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Cosmic Ray Anisotropy Over Time

Presentation #125.02 in the session “GW and MMA. Strong/Weak Gravitational Lensing, Relativistic Astrophysics”.

Published onJan 11, 2021
Cosmic Ray Anisotropy Over Time

IceCube is primarily a neutrino observatory, but it detects many more cosmic rays. Even before the detector was completed, a small anisotropy in cosmic ray arrival direction was found. This just means that cosmic rays are not arriving uniformly from every region of the sky. One way to parse out possible sources of this anisotropy is to look at data in different reference frames. These include the solar and sidereal frames, which can help us distinguish between effects due to Earth’s motion around the Sun, and effects that are more distant and ambiguous. Anisotropy in solar time is expected to be a dipole due to a Compton-Getting effect, but we found a slightly better fit when we included an additional quadrupole component. This could hint at small underlying sources, like a possible effect due to atmospheric tides. Even frames that do not correspond to a physical meaning, like the anti-sidereal and extended-sidereal frames, are useful. After incorporating the last few years of IceCube data, the anisotropic patterns that we observe are consistent with earlier findings. With fewer years included, earlier work could not determine the best fit for the extended-sidereal distribution. However, using data collected between 2011 and 2019, we found that the extended-sidereal distribution is not flat with high statistical significance. This means that anisotropy in the sidereal frame is modulating over time, which also creates a spurious solar signal that can be modeled and accounted for. Both the extended-sidereal and anti-sidereal distributions that we observe in the 2011 to 2019 data could be imprints of solar and sidereal interaction over a year. This work was supported by the National Science Foundation’s REU program in Astrophysics through NSF award AST-1852136.

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