Neutron Star Collapse: Q&A on Physics and Observation

[anorlunda]

I had a question and I found a thread on PF with a nearly identical question ---Slowly add mass to a neutron star till it collapses. I learned some very interesting physics from that thread, namely the Tolman–Oppenheimer–Volkoff limit, and the significance of "9/8 of its Schwarzschild radius"

But I still have my original question. I presume that density would be greater at the core of the neutron star, and that collapse would begin with collapse of the core, blowing away the outermost matter in a manner analogous to a type II supernova. Then the freed outer neutrons would undergo beta decay. Is that reasonable?

A second question: presume a stable neutron star that acquires mass and undergoes gravitational collapse. Has astronomy ever observed an event like that?

 

[mfb]

Once the neutron star core collapses, it does not provide pressure for the outside any more (this is different from core-collapse supernovae) and everything starts to collapse. The process happens way too fast for the lifetime of free neutrons anyway.

 

[Ken G]

I presume that density would be greater at the core of the neutron star, and that collapse would begin with collapse of the core, blowing away the outermost matter in a manner analogous to a type II supernova.

The density at the core is less-- to keep time from going backward. So it's not that the density behaves normally and time does appear to go backward, it is that the density behaves unusually.

A second question: presume a stable neutron star that acquires mass and undergoes gravitational collapse. Has astronomy ever observed an event like that?

A leading model for "short duration" gamma-ray bursts is merging neutron stars, which go above their maximum mass. As mfb said, there's nothing to create a "bounce" or a supernova, so these are hard to see and all you get is very high energy photons as stuff crashes together on its way in. It's believed you get some kind of highly relativistic accretion disk, and a beamed gamma-ray burst coming out its poles, but not an isotropic blast.

 

[anorlunda]

Thank you mfb and Ken G for educating me. A follow up question please.

If a pulsar accretes matter and approaches the Schwarzschild radius, it pulses should begin to red shift with time (assuming that the pulses are emitted from the surface.) Has anything like that been oberved?

 

[Ken G]

It's complicated, because there's also the rotation rate to consider, so it's not guaranteed it will appear to slow down or speed up as it gets closer to its Schwarzschild radius. Also, although there will be redshifts, there's also higher energy phenomena and stronger magnetic fields, so it's not clear if you will observe lower or higher energy emissions. I guess we'd have to look at what is being observed about these objects, and connect it to their inferred masses.

 

[mfb]

The accretion disk does not reach the event horizon - once matter is beyond the innermost stable orbit, it falls in quickly. This can still lead to large redshifts - iron lines at a few keV are common tools to study it (see these slides for example).

 

[Chronos]

Keep in mind typical neutron star mass is well below the TOV limit - which is itself uncertain. The most massive known neutron star, PSR J0348+0432, is right at 2 solar masses. This sets an observational constraint on the lower TOV limit. The equation of state for condensed matter is a wild card here. The calculated upper TOV limit is somewhere between 2.2 and 2.9 solar masses; re: http://arxiv.org/abs/astro-ph/9608059, The Maximum Mass of a Neutron Star. The circumstances under which a neutron star could accrete sufficient mass to collapse into a black hole are unusual and, no known observational examples exist. A more likely route is via neutron star mergers - which are widely believed to be the source of short GRB's. It is unclear if short GRB's expel significant mass and there are no known examples of ejecta associated with a GRB. This is no surprise given GRB's at a distance near enough to be studied in detail have [fortunately] not been observed. It is certainly possible the extreme gravity of a neutron star could result in mass infall as a one way trip. Another curious issue is the mass gap between the most massive known neutron star and the least massive known black hole. XTE J1650-500 was thought to hold low mass black hole honors at about 3.8 solar masses. This claim was subsequently retracted, re: http://arxiv.org/abs/0902.2852. It appears the current low mass observational limit is set by GRO J1655-40 at 6.3 solar masses. This is known as the black hole mass gap, e.g., http://astrobites.org/2011/10/17/mind-the-black-hole-mass-gap/.

 

[Ken G]

But don't forget that the minimum mass of a neutron star is about 1.4 solar masses, so it's quite likely that almost any neutron star mergers will collapse into black holes rather than make a larger neutron star-- unless there is significant mass ejection, and I agree there isn't a lot of evidence for that. So it sounds to me like short gamma-ray bursts are likely to result in black holes, though I don't know if that is generally expected and it isn't observed as you say. But the absence of low-mass black holes, near 3 solar masses, could just be that they are hard to observe. It generally requires a companion to light up a black hole via accretion, so when two neutron stars merge, their might not be anything left to help us see the resulting black hole.

 

[Matterwave]

But don't forget that the minimum mass of a neutron star is about 1.4 solar masses, so it's quite likely that almost any neutron star mergers will collapse into black holes rather than make a larger neutron star-- unless there is significant mass ejection, and I agree there isn't a lot of evidence for that. So it sounds to me like short gamma-ray bursts are likely to result in black holes, though I don't know if that is generally expected and it isn't observed as you say. But the absence of low-mass black holes, near 3 solar masses, could just be that they are hard to observe. It generally requires a companion to light up a black hole via accretion, so when two neutron stars merge, their might not be anything left to help us see the resulting black hole.

I believe one of the problems with neutron star mergers as candidate sites for the r-process is that current simulations show no ejecta...at all...everything seems to just fall into the black hole that's created. Of course, these are PPN simulations and are not fully GR, but still.

 

[Chronos]

The low mass limit for neutron stars is also rather loosely constrained. 4U 1538-52 at 1 solar mass, re: http://arxiv.org/abs/1305.3510, is probably the tightest observational limit. In a core collapse event it is fairly well established the core mass must be near the Chandrasekhar limit [1.4 solar], but, it appears some of that mass can be ejected. There are nearly as many neutron stars below the Chandrasekhar mass limit as there are above it.