The IceCube detector can search for neutrinos from gamma-ray bursts. But, surprisingly, none have shown up.

Neutrinos are generated by a lot of basic physics processes. Almost any nuclear reaction, many particle physics collisions, and lots of astrophysical processes create neutrinos. There are enough coming from the Sun that trillions pass through your body every second without you noticing them.

Physicists create models of astrophysical processes because they can’t measure them in action directly, and compare the predictions of the models with the observations they can make with telescopes. But now there is a surprise from gamma-ray bursts.

Gamma-ray bursts are essentially a huge fireball in space thought to generate the highest energy cosmic rays and gamma-rays that we observe from space. The fireball accelerates protons and electrons to extremely high energies. The protons escape the fireball and we detect some on Earth as cosmic rays. The electrons are made to curve sharply in the strong magnetic fields that exist in the fireball and whenever electrons are made to curve sharply, they give off synchrotron radiation, in this case in the form of gamma-rays. Those are the gamma-rays we detect and the reason for the name gamma-ray bursts.

When the protons and gamma-rays are still inside the fireball, it is quite possible for them to interact and that particular reaction could give off neutrinos. That is built into many of the models of gamma-ray bursts. The trouble is, gamma-ray bursts are so far away from us that not many of the neutrinos reach us, and neutrinos are extremely hard to detect anyway. We never had a neutrino telescope that could expect to detect neutrinos from gamma-ray bursts. Until now.

IceCube is quite a feat of engineering. It consists of 80 strings of photodetectors buried between 1.5 and 2.5 km deep in Antarctic ice. As neutrinos pass through the ice, they occasionally collide with ice molecules, creating muons that travel faster in ice than light can travel in ice. That causes Cerenkov radiation, which travels outward in cones around the path of the muon. Those cones of light can be picked up by the photodetectors, revealing information about the original neutrinos such as the direction they came from and their energy.

When IceCube had half it’s detector strings in place, it was already collecting quite a lot of data. Enough data that it is sensitive to the neutrinos from gamma-ray bursts. And so it waited and watched.

During the watching period, there were 129 gamma-ray bursts observed by the Fermi Gamma-ray Space Telescope, Konus-Wind, Suzaku WAM, and Swift. A few were left out of the data set because IceCube had 59 strings in place by then and they wanted to keep that data for a better analysis later. In the end, the physicists ended up with 117 gamma-ray bursts in their catalog.

They then sifted through their neutrino data looking for neutrinos that arrived at the same time and, in a second study, somewhat earlier or later. They checked for the expected energy of the neutrinos to be correct and that they were coming from the same locations in the sky as the gamma-ray bursts themselves.

The most commonly used gamma-ray burst models predicted that IceCube should see 2.99 neutrinos in that observing period. Yep, just 3 of the sneaky little things. However, IceCube saw none at all that matched predictions. The scientists even erred on the side of including as many neutrinos as possible. This means that there seems to be something wrong with the gamma-ray burst models!

Of course, there are statistics involved in these predictions so there could be some fluke chance going on but the statistics give 90% confidence that the models must be incorrect about neutrino emissions from gamma-ray bursts.

Overall, this is one of those cases where a null result is quite interesting. The machine observed nothing, but that nothing is surprising in itself. Does it mean that we need to throw out all our gamma-ray burst models. Not quite yet, but it is making physicists ask serious questions about them. In particular, it means that the mechanism by which protons are accelerated to high energies and arrive at Earth as cosmic rays might need to be re-thought.

As IceCube collects more data with all of its strings in place, it should be either able to observe neutrinos coming from gamma-ray bursts or more conclusively rule out the theoretical models.

Ref: Phys. Rev. Lett. 106, 141101 (2011), arXiv:1101.1448