[syhprum (OP)]

how quickly would it happen ?, would there be much radiation of photons and neutrinos before the event horizon trapped everything ?.

[evan_au]

A recent supercomputer simulation produced a video of what might happen in a neutron star collision:

• The neutron stars will approach each other as gravitational waves radiate away their angular momentum
  
• If we had a sensitive-enough gravity-wave telescope, we would see a "chirp" of increasing frequency, as the neutron stars spin around each other, faster and faster.
  
• The final merger takes around 30ms
  
• The two neutron stars are distorted as they approach each other; as they touch, they shatter and spray superheated "neutronium" into an accretion disk around them
  
• Intense magnetic fields can sweep up material from this accretion disk and accelerate it to nearly the speed of light; if we happen to be in line with this jet, we see it as a short gamma ray burst, lasting less than 1 second. It is one of the most intense events in the universe.
  
• The neutronium in the jet and accretion disk, no longer compressed by the intense gravity of the neutron star will start to decay into protons, electrons and neutrinos; the protons & neutrons will form atomic nuclei, although the temperature will be initially too high to form atoms.
  
• It is thought that most of the elements heavier than iron are formed in neutron star collisions
  
• If the combined mass of the two neutron stars exceeds 3-4 times the mass of the Sun, it will collapse into a black hole. Otherwise it will just form a single massive and extremely hot neutron star.
  

Neutron star collisions will not be a major source of neutrinos, since the majority of matter is already neutrons, and stays in the form of neutrons until it is swallowed by the black hole. Core-collapse supernovae like SN1987a produce more (anti)neutrinos, since a large amount of "normal matter" (protons & electrons) is rapidly crushed into neutrons, releasing an intense blast of antineutrinos.