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tokens: 40.7 Mpc with a standard deviation of 2.36 Mpc, corresponding to the measured distance and quadrature sum of statistical and systematic errors reported in Cantiello et al. ( 2018 ). Here we have assumed a Gaussian distribution on this distance measurement, which we deem valid for a measurement of this precision and for the purpose of exploring upper and lower bounds. Using the EM observations as the prior on the luminosity distance results in significantly narrower posteriors on inclination angle and source-frame chirp mass shown in Figure 2 . The improved chirp mass measurement is due to the reduced error on d L , as the d L posterior samples are used to convert from the measured detector-frame chirp mass to the source-frame chirp mass (Schutz 1986 ; Finn Chernoff 1993 ). However, the improved precision on distance has no effect on our measurements of the component masses or spins, because at leading order the mass ratio q = m 1 / m 2 and spin parameter χ eff (Cutler Flanagan 1994 ) are not correlated with distance. With the EM observations as the prior on d L and a prior on the inclination angle uniform in , we find that the viewing angle is degrees (90% confidence). Figure 2. Comparison of posterior probability distributions without and with combined EM information. The black contours show the results for using only the GW signal, the blue contours show the results for the fixed sky location of NGC 4993, and the red contours show the results for both the fixed sky location and a Gaussian prior on distance of 40.7 ± 2.36 Mpc from Cantiello et al. ( 2018 ). These analyses used a prior on inclination angle that is uniform in . For each parameter, we quote the median value and the 90% credible interval (shown with vertical solid and dashed lines, respectively, on the posterior plot of each parameter). The EM information on the distance measurement greatly improves the precision with which we measure the inclination angle (and significantly reduces the uncertainty on the source-frame chirp mass). Errors in the calibration of the GW detectors can cause errors in the measured amplitude of the GW signal and hence in the inclination angle of the binary. We treat this as a systematic error, which we measure by shifting the amplitude calibration of the LIGO and Virgo detectors by the 1 σ uncertainty bounds for LIGO/Virgo’s second observing run (Cahillane et al. 2017 ). We find that shifting the calibration to its most sensitive and least sensitive extremes results in a ±17 shift in the peak of the viewing angle when using a prior on inclination angle that is uniform in . We quote this shift as the systematic error on our measurement. Changing the phase error of the calibration within the bounds reported in Cahillane et al. ( 2017 ) produces a negligible effect on the inclination angle. A prior uniform in goes to zero as the viewing angle approaches face on (or face off), so we repeat our analysis using a prior uniform in ι . Figure 3 shows a