Friday, April 20, 2012

"Fireballs" Snuffed

This post was chosen as an Editor's Selection for

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Ok, I admit, the title is a bit misleading. That's journalism, right?
So the story begins thusly: somewhere out there in the universe, something is producing really high energy cosmic rays. I mean, really high energy. Energies above 10^18 electronvolts (that's a one followed by eighteen zeros). That's nearly a million times more energetic than the LHC upgrade. Boggles-the-mind high energy.
We've detected those high energy cosmic rays here on Earth. And we want to know where they come from.
There aren't many things we know of in the universe that are powerful enough to produce such high-energy cosmic rays. Neutron star mergers, perhaps. Supernova explosions. But gamma-ray bursts (GRBs) seemed like a really good candidate. These astrophysical explosions are tremendously energetic, and so, it was proposed, they could be producing the seriously high energy cosmic rays we've been seeing. But the IceCube collaboration published results this week that seem to indicate "no such luck."
In the fireball model of the physics taking place inside a gamma-ray burst, the production of the high-energy cosmic rays is accompanied by a flux of neutrinos. The number of neutrinos produced is related to the number of proton-photon interactions inside the GRB, and this is in turn related to the energetic cosmic rays produced (the cosmic rays observed are generally protons or neutrons). So if the model was correct, and GRBs are the source of the cosmic rays, then each cosmic ray event should be associated with a flux of neutrinos, coming from the same location in the sky and at a specified time and energy. And IceCube set out to look for just that.
The IceCube detector is itself a wonder of the universe (in my opinion, anyway!). Strings of sensitive electronics extend down into the polar ice cap over Antarctica to a depth of one and a half miles. These detectors look for the tiny flashes of blue light produced when a neutrino interacts with the ice. It doesn't happen very often - neutrinos are notoriously hard to detect, because they don't like to interact with matter all that much - but when you have a cubic kilometer of ice to work with, you're bound to see something!
That's where the results come in. The IceCube collaboration saw... nothing. No flux of neutrinos from gamma-ray bursts. In fact, they saw so much nothing that they were able to put an upper limit on the number of possible neutrinos that was nearly a factor of four lower than what the fireball models suggested. So either the fireball models are (at least partially) incorrect, or gamma-ray bursts are not the source of the high energy cosmic rays.
Something to keep in mind: scientists appreciate a negative result just as much as a positive one. Not seeing what you expected to see is still just as important (perhaps more so) than seeing what you expected to see. It means you have to go back to the drawing board, think through everything again, and potentially come up with a brand new model to explain what you did see.
As the authors of the study themselves say, "all such models - in which all extragalactic cosmic rays
are emitted from GRBs as neutrons - are now largely ruled out.... either the proton density in GRB fireballs is substantially below the level required to explain the highest-energy cosmic rays or the physics in GRB shocks is significantly different from that included in current models. In either case, our current theories of cosmic-ray and neutrino production in GRBs will need to be revisited."

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  1. See that and raise with this.

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