Gamma ray bursts were first detected by satellites that had been designed to watch for nuclear weapons on Earth. They kept reporting super high energy radiation events-- bursts -- but there were no other signs of nuclear explosions. They were seeing light from outer space.
The short duration of these bursts, along with the extreme energy level of the photons, impose constraints on the type of event that could emit such light.
Light moves really fast, but it doesn't move infinitely fast, and nothing else can happen faster than light. Anything violent enough to shine mostly in gamma rays is going to be pretty much the only thing going on at the source: a big explosion. A burst two minutes long generally means that the source is two light-minutes wide. Something that would fit inside the orbit of planet Mercury. For example. It gives you an idea of limits that can be placed on the phenomena that might cause such a thing.
NOT_IMPOSSIBLE + REALLY_UNUSUAL = SCIENCE!