Estimate of the Fermi large area telescope sensitivity to gamma-ray polarization

High Energy Gamma-Ray Astronomy: 6th International Meeting on High Energy Gamma-Ray Astronomy, Heidelberg, Germany, 11 Jul 2016 - 15 Jul 2016; AIP conference proceedings 1792(1), 070022 (2017). doi:10.1063/1.4969019
Although not designed primarily as a polarimeter, the Fermi-Large Area Telescope (LAT) has the potential to detect high degrees of linear polarization from some of the brightest gamma-ray sources. To achieve the needed accuracy in the reconstruction of the event geometry, low-energy (≤ 200 MeV) events converting in the silicon detector layers of the LAT tracker have to be used. We present preliminary results of the ongoing effort within the LAT collaboration to measure gamma-ray polarization. We discuss the statistical and systematic uncertainties affecting such a measurement. We show that a 5σ minimum detectable polarization (MDP) of ≈ 30 − 50% could be within reach for the brightest gamma-ray sources as the Vela and Crab pulsars and the blazar 3C 454.3, after 10 years of observation. To estimate the systematic uncertainties, we stack bright AGN, and use this stack as a test source. LAT sensitivity to polarization is estimated comparing the data to a simulation of the expected unpolarized emission of the stack. We measure a 5σ sensitivity limit corresponding to a polarization degree of ≈ 37%. This is in agreement with a purely statistical estimate, suggesting that the systematic errors are likely to be small compared to the statistical ones.
Published by AIP, Melville, NY