Search for Spatial Extension in High-Latitude Sources Detected by the Fermi Large Area Telescope

We present a search for spatial extension in high-latitude ($|b|>5^\circ$) sources in the Fermi Large Area Telescope Third Source Catalog (3FGL) and Third Catalog of Hard Sources (3FHL) using the latest Pass 8 software and data set. The result is the Fermi High-Latitude Extended Sources Catalog (FHES) that provides source extensions (or upper limits thereof) and likelihood profiles for the tested source morphologies, i.e., a two-dimensional Gaussian, a disk, and a two-dimensional disk superimposed with a point source (halo). We find 24 extended sources of which 19 sources were not previously characterized as extended. These sources include ones with potential associations with supernova remnants and star forming regions. We also found extended $\gamma$-ray emission in the vicinity of the Cen A radio lobes, and, for the first time at GeV energies, spatially coincident with the radio emission of the SNR CTA 1, and from the Crab Nebula. We also search for halos around active galactic nuclei, which is predicted in the scenario where the $\gamma$ rays that produce the $e^+e^-$ pairs in interactions with background radiation fields induce an electromagnetic cascade. We do not find evidence for extension in individual sources or in stacked source samples. This enables us to place limits on the flux of the extended source components, which are used to derive constraints on the intergalactic magnetic field (IGMF) using spectral and spatial templates of the cascades. This allows us to constrain the IGMF to be stronger than $3\times10^{-16}\,$G for a coherence length $\lambda \gtrsim 10\,$kpc even when conservative assumptions on the source duty cycle are made. This improves previous limits by several orders of magnitude.