Why can there be several seconds of lag between a binary neutron star merger and the emission of gamma rays from the same area?
The paper (section 5.1) discusses three possibilities in the context of a relativistic fireball model, where some of the kinetic energy in relativistic jets of material emerging from the explosion is converted into gamma rays. The gravitational wave emission is always &quot;prompt&quot; since any surrounding material is transparent to gravitational waves. In contrast, the gamma ray emission may be delayed because:

The gamma rays are produced by shocks. There is a delay in the jets reaching the radius where the shocks form, either generated internally in the fireball by components with different speeds or just through deceleration in the interstellar medium.

The fireball is initially opaque to gamma rays and it is not until it has expanded sufficiently that it becomes transparent, allowing the gamma rays to escape.

It may take some time for the jet to punch out of the dense inner regions of the explosion.