Novel Search for TeV-Initiated Pair Cascades in the Intergalactic Medium

Abstract

TeV emission from blazars can be used to probe the intergalactic magnetic fields and measure their intensity, coherence length, and helicity. Intergalactic magnetic fields deflect the electron-positron pairs produced by very-high-energy gamma-rays from blazars, resulting in broadened beams of cascade gamma-rays developing along the projected direction of the blazar jet. We present an analysis that uses for the first time the jet orientation of 12 high-synchrotron-peaked (HSP) BL~Lac-type blazars from VLBA radio observations to search for signatures of pair haloes in the Fermi-LAT data. Our search improves the sensitivity of previous studies by taking the asymmetry of the pair haloes into account, increasing the signal to noise and reducing the possibility of systematics in determining the point spread function. Although there is no significant detection, a hint for an offset halo with a global significance of 2$\sigma$ is found in the $30-300$ GeV energy range, corresponding to an intergalactic magnetic field with $\sim 10^{-15}$ Gauss, consistent with the inferred field from a prior study using stacked HSP BL Lac objects.