}_ ext{GW}$ at 50 Hz is 5.2×10⁻⁸
- Upper limits on angular power spectra $C_ ext{ℓ}$ are reported for three broadband integrations
- First attempt at a model-independent estimate of SGWB and its anisotropies through spectral binning

## 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.