Search for Cosmic-Ray Electron and Positron Anisotropies with Seven Years of Fermi Large Area Telescope Data

The Large Area Telescope on board the Fermi Gamma-ray Space Telescope has collected the largest ever sample of high-energy cosmic-ray electron and positron events since the beginning of its operation. Potential anisotropies in the arrival directions of cosmic-ray electrons or positrons could be a signature of the presence of nearby sources. We use almost seven years of data with energies above 42 GeV processed with the Pass 8 reconstruction. The present data sample can probe dipole anisotropies down to a level of ${10}^{-3}$. We take into account systematic effects that could mimic true anisotropies at this level. We present a detailed study of the event selection optimization of the cosmic-ray electrons and positrons to be used for anisotropy searches. Since no significant anisotropies have been detected on any angular scale, we present upper limits on the dipole anisotropy. The present constraints are among the strongest to date probing the presence of nearby young and middle-aged sources.

Figure 1

Method 1 (left) and method 2 (right) APS as a function of the multipole $l$ for ideal detectors with a 50° FOV radius based on 1000 independent simulations. The colored bands show the regions corresponding to different quantiles at $\pm 1\sigma$ (green), $\pm 2\sigma$ (yellow), and $\pm 3\sigma$ (gray), respectively. The blue lines show the calculation from the white noise distribution at the same quantile values. The fluctuations outside the $2\sigma$ region are due to the limited number of simulations.

Figure 2

Dipole anisotropies as a function of energy. Top panel: simulated isotropic data using methods 1–4. Bottom panel: real data using methods 1 and 2. The markers (median energy value calculated for a power-law flux with a spectral index of -3) show the results and the horizontal error bars indicate the energy bin width. The colored bands show the expected central confidence intervals of the white noise at 68% and 95%.

Figure 3

Top panel: CRE spectra measured by the Fermi-LAT [6] and AMS02 [17]. Blue line: galactic CRE evaluated with dragon-fluka [21]; Green line: Monogem alone (dashed) and total (solid). Cyan line: Vela alone (dashed) and total (solid). Bottom panel: Upper limit at 95% C.L. on dipole anisotropies as a function of energy. The markers in this panel show the actual measurements.