University of Wisconsin-Madison

The cosmic-ray Moon shadow seen by IceCube

Cosmic rays, very high-energy particles generated somewhere in outer space, continuously bombard the Earth from all directions. The Moon, however, absorbs those particles that reach its surface after traveling over astronomical distances. Earth-based telescopes such as IceCube use the cosmic-ray shadow cast by the Moon to calibrate their angular resolution and their pointing accuracy for identifying point-like sources. The first impacts on the size of source signal in the detector, while the second constrains the precision with which the detector can estimate the direction of the incoming particles.

The observation of a cosmic-ray deficit from the direction of the Moon with the IceCube Neutrino Observatory is thus an important milestone in proving its potential in the search for point-like sources of astrophysical neutrinos. A recent measurement of the Moon shadow in TeV cosmic rays with the IceCube telescope sets an upper limit on the detector’s absolute pointing accuracy to 0.2 degrees. The Antarctic telescope has observed the Moon shadow with a high significance (over 6 sigma), and the Moon’s center has been measured to be statistically consistent with its actual location. The IceCube Collaboration presents these results in a paper submitted today to Physical Review D.

Contour plot of the muon deficit as measured by IceCube in the region around the Moon's position, the so-called on-source region. Image:IceCube Collaboration
Contour plot of the muon deficit as measured by IceCube in the region around the Moon's position, the so-called on-source region. Image:IceCube Collaboration

The search for neutrinos coming from gamma-ray bursts (GRBs), for example, is very sensitive to the absolute pointing accuracy of IceCube. "In this case, the position in the sky of the GRB is compared to nearby events detected within a time window around the burst. If off by a few degrees, the sensitivity of IceCube to detect these potential sources of high-energy neutrinos would be significantly reduced," explains Marcos Santander, an IceCube researcher at the Wisconsin IceCube Particle Astrophysics Center (WIPAC).

The importance of the pointing accuracy is also highlighted in studies such as the cosmic-ray anisotropy map, which IceCube measured for the first time in the Southern Hemisphere. The new small-scale structures reported by the IceCube Collaboration rely on the angular resolution of the detector, now very accurate according to the results of the Moon shadow measurements.

Even though the search for point-like sources of astrophysical neutrinos uses data samples with a different energy distribution, the Moon shadow results indicate that high-energy neutrinos detected with IceCube will allow for identifying astrophysical point sources if enough statistics are available.

Two methods and two detector configurations

Two independent analysis methods were applied to data taken between April 2008 and May 2010, before the completion of the IceCube neutrino telescope. Both methods and data samples show consistent results, and they are the first statistically significant detection of the shadow of the Moon using a high-energy neutrino telescope.

The Moon shadow was measured as a deficit of downgoing muon events, which are the majority of events detected by IceCube and are generated by the interaction of cosmic rays with the Earth’s atmosphere. Only muons reaching the detector when the Moon was at least 15 degrees above the horizon were selected for analysis. Over 300 million events passed this initial cut, with about 68% of the events estimated to be produced by proton cosmic rays and another 23% by helium cosmic rays.

The width of the Moon shadow as measured by IceCube in its partial 40-string and 59-string configurations was around 0.7 degrees, in agreement with simulation studies. "The good news from this measurement is that it allows IceCube to trust its estimate of the angular resolution for a given point-like source based on simulation models and techniques", says David Boersma from Uppsala University who also works at IceCube.

+ info "Observation of the cosmic-ray shadow of the Moon with IceCube," IceCube Collaboration: M.G. Aartsen et al. Submitted to Physical Review D, arXiv:1305.6811