Most reports of superluminal effects are not hard to debunk. This one is more difficult.
https://www.space.com/41724-neutron-star-merger-superfast-jet.html All one can say about this reasoning, or lack thereof, is “lame”… or to be more precise: “invented.” The conclusion of superluminal emission can probably be debunked eventually but not by inventing a “cocoon” which had to be travelling even faster in order to be in place to distort the measurement… Maybe it is simply poor technical writing.
