Would the collision of a neutron star and an anti-neutron star destroy the galaxy it happens in? My question is a follow up to this one: Does the collision of a neutron and anti-neutron produce energy?.
Quoting from an answer:

"The collision of a neutron and antineutron star would initiate a terrible strong annihillation. The result would be similar to a supernova with an extreme gamma photon flash. It is hard to say, what would be the result. In the Universe, no significant amount of antimatter exists."

Let's say this is the smallest possible neutron star and smallest possible anti-neutron star. Sure, there would be an extreme gamma photon flash. But I want to know how devastating this will be. Will it destroy the entire galaxy with material spewed out into inter-galactic space? Or will it be like any other supernova explosion? And will this result in a black hole or just a big explosion followed by nothing?
 A: Probably not even close. A back of the envelope calculation indicates that a neutron and an anti-neutron star colliding and completely annihilating would release energy of about $10^{47}~\rm J$. This is a staggering amount of energy, but it's also only a few hundred times larger than the largest supernovas we've seen.
Even if you add a couple orders of magnitude to this (to compensate for the fact that the vast majority of a supernova's energy is lost to neutrinos), when you include the $1/r^2$ factor for the explosion, the blast radius is "only" ~100 times larger than a supernova. This would be dramatic, no doubt, but it doesn't come close to the ~50,000 light years of the Milky Way.
Besides which it's very likely that the annihilation would blow the stars apart prematurely, and relatively little of their mass would actually annihilate.
A: Let us assume two minimal mass, $1.2M_\odot$ neutron and anti-neutron stars collide, totally annihilate, and produce a flash of GeV gamma rays.
The energy released will be $2.4M_\odot c^2 = 4\times 10^{47}$ J.
The gravitational binding energy of the inner part of the galaxy (excluding the dark matter halo that can't absorb gamma rays) is about $GM^2/R$, where $M\sim 10^{11}M_\odot$ and $R\sim 10$ kpc. This amounts to $\sim 10^{52}$ J.
Thus, even if the gamma rays were all absorbed (they wouldn't be and also some fraction of the annihilation energy is lost in the form of weakly interacting neutrinos), this falls short by around 5 orders of magnitude from injecting enough energy to unbind the galaxy.
