Fermi Gamma-ray Space Telescope

A New Template Method to Subtract the Earth Limb for Pass 8 Analysis

E. D. Bloom
W. Mitthumsiri, R. Rando, M. Wood, Representing the Fermi-LAT Collaboration

Abstract:

Contamination of the celestial photon sample by photons from the Earth Limb is a major challenge for Fermi LAT Pass 8 analyses utilizing data below ~100 MeV. Owing to the broad point-spread function at low energies, the standard zenith angle cuts have only limited effectiveness, and applying tighter cuts can severely reduce the remaining exposure. We present a technique for creating templates for the residual Earth Limb contamination using a phenomenological model of the Earth Limb emission, which is based on Pass 8 Earth Limb gamma-ray data collected by the LAT. To implement this technique we are developing a Limb Tool (analogous to the Solar System Tools available in the FSSC) specific to Pass 8 analyses, which uses this phenomenological Earth model (intensity as a function of energy and nadir and azimuth angles) to build templates of the Earth Limb contamination for any chosen time or zenith angle selection. In order to subtract the Limb contribution with sufficient accuracy at lower energies, a better understanding of the limb gamma-ray emission reconstructed in the far tails of the PSF is needed as compared to the current phenomenological model, which we have been developing for this new Limb Tool. We will report on our progress toward the development of the Limb Tool and the resulting new Earth Limb templates that could be used for Pass 8 analysis.