A gamma ray is just light, we would expect the signal to get attenuated early on in the atmosphere. This particular one had enough oomph to punch down to ground level instruments and actually generate a response. Either it lucked out statistically and had a momentary straight shot to the ground (I mean, it could happen), or the thing had so much energy it ploughed straight through anything that got in its way before dumping the rest of its energy in the detector/ground beneath it.
Regardless, that is one hell of a signal. The worst case of starburn imaginable. Short of death by neutrino flux in proximity to a supernova any way.
We are talking about single particles of light having energies between ~ 50 GeV and up to 10 TeV (for this GRB, other sources go even higher).
At these energies, the atmosphere is completely opaque and the gamma rays are absorbed at around 20 km height.
However, the process kicks of a cascade of particles, the gamma ray is absorbed by creating an electron-positron pair, each with half the energy. These particles then create new gamma rays via bremsstrahlung and those gamma rays again are absorbed via pair production creating a cascade of high energy particles.
The charged part of this cascade (electrons and positrons) are moving faster than the speed of light in air and are thus producing Cherenkov radiation, which is in the UV to the visible range.
It is this light our telescopes detect and from this light we reconstruct the original properties of the first gamma ray that entered the atmosphere.
Thank you for that. I figured there were processes in there I wasn't aware of,and that fits with my mental model a lot better than something I kicked out while falling asleep.
If you have multiple telescopes observing the air showers this gets much better. MAGIC uses two 17m telescopes, VERITAS 4 12 m telescopes and HESS 4 12m and one 28 meter telescope.
CTA is currently in the planning and construction phase and will build around a hundred telescopes at two sites, Chile and La Palma.