[@2009MNRAS.398..293M; @2010MNRAS.409L.132Z; @PhysRevD.72.084001; @PhysRevD.73.104024].

*Data*— We analyze data from Advanced LIGO’s second observing run, which took place from 16:00:00 UTC on 30 November, 2016 to 22:00:00 UTC on 25 August, 2017. We cross correlate the strain data measured by the two Advanced LIGO detectors, located in Hanford, WA and Livingston, LA in the United States [@aLIGO_2015]. Linearly coupled noise has been removed from the strain time series at Hanford and Livingston using Wiener filtering [@Noise_sub_O2; @Wiener_O2], see also [@Noise_sub_1; @Noise_sub_2; @Noise_sub_3]. By comparing coherence spectra and narrowband estimators formed with and without Wiener filtering, we additionally verified that this noise subtraction scheme does not introduce correlated artifacts into the Hanford and Livingston data.

Virgo does not have a significant impact on the sensitivity of the stochastic search in O2, because of the larger detector noise, the fact that less than one month of coincident integration time is available, and because the overlap reduction function is smaller for the Hanford-Virgo and Livingston-Virgo pairs than for Hanford-Livingston. Therefore we do not include Virgo data in the O2 analysis.

The raw strain data are recorded at 16,384 Hz. We first downsample the strain time series to 4096 Hz, and apply a 16th-order high-pass 