Re: Can't neutron stars grow bigger from accretion?
When neutron stars merge, as when anything merges, they tend to shed mass in spectacular fashion. (Astrophysics: the science for people who like big explosions seen from a safe distance.)
Accreting material gets burnt in a jet of X-rays. I'm not sure you could get enough mass from the ash to bulk up the star. But if it got too heavy, it would suffer explosive collapse which might produce a black hole or might produce another neutron star.
The real problem is we don't have a solid value for the upper limit for neutron stars. If Wikipedia is to be believed, this object is slightly lighter than the heaviest neutron star known (2.74M☉) And that's in pretty good agreement with the lightest stellar black hole (ibid - scroll down). But there's no theoretical minimum mass for a black hole and merging objects could, after shedding some mass, produce one that was, ahem. lighter than the upper limit for neutron stars. So it's probably a black hole