Main Question or Discussion Point
when two neutron stars collide, do they creat both a balck hole and release gamma rays or does it happen in stages?
The general belief the last time I checked was that the black hole limit was on the low side. What happens is that as long as something is a neutron star anything that hits it can get blown off, but once something turns into a black hole, it just keeps getting bigger and bigger.The great majority of neutron stars detected to date are less than 1.5 solar masses. If the actual TOV limit is on the high side [~3], colliding neutron stars would generally be incapable of forming a black hole. The least massive black hole detected thus far is around 3.8 solar masses.
There is a natural explanation for this. If you have a black hole just sitting by itself, you are never going to see it, while you *do* see neutron stars that are sitting by themselves. You only see black holes when stuff has been falling into them or when they are a companion to some binary star. If this is happening, then chances are that the black hole has been gobbling stuff for a while and so has gotten big.A curiousity is the large discrepancy the least massive black hole and most massive neutron star - about 2 solar masses. As twofish noted, the equation of state may favor lower masses [~ 2 solar] for black holes, and, this discrepancy is due to difficulty in detecting black holes near the lower mass limit.