Ultrahigh-energy photons up to 1.4 PeV from 12 γ-ray Galactic sources (2021)

The state of the art observatory for ultra high energy cosmic rays is the Pierre Auger Observatory in Argentina (auger.org). Unfortunately the funds for building a 7x larger northern observatory in Colorado were scraped but the NSF during the crisis in 2008. :(

The PAO published an article on the highest energy arrival directions in 2007 (https://arxiv.org/abs/0712.2843) and I was very sad that I wasn't on the author list (yet) at the time since I hadn't been around for a full year. The discovery was a correlation in arrival directions with the known location of supermassive black holes in the center of other galaxies ("active galactic nuclei"). I was less sad when it appeared to have been a statistical fluke a couple of years later (about 3 sigma worth, enough for a discovery claim in astronomy but not in particle physics - PAO is a mix of both disciplines and the arguments were always thrilling).

We then went back to data collection and analysis and became much more cautious about publication of correlation analysis. (I moved on in 2010.) More recently (2018) it looks like there's a higher significance correlation study: https://arxiv.org/abs/1801.06160. An outstanding breakthrough, yet I think in principle this surprises nobody in the field. They also published a paper purely about the anisotropic arrival directions (no correlation analysis with known objects) that showed that far more of the highest energy events arrived from the side facing away from the milky way. This shows (again to no surprise of anybody in the field, but you need to prove it anyway) that these particles are of extragalactic origin. A mechanism that puts this much energy into a single particle might not be conducive to life nearby (ie. in the same galaxy).

With that background back to your topic of inquiry: Regarding origin paths of such high energy particles.

~All ultra high energy cosmic rays are believed to be charged particles. That means their path is changed by galactic and intergalactic magnetic fields. (Mostly the latter, they are weaker but the distances are so very much longer.) That means the higher the particle's energy, the less it's path will be bent by the magnetic fields. At lower energies, they are basically arriving from random directions (isotropically) because their paths are bent into curves. We don't have a great way to get super accurate models of these magnetic fields (I lack the detailed background to comment on the theory here). Still, at these energies and with the upper limits that we believe about the strength of the fields, and with the measurement accuracy of the more modern PAO detectors (angular resolution better than 1 degree) we get something on the order of a few degrees or so (didn't have a chance to reread entire paper, this is from memory) in terms of uncertainty on the arrival direction.

The upshot of that is that a single event isn't really useful on its own. There's also significant uncertainty in the energy measurement, so again, a single event like the flys eye event doesn't make for finding an origin. But the larger dataset does, if you match it against potential sources. (In case you're wondering: yes, one has to be careful with bias in those analyses.)

It's maybe worth mentioning that the beams at CERN are made of quite a few particles and there was as much energy in the beam as in a high speed train (and same in the magnets), before the high luminosity upgrade. The beam dumps are massive pieces of graphite and concrete with water cooling.

Also at these energies the wavelength is very small so the energy can be quite concentrated in a small region of space-time.

Perhaps it would sound something like the house settling.

Question: how come a photon can carry a variable amount of energy? (Layman here, I would have guessed that photons travel at the speed of sound, and are of fixed size and energy, like electrons.)

Photons do indeed all travel at the same speed, but are not all the same size, and what affects their energy level is their wavelength.

Shorter wavelengths have more energy per photon, so e.g. a photon of blue light has about half the wavelength of a photon of red light and twice the energy.

I’m curious if you really meant “speed of sound” in your question? Photons all travel at the speed of light, which is quite a lot faster than the speed of sound. But I’ve seen this several times from different places, and never found out if it’s basically just auto-complete in the speaker’s mind, or a genuine misconception.