Mystery #3: Where do cosmic rays come from?

Cosmic rays are one of the most wonderfully-baroque, mad-science things in astronomy. They’re exotic. They’re mysterious. They offer no quarter to common sense. And they’re also misnamed.

They’re actually, strictly speaking, particles rather than ‘rays’. Around 90% of them are free protons, with most of the rest being helium nuclei. There’s just a whiff of other stuff in there, namely heavier elements. They come in from space, zap themselves into our atmosphere, smash into the atoms therein and create particle showers that things like the MAGIC telescopes can pick up.

In the early 20th Century, it was believed that they actually came from inside the Earth, released by the radioactive decay of elements in its crust. Then an Austrian physicist called Victor Hess did a remarkable experiment involving an early scintillation counter and a hot air balloon. He showed that counts actually went up with increasing altitude, which was entirely inconsistent with the ‘Earth radioactivity’ hypothesis. This meticulous work ultimately earned him the Nobel Prize.

(Also, he did useful physics from a hot air balloon. Epic mad-science win?)

The space origin of cosmic rays is no longer controversial. You can see further evidence of it in their composition. The free protons can be considered as ionised hydrogen, in which case the H/He ratio is tending vaguely toward that of the universe as a whole. The ‘metallicity’ of cosmic rays is arguably a little low, but on the other hand many of these things will have been travelling for billions of years and thus will have set off from before much of current star formation had happened.

But where have they set off from, exactly?

Some cosmic rays definitely come from supernovae. Some probably come from stellar winds. Some are believed to be extragalactic in origin. And some are just plain weird.

The highest-energy cosmic ray ever detected was the so-called ‘Oh-My-God’ particle. It paid us a visit on the 15th of October, 1991, where it zapped into our atmosphere somewhere over Utah. It consisted of one proton, with an estimated energy of over 3 x 10^20 electron volts – or about the same as a mid-flight tennis ball. Consider – a single proton, packing in as much kinetic energy as a tennis ball. In principle, if this thing decided to pass through you, it could actually injure you. Remember – that’s one proton.


But the sources of these so-called ‘utra high-energy’ cosmic rays are not well-understood. Or understood at all, in fact. There exists a suspicion that they’re accelerated in the hearts of galaxies, by supermassive black holes. In principle, this should be simple to test – look at the active galaxies in the sky, and see if they correlate with cosmic ray detections.

Unfortunately, it doesn’t work like that.

The problem is magnetic fields. Cosmic rays are charged particles, so they will interact with magnetic fields. And space is full of magnetic fields, even intergalactic space. So, if a cosmic ray travels more than few megaparsecs, its trajectory will almost certainly be scrambled beyond recognition. And sure enough, this is what we see on the sky. The distribution of possible extragalactic cosmic rays is basically random. There exists one small correlation; that’s a cluster of high-energy detections associated with the radio galaxy Centaurus A. It’s located at a distance of 11 megaparsecs, so it’s arguably just barely close enough that a few particles might make it to us undistorted. Other than that, the high-energy cosmic ray sky is just a featureless blur, aside from some activity at the centre of our own galaxy.

So, sadly, the origins of extragalactic cosmic rays are probably never likely to be well-known.


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