Suppose you have a neutron star that's as close as can be imagined to the required mass to become a black hole, perhaps just one proton mass away from this limit, when it collides with a dust grain from space.
Is the collapse instantaneous? Is the event horizon formed in the center and moves outward as more mass gets pulled in? Are there gravitational waves as a result of the sudden shift in the spacetime geometry?
Any details that are known about such an event would be greatly appreciated! Thank you
Edit: As requested I'm limiting my question to be more specific to what most interests me - Does the event horizon form at the center of the neutron star? Is there a specific "initial size" that grows?
