Neutron Stars with Less Mass Than a White Dwarf Might Exist

Neutron star-black hole mergers (NSBH; yes sorry the name is the other way around from the easier way to think about the mass ratio) mergers are relatively rare in our detectors. Detections GW200105 and GW200115 were moderate mass ratios, ~9:1.9 and ~6:1.5 solar masses respectively. In both cases there was nearly no optical signal, which is easy to interpret as the NS essentially falling right into the BH.

Astrobites, whose writers are graduate students who summarize papers in (or at least close to) their fields in a way meant to be accessible for undergrads, has a 2021 article at <https://astrobites.org/2021/06/30/first-nsbh-merger/> which includes a link to a video simulation of a NSBH. Another simulation, explicitly compatible with the GW200115 (6.1:1.4) detection, can be seen at <https://www.youtube.com/watch?v=Rd3p3xPtWn4> which also shows the NS deformed into a prolate shape which falls right into the BH in this way. The prolation does not rise to a tidal disruption, and essentially no mess is left around outside the BH to influence the ringdown (GW waveform traced on bottom).

Higher mass ratios are even less likely to produce an optical component -- as one takes the mass ratio higher, the NS's tidal deformation (the prolation) will effectively vanish into negligibility.

Smaller mass ratio NSBH mergers are likely to disassemble the NS in a "kilonova" event comparable to a collision between two neutron stars.

There is a small zoo of simulations at <https://www.aei.mpg.de/145347/gravitational-waves>. The same group did <https://www.youtube.com/watch?v=-pdNYuWWN_w> (which does not seem to be in the "zoo"), which shows a tidal disruption in a low-mass-ratio NSBH merger. The matter left outside the BH will initially be extremely hot and optically bright. AFAIK there is no known NSBH merger with a definitive kilonova; the low mass ratio GW230529 had a significant optical signal but it is not entirely clear that the more massive of the pair was a black hole.

> lose degeneracy

Oh yes, binary neutron star mergers (BNS) are very busy with nuclear physics, producing all sorts of heavy atomic nuclei in the ejecta. Gold is certainly one of those elements (see e.g. <https://en.wikipedia.org/wiki/R-process>).

> "slice" the degenerate matter away

The apparent horizon is not a razor.

Apparent horizons are not often at the same location in spacetime as event horizons in these mergers. An event horizon is not even in-principle measurable by us: you'd need know the entire contents of the future light cone out to the infinite future. An apparent horizon is physically measurable locally (but not too locally, because General Relativity guarantees an ultralocal patch of flat spacetime at every point in a general curved spacetime), essentially by measuring whether a beam of light (visible by being shot through a dust, like how one can see the beam from a laser pointer shining through a mist of water or a cloud of chalk dust) tangent to the emitter's trajectory curves inwards.

The self-gravitation of a neutron star is too great for a larger BH to overcome outside the horizon except when the BH is very small (low mass ratio approaching unity, and there are observational and astrophysical-theory reasons to suspect that BHs with NS masses are rare, which makes studying GW230529 exciting). However tidal deformations can heat the NS so there is the possibility of exciting standard model physics inside it during its final pre-merger orbit even for moderate mass ratios, given a highly noncircular NSBH orbit.