I don't think "external sources" necessarily means "multi-messenger." GW detectors are sensitive to local interference, so we have three of them spread across t

I don't think "external sources" necessarily means "multi-messenger." GW detectors are sensitive to local interference, so we have three of them spread across the globe with non-overlapping blind spots. A GW must be detected in at least two of the detectors to be confirmed. I'm not sure if that's what @dogma1138 meant, but I'm pretty sure that's how the past GW detections have been confirmed.