Gamma rays belong to the electromagnetic emissions realm, just like broadcast radio, visible light and X-rays. As such, they are photons, those little speedy things that wear a wave or particle hat depending on how you look at them.
Only they are at the upper extreme of the electromagnetic spectrum, that is, gamma rays photons are all those generic ones with energies beyond the more mundane X-rays, those capable to whizz through your chest and imprint pretty skeletal shadows on a photosensitive plate.
As much as prolonged exposure to X-rays is not to be indulged, exposure to gamma rays may be outright lethal unless cleverly focused, for example, on malignant tissue in one of the standard cancer treatment procedures.
As may be expected, gamma rays are telltale byproducts of extreme phenomena. All radioactive matter (basically all matter to some degree) emits gamma rays in varying amounts, a consequence of very energetic yet few and far between nuclear decay events.
When nuclear fission or fusion happens in a prolific and coordinated way, the gamma ray flux becomes much higher as it is the case in nuclear reactors and weapons.
VELA blows the whistle.
So, when the nuclear arms race heated up, it was readily apparent some basic agreement had to be reached less radioactive fallout just from test blasts should imperil life planet-wide.
This is why the atmospheric nuclear test ban was reached and - of course, given the wicked human nature - means to verify its compliance. Gamma rays came handy as proof of violations, a high flying satellite fitted with adequate detectors could easily pinpoint a blast Nature does not make by herself - at least that's what scientists thought by the time - and the VELA series of surveillance ships was put into orbit by the US.
Surprisingly, gamma flashes begun to pop daily, fortunately from elsewhere but the Earth's surface. Given their intensity, it was immediately assumed their origins were in the immediate neighborhood, though the almost null spatial resolution of those early detectors could not pinpoint their direction but only broadly.
Gamma rays are so energetic they cannot be bent like ordinary light by lenses and mirrors; they simply fly through or get absorbed, and the same happens to X-rays, though to a lesser degree. This is why the Chandra X-Ray telescope makes use of a long delicate structure of tapered metal tubes to gently nudge, ever so slightly, photons to focus. Gamma rays cannot even be subject to this.
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