3 ms·
This is not really how things work. We are talking about single particles of light having energies between ~ 50 GeV and up to 10 TeV (for this GRB, other sourc
by maxnoe 7y ago
This is not really how things work.
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.
- salawat 7y agoSweet. Figured someone would be along with a more reasonable explanation shortly. 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.
- strainer 7y agoAh so effectively none of the Hev photons reach the ground. I see the papers include the "Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes" data. I wouldn't have expected imaging to be practical because cherenkov is kind of scattering, but they seem to be able to triangulate individual collisions. Thanks for the correction.
- maxnoe 7y agoWe basically see a blob of light from which we can deduce origin, energy and particle type of the particle that created the air shower. 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.