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My understanding of Type Ia supernovae is that they are expected, in most cases, to destroy the white dwarf(s) that went in to them, leaving behind no high density remnants (i.e. no white dwarf, neutron star, or black hole). Black hole/black hole collisions are expected to leave behind a black hole, of course, with less mass than the sum of the masses of the black holes that went in to the collision. Do we expect a kilonova to leave behind nothing but gas and radiation or some kind of stellar remnant? If it leaves behind a remnant, what class to we expect it to be (white dwarf, neutron star, or black hole) and what mass? The addition of mass seems redundant, but the dividing lines between the masses of these objects are based on upper limits on the mass of the less dense class (Chandrasekhar limit for white dwarfs, Tolman–Oppenheimer–Volkoff limit for neutron stars), and don't actually apply as lower limits for the mass of the high density class. For example, neutron stars are sometimes referred to as giant nuclei, which would put the lower limit of their mass at $1$ or $2$ atomic mass units, depending on whether the presence of a neutron and stability to radioactive decay are requirements. Yes, those times people are being poetic due to the difference in stabilization mechanisms (gravity vs nuclear forces), but the point remains that it may be possible for a neutron star to be theoretically stable at less than $1.4M_\odot$. The only lower limit to black hole mass I know of would be the lifetime limit from black hole evaporation.

Put another way, do we think the density at any point in either of the inspiraling neutron stars gets high enough to form an event horizon? If that happens, this seems like a plausible way for producing black holes that are smaller than those produce by core collapse supernovae, kind of like how critical mass for atom bombs can be achieved by either bringing together enough fissile material or compressing the available material enough (e.g. the fat man detonation mechanism vs the little boy one).

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    $\begingroup$ I don't think we know for sure what kind of high mass remnant is there, but from this article it looks like scientists think so: seeker.com/space/astrophysics/… $\endgroup$ – Phil N DeBlanc Oct 18 '17 at 19:35
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    $\begingroup$ From why I read of the recent gravitational wave detection, it is expected that there will be a object at the end and it continues to generate gravitational waves due to irregularities on it's surface (mountains). $\endgroup$ – A. C. A. C. Oct 18 '17 at 20:24
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I think it is generally expected that the merger of two neutron stars will lead to the formation of a black hole. What is more uncertain is whether that black hole forms straight away, or an intermediate state of a hyper(/supra)massive neutron star forms (see, e.g. Sec IIC of Hotokezaka et al, PRD, 044026, 2013 for a definition of hypermassive and supermassive neutron stars), which then collapses to a black hole after a short time. A few papers looking at the stability of such a hypermassive neutron stars formed from a binary merger are here, here, here and here (check out the references and citations to them for more info). In the first three reference I think the collapse to a black hole is expected within a second (see Table 2 in Hotokezaka et al, PRD, 044026, 2013 for expected lifetimes before collapse times for various neutron star equations of state and masses), whilst in the final one a range of collapse times from fractions of a second up to several tens of thousands of seconds are found.

A search for a post-merger remnant from the binary neutron star merger GW170817 is now available here. In the introduction of this paper it describes in more detail some of the post-merger remnant possibilities that I mentioned above. The searches don't find any evidence for a signal, but given their sensitivities this isn't surprising (they'd require more than the total mass of the system to have been converted to gravitational waves to see anything!)

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