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The Fermi bubbles in our galaxy are the dense red spots above and below the center of this image.
Enlarge / The Fermi bubbles in our galaxy are the dense red spots above and below the center of this image.

Cosmic rays are a nuisance, a source of mystery and a window to the galaxy. Cosmic rays are bursts of high energy particles, usually protons. These slam into the atmosphere, generating showers of other high-energy particles. Whether we like it or not, we see them as streaks in images or errors in computer memory. They are, to put it bluntly, a minor nuisance to any researcher. They are a major nuisance to dark matter and neutrino detectors around the world. The sound of cosmic rays is so bad that physicists have been prompted to live at the bottom of mineshafts, muttering to escape it.

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

That explains most, but not all. Cosmic rays with even higher energies have been detected and they have remained a mystery. Now it seems that they can also come from supernova, but not by a direct route. First they have to leave the galaxy and then come back hot and angry after getting the fright of their lives.

Big bubbles

A 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 measurements on Earth to estimate the total flux of cosmic rays, you find that almost all cosmic rays should have a supernova origin . Because the highly energetic rays are so rare (1015-1019eV), adding this to the supernova bill is almost a rounding error.

But this simple math faces a big problem. The higher the energy of a cosmic ray, the more it is scattered, so cosmic rays with very high energy should get up close. We should have observed other radiation from that supernova and located the source, otherwise the supernova would have left Earth a smoking ruin and no one would care.

According to recent research, the solution to this problem had to wait for the discovery of Fermi bubbles. After the Fermi Gamma telescope launched, it noticed (actually a German X-ray telescope found it first, but who’s counting?) two bubbles radiating from the center of the galactic disk. Gamma rays were emitted from these bubbles as electrons were accelerated to very high energy. So we seem to have found a natural particle accelerator.

Fermi bubbles are now thought to be created by the shock waves of massive events near the galactic center. These are events such as stars being sucked into black holes or a burst of star formation. The shock front of the event spreads rapidly from the center. As it passes through it, it accelerates any charged particles in its path. So is accelerating already fast-moving supernova cosmic rays.

Speeding up the swift

You may be wondering how a shock wave can overtake fast moving cosmic rays? Cosmic rays travel fast, but don’t move in straight lines. Because they are charged, they spread over everything, including the cosmic microwave background radiation. So they zigzag around and lose energy in the process.

So the idea is that huge shock waves from the galactic center catch up with these jets and accelerate them again. Some of these go back to our galaxy, where they puzzle us.

To check this idea, researchers created a simplified model of a shock wave and examined how cosmic rays would behave if they were reaccelerated 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 their one-free-parameter model fits very well with the observed flux for cosmic ray energies up to about 1019eV.

The model is still unable to explain the very rare cosmic rays with energies higher than 1019eV, but the cosmic ray map begins to fill up as the “here be dragons” tones are erased. Of course, it is also not possible to say with certainty that this model is actually correct. Confirmation will depend on better observations of Fermi bubbles and a better understanding of how the bubbles form.

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

By akfire1

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