One of two things happens when neutron stars collide: they merge together to form a new, larger neutron star, or they collapse into a black hole. But which happens when? That leads to another, trickier question: How big can a neutron star get?
This is going to sound weird, but mathematically speaking, we aren’t entirely sure what a neutron star is. More specifically, we don’t know how big a neutron star can possibly get before it hits an upper mass limit and collapses into a black hole.
We also aren’t exactly sure how often dual neutron star collisions happen, but it’s probably somewhere between 1 and 10 times in a billion years.
Luckily, we can tell the difference between the two possible outcomes of a neutron star collision observationally. If we manage to spot a pair of neutron stars colliding and they collapse into a black hole, they’re going to emit a blinding gamma ray burst across the galaxy. But if they instead merge into neutron star, that same jet is going to get loaded down with baryons and choke out without ever producing a gamma ray burst.
Again, this is where uncertainty comes in. We’re trying really hard to track gamma ray bursts around the galaxy with rapid-response telescopes specifically tasks with rapidly whirling around to point at any suspect events, but it’s not perfect. Our understanding of how often gamma ray bursts happen will improve as we role out LIGO, Virgo, and LISA to search for gravitational waves associated with the bursts.
With all these unknowns, you can see why it’s hard to say for certain what’s going on when neutron stars collide! But this is where the new research comes in: a team of researchers led by Chris Fryer ran a population study simulating what happened if you slammed together combinations of neutron stars of different characteristics.
The details of all the possible outcomes get decidedly complicated, but the bottom line is to stick with both current theories on the frequency at which these collisions happen and the rate at which we observe gamma ray bursts, then neutron stars need to collapse into black holes after they reach a specific maximum mass. Again, oversimplifying, that maximum mass is 2.3 to 2.4 times the mass of our Sun, unless the neutron star is hot or spinning which can bump that upper limit higher.
This is really cool because just from looking at how often a neutron star collision collapses into a black hole spewing gamma rays verses when it more sedately merges into a larger neutron star, we suddenly have a new constraint on how to describe neutron stars. The best part of this is that as our data gets better from LIGO and the rest of the new observatories, we’ll be able to keep constraining this limit on how big a neutron star can grow.
Need one more twist to bend your brain a tiny bit more? The math works out exactly the same if instead of having two neutron stars collide, it’s a neutron star colliding with a black hole. That means that sometimes, under the right conditions, a neutron star can eat a black hole and come out still looking like a star (albeit a very weird one). Trippy!