Neutron Star Collisions: Effects & Stages

[josh_c7]

when two neutron stars collide, do they creat both a balck hole and release gamma rays or does it happen in stages?

 

[Chronos]

Neutron star collisions might or might not result in a black hole. The Tolman–Oppenheimer–Volkoff [TOV] limit on minimum black hole mass is 1.5 - 3.0 solar masses. This range is due to uncertainty over the equation of state [EOS] for degenerate neutron matter. 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.

 

[josh_c7]

thank you for your reply, i know see that not all collisons result in a black hole, but if the solar masses were large enough, would you have gamma rays and black holes at the same time?

 

[twofish-quant]

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.

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 idea behind one type of gamma ray bursters is that when you have two neutron stars collide, the powerful magnetic fields cause the debris to get spun around creating huge amounts of gamma rays, and then they become a black hole which eats up everything so that it all disappears without a trace.

 

[josh_c7]

thanks you for your reply also, i see what you mean, i was just wondering about if there were to be gamma ray burst and black holes at the same time, how would the gamma ray escape the black hole, but i see what you mean now with the black hole coming in after the bursts, thank you both for you replys :)

 

[twofish-quant]

Also there appear to be different types of gamma ray bursters. There is one type in which you have a burst that flashes and then nothing, and those can be explained by the black hole eating the debris. There are others that last for longer periods that there probably aren't black holes there.

 

[Chronos]

A curiosity 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.

 

[twofish-quant]

A curiosity 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.

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.

You'll only see a light black hole if it's in the middle of nowhere and hasn't been gobbling up gas and dust, but if it's in the middle of nowhere, it's not going to attract any attention at all.

 

[Chronos]

True, but, about 5% of neutron stars are members of binary systems. It would be reasonable to assume about the same occurence rate for stellar mass black holes. That suggests we should have observed about 100 stellar mass black holes in our galaxy. We are nowhere near that number.

 

[twofish-quant]

It could be an observational artifact. If you have a neutron star in the binary system, it's going to be very loud. Matter is going to interact with the neutron stars in ways that are going to cause you to see something interesting happening. Matter that's falling into a black hole is going to attract a lot less attention. (At that's that what I remember people telling me. If there are some accretion disk people that say otherwise speak up.)

Or there could be something interesting happening with supernova or binary star physics.

One other thing that I find interesting is that the black hole candidates that we see are in the 4-10 solar mass range. If you had a massive stars collapse directly to black holes without anything else happening, then you should be seeing 30 solar mass black holes.

This means that there could be some interesting supernova physics going on. It's possible that nature let's you form a two solar mass black hole just fine, but that these never get produced because of some quirk of the way supernovas happen or how binary stars evolve. Now what we should be able to do is to just run supernova simulations for different mass stars and see what happens, which then gets into the problem that no one has gotten supernova explosions that work well enough so that you can do this.

As far as why people tend to think that the black hole limit is on the low side. Basically you figure out whether something is soft or stiff by counting the number of different ways that it can change in response to pressure. If you hit a brick, it's not going to change much under pressure so it's stiff. If you hit a bean bag, it's going to change a lot under pressure so it's soft. The most recent nuclear physics seems to suggest that he really high pressure there are all sorts of ways that nuclear matter can change (the term "quark soup" comes up) so that suggests a soft equation of state rather than a stiff one.