  Advanced LIGO and Advanced Virgo are actively monitoring the sky and
collecting gravitational-wave strain data with sufficient sensitivity to detect
signals routinely. In this paper we describe the data recorded by these
instruments during their first and second observing runs. The main data
products are the gravitational-wave strain arrays, released as time series
sampled at 16384 Hz. The datasets that include this strain measurement can be
freely accessed through the Gravitational Wave Open Science Center at
http://gw-openscience.org, together with data-quality information essential for
the analysis of LIGO and Virgo data, documentation, tutorials, and supporting
software.
