Quite a while ago I wrote a post talking about the IceCube neutrino telescope and its potential to become the first detector to observe sources of high energy neutrinos in the sky. IceCube, located at the South Pole, detects neutrinos not by observing them directly, but by detecting the particle that is created when a neutrino interacts with the ice that surrounds the telescope or the rock underneath it. This charged particle, a muon most of the times, emits a bluish light called Cherenkov radiation which can be detected by the array of light sensors that make up IceCube.
This is how neutrino detection goes, but it turns out that not only neutrinos are able to produce muons that reach the pristine ice that surrounds IceCube. Cosmic rays (charged particles coming from the cosmos), in fact, account for most of the events seen by IceCube. In general, for every million of cosmic ray events seen by IceCube there is only one neutrino in our data set (which is most of the times produced by another cosmic ray on the other side of the Earth!) This is my very convoluted way to say that besides being a very nice neutrino detector, IceCube is also an amazing cosmic ray detector.
So, is there anything interesting that we can do with these cosmic rays that light up the detector at a rate of about 2000 events per second? Maybe we could plot their arrival directions in a sky map and see if they’re pointing us back to their sources.
Well, there’s a problem. We know that permeating the vicinity of the Solar System there’s a magnetic field that bends the trajectories of these protons (that have energies of tens of TeVs) in pretty much the same way that the LHC magnets bend the trajectory of protons around the collider ring. The main difference here is that the magnetic field in the solar neighbourhood is not so organised and neat as that of the LHC, and these protons would follow pretty chaotic paths before they reach the Earth, at which point they would not be pointing back to the source that originated them. This is why we should expect to see a completely featureless sky if we were to just plot the incoming direction of these TeV cosmic rays.
But you know that I would be writing about this if this were the end of the story. Last year, IceCube published its first map of the TeV cosmic ray sky, and we found that, actually, there are significant features in it. These features are very weak, with the “hottest” spots in the sky differing from the number of events detected on the “coldest” spots by only parts in thousands. This is where a data set with a huge number of cosmic rays becomes handy; with IceCube gathering billions of cosmic rays events every year, we can measure these minute differences very accurately. This study, performed by fellow UW-Madison colleagues Rasha Abassi, Paolo Desiati, and Juan Carlos Diaz Velez with data taken when the detector was only one-quarter of its final size, revealed that the cosmic ray sky is anisotropic, and that the excess and deficit regions that are visible take about half the sky each.
Large scale anisotropy of cosmic rays as seen with the IceCube detector in its 22-string configuration. The red colour in this map indicates a deviation of 0.2% from a flat sky, while the blue indicates a deficit of the same strength.
The next question that we asked ourselves was: is that all that there is to it? Is this half-and-half feature the only remarkable thing about this cosmic ray sky? This is the question that we’ve been trying to answer for the past year with the group that I work with. Our group (Dr. Simona Toscano, Dr. Segev BenZvi, Prof Stefan Westerhoff, and myself) has focused its attention on the search for smaller structures in this cosmic ray sky, to see if there are features that are smaller in size than those previously reported. And here again the answer was yes!
Calculating the angular power spectrum of the sky map that we got for data taken with IceCube in its 59-string configuration (about 2/3 completed) we obtained the blue points shown in the graph below. The y axis shows a value that gives an idea of how strong the features in the sky are at a certain angular scale (given by the upper x-axis) We knew from the previous analysis that structures that are large (with sizes between 90 and 180 degrees in the sky) were present, but as you can see the blue points don’t go immediately into the grey bands which indicated what we should expect for a “featureless” sky but rather remain away from them up to angular sizes of 15 degrees.
This tells us that besides the large scale structure already reported by IceCube, there must be regions of excess and deficit of cosmic rays that have typical sizes of ~ 20 degrees in the sky (~40 times the size of the Full Moon.)
The power spectrum of the cosmic ray anisotropy detected by IceCube. The presence of a large scale structure is evidenced by the peak to the left, while smaller structure can be seen as a departure from the gray band regions for angular scales between 15 and 35 degrees. After the subtraction of the large scale structure, the small scale structure persists (seen in red dots), which indicates that the presence of smaller structures is not due to an artifact caused by the presence of the large scale anisotropy.
Using a technique that allows us to filter out the large scale structures to focus only on the smaller regions, we got the map shown below, where we can see localised regions of excess and deficit of cosmic rays coming across the Southern sky. We also see that, as we were expecting, these regions are about 20 degrees in size. The causes of these “hotspots” are still unknown, but we’re working to see if we can determine what’s causing them. Possible reasons include nearby pulsars, the configuration of the local interstellar magnetic field, or a combination of these two factors, but more information is needed to determine what the possible sources of this anisotropy may be.
This is how the Southern sky cosmic ray sky looks like at TeV energies once structures larger than ~60 degrees have been filtered out. Both regions with an excess (red) or deficit (blue) of cosmic rays when compared to an isotropic sky are clearly visible.
Similar excesses and deficits have been observed in the past by experiments located in the Northern hemisphere, but this is the first detection of this kind of structure in the Southern sky. You can take a look at the preprint of the paper we submitted to the Astrophysical Journal here. We’re right now trying to organise a workshop in October where we will discuss possible theories and the details of observations made in the North with colleagues from other experiments.
Interesting times ahead!
