We can consider sky localization in greater detail by comparing areas on an event-by-event basis and not just the cumulative distribution across the population. Doing this, we confirm that sky localization is consistent between approaches for any given event. We use the medium-latency non-spinning TaylorF2 analysis as a reference point and compare the ratio of sky areas. To summarize the variation in sky areas computed in different analyses, we use the log ratio \[\mathcal{R}_A^X = \log_{10}\left(\frac{A^X}{A^\mathrm{NS}}\right),\] where \(A^X\) is a credible region or the searched area as determined by method \(X\) and \(A^\mathrm{NS}\) is the same quantity from the non-spinning analysis. The log ratio \(\mathcal{R}_A^X\) is zero when analysis \(X\) agrees with the non-spinning results. Considering all \(250\) events, the mean and standard deviation of the log ratio is given in Table \ref{tab:sky-ratio}. For the purposes of EM follow-up, there is no significant difference between analyses.1 The computationally expensive fully spinning analysis does not improve sky localization: there is no disadvantage in using the lower-latency results for EM follow-up of slowly spinning BNSs.


  1. The non-spinning analysis was performed using LALInference_nest while the spinning analysis was performed using LALInference_MCMC \citep{Veitch_2014}; therefore, the consistency between analyses additionally shows the consistency of results from different sampling algorithms.