Neutron star merge why didn't all EM radiation came at once?

[Borek]

Bear with me, I am just a chemist.

Observations took several days (up to two weeks if memory serves me well). What I wonder is - why had different types of the EM radiation came at different times? Gamma burst was observed at almost exactly the same time gravitational waves were detected, but visible light, IR and X-rays came days later.

Radiation traveled mostly through a high vacuum, so the speed should be identical and the arrival of radiation in all wavelength should be simultaneous. The most obvious answer is that we observed what was produced at different moments of the merger due to its mechanism (combined with cooling of the expanding matter ejected in the collision). But is it all, or is there more to it? Is the interstellar/intergalactic vacuum dense enough to slow down different wavelengths in an observable way?

 

[phyzguy]

It took about 11 hours for the optical telescopes to start looking, mostly because they were waiting for it to get dark. As soon as they started looking, the optical radiation was there, but it shifted to longer wavelengths as the ejecta cooled off over time.