Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV

Abstract

We present the results of our analysis of cosmic-ray electrons using about 8 × 10 6 electron candidates detected in the first 12 months on-orbit by the Fermi Large Area Telescope. This work extends our previously published cosmic-ray electron spectrum down to 7 GeV, giving a spectral range of approximately 2.5 decades up to 1 TeV. We describe in detail the analysis and its validation using beam-test and on-orbit data. In addition, we describe the spectrum measured via a subset of events selected for the best energy resolution as a cross-check on the measurement using the full event sample. Our electron spectrum can be described with a power law ∝ E −3.08±0.05 with no prominent spectral features within systematic uncertainties. Within the limits of our uncertainties, we can accommodate a slight spectral hardening at around 100 GeV and a slight softening above 500 GeV.

In-flight measurement of the absolute energy scale of the Fermi Large Area Telescope

“…Electrons and positrons are a good calibration source because they interact in the LAT CAL in the same way that photons do: they both produce electromagnetic showers. The energy measurement bias coming from the slight differences between CRE and photons interactions in the LAT is much less than 1% [4]. The capabilities of the LAT to measure the CRE spectrum have been well demonstrated [4,5].…”

“…The energy resolution for all three available energy reconstruction methods was measured to be consistent with expectations. However despite these improvements, the reconstructed energy in the CU was consistently higher than the beam energy by ∼9%, on average, with further fluctuations of ∼5%, depending on incoming beam energy (5-282 GeV), angle (0-60 degrees) and position of the crystal within the shower [4,11].…”

“…The energy measurement bias coming from the slight differences between CRE and photons interactions in the LAT is much less than 1% [4]. The capabilities of the LAT to measure the CRE spectrum have been well demonstrated [4,5]. The cutoff rigidity 1 can be predicted by numerically tracing particle trajectories in the Earth's magnetic field, and the comparison of the predicted and measured values provides the opportunity to perform this validation.…”

“…We selected energies of the test particles according to the power-law spectrum measured by the LAT [4], and generated both electrons and positrons in the abundance ratio measured by PAMELA [13]. We use the tracer code to determine for each test particle whether it could have originated from outside the geomagnetosphere.…”

“…We selected electron and positron data from the first year of Fermi LAT data taking (Aug 2008 through July 2009) using the methods described in [4] for the 100 MeV to 100 GeV energy range. In figure 2 are the distributions of the reconstructed angle with respect to local zenith (upper panel) and azimuth (lower panel) for the flight data (black) and for the tracer output (red).…”