When 2 neutron stars collide to form a blackhole, the resulting emission of high-energy photons in the form of gamma rays is what's referred to as a "Kilonova" or "Kilonovae" for plural.
Image credit: NASA
If you remember from my other post "Neutron stars & Pulsars - what are they, and how are they different?", then you'll know that a neutron star is essentially 1 massive atomic nucleus.
That's a good way of thinking of them at least....
Electrons and protons have been smashed together during the collapse of a massive star to produce a body completely comprised of degenerate neutrons, leaving behind a neutron star (hence the name...).
Neutron stars can merge when they're held together by each others gravity, both orbiting around a central point that's levied towards the heavier of the two (or more).
These systems are referred to as "binary neutron star systems", and are extremely rare phenomena throughout the Observable Universe, with only 10 systems suspected to be scattered throughout our own galaxy, The Milky Way, which comprises of hundreds of billions of stars. In contrast, 85% of stars are contained within a binary system of at least 2 stars.
A kilonova can also be produced via the merger of a black hole with a neutron star, which is even rarer!
A single kilonova or multiple kilonovae are produced when either two neutron stars or 1 black hole and 1 neutron star collide and merge together. During the merger, highly energetic photons are realised in the form of gamma rays that can be detected with certain telescopes.
For more information on what gamma rays are exactly, feel free to check out this article.
Thanks for reading!