band below 300 Hz, where the isotropic search is most sensitive. The narrow frequency binning of 1/32 Hz was needed to cut out a comb of coherent lines found at integer frequencies. A list of notch filters corresponding to lines which were removed from the analysis is also available on the public data release page [@o2_iso_data_behind_figures].

![The cross-correlation spectrum $\hat C(f)$ measured between Advanced LIGO’s Hanford and Livingston detectors during its second observing run. The estimator is normalized so that $\langle \hat C(f) \rangle = \Omega_{\rm GW}(f)$ for tensor-polarized gravitational waves. The black traces mark the $\pm 1\sigma$ uncertainties on the measured cross-correlations. Coherent lines that were identified to have an instrumental cause have been removed from the spectrum. The loss in sensitivity visible at approximately 64 Hz is due to a zero in the tensor overlap reduction function $\gamma_T(f)$. []{data-label="fig:Y_sig"}](Fig1.pdf){width="49.00000%"}

*O2 Results*— In Figure \[fig:Y\_sig\], we plot the observed cross-correlation spectrum $\hat C(f)$ and uncertainty $\sigma(f)$ obtained from Advanced LIGO’s O2 run. We only plot the spectrum up to 100 Hz to focus on the most sensitive part of the frequency band. These data are also publicly available on the webpage [@o2_iso_data_behind_figures], and can be used to search for  