A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope

Abstract

The Fermi Large Area Telescope (LAT) has amassed a large data set of primary cosmic-ray protons throughout its mission. In fact, it is the largest set of identified cosmic-ray protons ever collected at this energy. The LAT's wide field of view and full-sky survey capabilities make it an excellent instrument for studying cosmic-ray anisotropy. As a space-based survey instrument, the LAT is sensitive to anisotropy in both R.A. and decl., while ground-based observations only measure the anisotropy in R.A. We present the results of the first-ever proton anisotropy search using Fermi LAT. The data set was collected over eight years and consists of approximately 179 million protons above 78 GeV, enabling it to probe dipole anisotropy below an amplitude of 10 −3 , resulting in the most stringent limits on the decl. dependence of the dipole to date. We measure a dipole amplitude δ=3.9±1.5×10 −4 with a p-value of 0.01 (pretrials) for protons with energy greater than 78 GeV. We discuss various systematic effects that could give rise to a dipole excess and calculate upper limits on the dipole amplitude as a function of minimum energy. The 95% confidence level upper limit on the dipole amplitude is δ UL =1.3×10 −3 for protons with energy greater than 78 GeV and δ UL =1.2×10 −3 for protons with energy greater than 251 GeV. Unified Astronomy Thesaurus concepts: Galactic cosmic rays (567); High-energy astrophysics (739) 54 Funded by contract FIRB-2012-RBFR12PM1F from the Italian Ministry of Education, University, and Research (MIUR). 55 This is in contrast to ground-based detectors, which have poor composition resolution. 56 The rocking angle of the instrument (angle between zenith and instrument axis) was 35°for the first year of data taking and increased to 50°thereafter.

Understanding the Phase Reversals of Galactic Cosmic-Ray Anisotropies

“…Figure 1 shows the calculated amplitudes and phases of the dipole anisotropies of GCRs in a wide energy range from 10 GeV to >PeV, compared with the measurements (Sakakibara et al 1973;Gombosi et al 1975;Alexeyenko et al 1981;Bercovitch & Agrawal 1981;Thambyahpillai 1983;Nagashima et al 1985Nagashima et al , 1989Swinson & Nagashima 1985;Andreyev et al 1987;Aglietta et al 1995Aglietta et al , 1996Fenton et al 1995;Mori et al 1995;Munakata et al 1995Munakata et al , 1997Ambrosio et al 2003;Amenomori et al 2005Amenomori et al , 2017Guillian et al 2007;Abdo et al 2009;Aglietta et al 2009;Alekseenko et al 2009;Abbasi et al 2010Abbasi et al , 2012Aartsen et al 2013;Bartoli et al 2015Bartoli et al , 2018Ajello et al 2019). The measurements made by the Fermi satellite are for protons (Ajello et al 2019), while the other measurements by underground muon detectors and groundbased air shower detectors are for all particle species in GCRs. Therefore, in each panel we show the results for protons (red) and all species (black) individually.…”

“…A natural expectation of this scenario is that there should be an additional phase reversal at low energies when the background streaming dominates again since the background GCR spectrum is softer than the local source. It is very interesting to note that the first direct measurement of the all-sky anisotropies by the space satellite Fermi seems to observe this predicted reversal of the dipole component at ∼100 GeV energies although the significance is not high enough to robustly claim a detection (Ajello et al 2019). The Fermi data show a dipole amplitude of (3.9 ± 1.5) × 10 −4 and a phase of −9.7 ± 1.5 hr in the R.A. when projecting to one dimension on R.A. for protons with E > 78 GeV (Ajello et al 2019).…”

“…It is very interesting to note that the first direct measurement of the all-sky anisotropies by the space satellite Fermi seems to observe this predicted reversal of the dipole component at ∼100 GeV energies although the significance is not high enough to robustly claim a detection (Ajello et al 2019). The Fermi data show a dipole amplitude of (3.9 ± 1.5) × 10 −4 and a phase of −9.7 ± 1.5 hr in the R.A. when projecting to one dimension on R.A. for protons with E > 78 GeV (Ajello et al 2019). The phase of the Fermi result differs by nearly 12 hr from those measured by underground muon detectors above 100 GeV and just experiences a reversal.…”

Recent status and results of the Dark Matter Particle Explorer

“…There were efforts to search for anisotropies of either CR nuclei or electrons/positrons by PAMELA [59], Fermi-LAT [60,61], and AMS-02 [62]. No significant anisotropies were detected except that PAMELA reported a potential detection of a dipole anisotropy using its calorimeter sample of all CRs [63].…”

Progresses of the Dark Matter Particle Explorer experiment

“…There were efforts to search for anisotropies of either CR nuclei or electrons/positrons by PAMELA [46], Fermi-LAT [47,48], and AMS-02 [49]. No significant anisotropies were detected except that PAMELA reported a potential detection of a dipole anisotropy using its calorimeter sample of all CRs [50].…”