"...light is only produced after the gravitational wave signal has ended, and may further be delayed by having to pass through the circumstellar material. This is why, even though light and gravity both travel exactly at the speed of light in a vacuum, the light we saw didn’t arrive until nearly 2 seconds after the gravitational wave signal ceased."
#astronomy
https://bigthink.com/starts-with-a-bang/light-gravitational-waves-arrive/