Jonathan S. wrote-

...aren't _changes_ in gravitational fields
supposed to propagate by gravity
waves?

O yes. I see what you're saying here, Jonathan.. since the radiating
object appears gravitationally quiescent, it shouldn't be radiating any
GWs.
I was going under the assumption of ringdown from
violent disruptions in the object's past, recorded by those outbound
pressure waves.
HOWEVER if the object has *always* been
gravitationally quiet, then there would be no GW signature, And the
pressure waves could only be interpreted as sound in the interstellar
gas.

Lacking long-term observations to determine the waves' actual velocity
(indicating whether they're propagating at c or some lower value), the
interpretation remains open.

Meanwhile, the subject provoked some lively discussion on gravity vs.
'gravity waves', "speed of gravity", BH ringdown etc.
oc

Anti-spam address: oldcoot88atwebtv.net
Change 'at' to@