Cosmic rays are a nuisance, a source of mystery, and a window into the galaxy. Cosmic rays are high energy bursts of particles, mostly protons. These slam into the atmosphere, generating showers of other high energy particles. We see them, whether we want to or not, as streaks in images or errors in computer memory. They are, to put it bluntly, a minor nuisance for every researcher. They are a major nuisance for dark matter and neutrino detectors the world over. The cosmic ray noise is so bad that has driven physicists to live at the bottom of mine shafts, muttering in order to get away from them.

But cosmic rays have also been a source of mystery for other physicists. Where do they come from? That question remained fairly contentious up until a few years ago, when supernovae were finally convicted of making neutrino scientists miserable. It turns out that as a star explodes, the magnetic field associated with the shock wave can accelerate particles up to energies of about 1015electronVolts (for comparison, the LHC works at about 1012eV).

That accounts for most, but not all of them. Cosmic rays at even higher energies have been detected, and they have remained a mystery. Now, it seems that they may actually also come from supernova, but not by a direct route. First, they have to exit the galaxy and then come back hot and angry after being given the shock of their lives.

Big bubbles

One piece of evidence for a common source comes from a fairly simple calculation. If you calculate the average amount of energy released by a supernova in the form of cosmic rays, and you use Earth-based measurements to estimate the total flux of cosmic rays, then you find that nearly all cosmic rays should have a supernova origin. Because the very high-energy rays are so rare (1015-1019eV), adding these to the account of supernova is nearly a rounding error.

But this simple math faces a big problem. The higher the energy of a cosmic ray, the more strongly it is scattered, so very high-energy cosmic rays should originate from nearby. We should have observed other radiation from that supernova and pinpointed its source, or the supernova should have left Earth a smoking ruin and nobody would care.

The resolution of this issue had to await the discovery of Fermi bubbles, according to recent research. After the Fermi Gamma-ray telescope was launched, it noticed (actually a German X-ray telescope found it first, but who's counting?) two bubbles radiating out from the center of the galactic disk. Gamma rays were being emitted by these bubbles as electrons were accelerated to very high energy. So we seem to have found a natural particle accelerator.

It is now thought that Fermi bubbles are created by the shock waves from massive events near the galactic center. These are events like stars being sucked into black holes or a burst of star formation. The shock front from the event propagates rapidly out from the center. As it passes through, it accelerates all the charged particles in its path. That includes accelerating already-fast-moving cosmic rays from supernova.

Accelerating the swift

You might be wondering how a shock wave can catch up to fast-moving cosmic rays? Cosmic rays travel fast, but they don't move in straight lines. Because they are charged, they scatter off of everything, including the cosmic microwave background radiation. So they zig-zag around, losing energy as they go.

So the idea is that massive shock waves from the galactic center catch up to these rays and re-accelerate them. Some of these head back to our galaxy, where they mystify us.

To check this idea, researchers created a simplified model of a shockwave and examined how cosmic rays would behave when re-accelerated by the shock front. From there, they used a model of cosmic ray propagation to estimate how the rays from the Fermi bubble would contribute to the observed cosmic ray flux at Earth's location in their model galaxy. They showed that with one free parameter, their model fits the observed flux very well for cosmic ray energies up to about 1019eV.

The model is still unable to explain the very rare cosmic rays that have energies higher than 1019eV, but the cosmic ray map is starting to fill up as the "here be dragons" notes are being erased. Of course, it is also not possible to say for certain that this model is actually correct. Confirmation will depend on better observations of Fermi bubbles and a stronger understanding of how the bubbles form.

EPJ Web of Conferences, 2017, DOI: 10.1051/epjconf/201714504004