An all-sky cosmic-ray proton anisotropy search with Fermi-LAT

Cosmic rays observed at Earth do not point back to their sources due to deflection in interstellar magnetic fields. So, despite over a century of research, direct evidence for the sources of cosmic rays remains elusive. By the time they are observed at Earth’s surface, cosmic rays are highly isotropic, i.e., uniform from all directions. Cosmic-ray anisotropy is any non-uniformity in their arrival directions—it can be thought of as excesses or deficits in the cosmic-ray flux from particular directions. It is important because it tells us about the distribution of sources of cosmic rays and even probes the structure of the local interstellar magnetic field.


Ground-based experiments have made precision measurements of cosmic-ray anisotropy, but each observatory on Earth sees only part of the sky. Furthermore, they use analysis techniques that do not allow all of the information about the anisotropy to be recovered. Specifically, the variation of the anisotropy along the direction of Earth’s rotation axis—called declination—has never been measured below the 1018 eV energy scale.


That’s where the space-based Fermi Large Area Telescope (LAT) can help. While it is primarily a gamma-ray telescope, most of the particles that Fermi-LAT records are cosmic rays. Being in space, it does not have the same limitations that afflict ground-based telescopes, allowing it to shed new light on the cosmic-ray anisotropy mystery.


Collaborators from this project realized that the abundance of cosmic-ray protons in the LAT data set might enable them to measure cosmic-ray anisotropy, and so they conducted the first search for cosmic-ray proton anisotropy using Fermi-LAT data. They present their results in a paper published last week in The Astrophysical Journal.

Upper limits on the dipole amplitude vs. minimum energy
Upper limits on the dipole amplitude vs. minimum energy. The dipole amplitude is a measure of the strength of the anisotropy, i.e., an excess/deficit in the flux of cosmic rays from a particular direction. The colored bands show the expected upper limits  given the sensitivity of the instrument, and the measured data points are consistent with this.  There is a slight excess for the lowest energy bin, which can be interpreted either as a signal with weak statistical significance or as a statistical fluctuation. Credit: Fermi-LAT Collaboration


Standard diffusion theory, which describes how cosmic rays travel through space, predicts that there should be a large-scale anisotropy in cosmic-ray arrival directions. This anisotropy has been consistently observed, though the size of the effect is at least an order of magnitude smaller than that predicted by standard diffusion theory. There is evidence that this discrepancy, along with the observed direction of the anisotropy, is the result of a confluence of effects: the distribution of local sources of cosmic rays, the local interstellar magnetic field, and systematic effects in the observations from ground-based observatories.


WIPAC researcher Justin Vandenbroucke realized the Fermi-LAT’s potential for measuring cosmic-ray proton anisotropy.  “Fermi has proven to be an incredibly flexible instrument, making breakthrough measurements of not only cosmic gamma rays, but also of cosmic electrons, positrons, and now protons.”


To conduct the search, the researchers first came up with a data selection algorithm to gather a large sample of cosmic-ray protons for the analysis—the larger, the better. However, since the LAT is optimized for gamma-ray analyses, they had to take extra care to understand the data and any effects introduced by the algorithms that reconstruct physical quantities, such as energy and direction, under the assumption that the particles are gamma rays


Because the anisotropy in cosmic-ray arrival directions is very small, researchers used a data-driven approach to measure it and created a “reference map,” or their best guess of what the sky would look like if it were isotropic, i.e., the same from all directions. They then compared the observed sky map to the reference map to search for deviations larger than the expected statistical fluctuations, which might be evidence of anisotropy.


“Since we know that cosmic-ray anisotropy is very small, the trickiest part of the analysis was to ensure that we understood the data set and reference map algorithm very well,” says Matthew Meehan, a recent WIPAC PhD graduate who, with Vandenbroucke, led the analysis and paper. Any uncertainties or irregularities in the reference map had to be smaller than 0.1 percent. The researchers also had to study systematic effects that could mimic an anisotropy, such as geomagnetic effects, and make sure those were understood to be less than 0.1 percent.


The researchers found that the cosmic-ray sky at 100 GeV is consistent with isotropy. While the analysis did reveal a slight excess (with statistical significance approximately 2.5 sigma), it may be due to statistical fluctuations. Because of this, they set upper limits on the strength of the anisotropy of cosmic-ray protons at these energies. The upper limits on the declination component of the anisotropy are the most constraining of any analysis by any instrument, for any energy range. “This is new constraining information about cosmic-ray anisotropy that hasn’t been measured before,” says Meehan.


Improving these limits would likely require years for another space-based observatory to acquire significantly more cosmic rays. In the immediate future, scientists can incorporate the information from these results about the full shape of the anisotropy to constrain the direction of the local interstellar magnetic field and its influence on cosmic rays.


“These results from the Fermi-LAT collaboration provide a new complement to ground-based measurements,” says Vandenbroucke, “not only by extending to a lower energy range, but by covering the full sky, constraining the declination component, and by applying purely to cosmic-ray protons without ambiguity as to which cosmic-ray nuclei are involved.”


+ info “A Search for Cosmic-ray Proton Anisotropy with the Fermi Large Area Telescope,” The Fermi-LAT Collaboration: M. Ajello et al. Published in The Astophysical Journal.