Parity-odd correlators of diffuse gamma rays and intergalactic magnetic fields

Abstract: We develop the connection between intergalactic helical magnetic fields and parity odd signatures in the diffuse gamma ray sky. We find that the location and the amplitude of a peak in a parity odd correlator, $Q(R)$, can be used to infer the normal and helical power spectra of the intergalactic magnetic field. When applied to Fermi-LAT data, the amplitude of the observed peak in $Q(R)$ gives $\sim 10^{-14}~{\rm G}$ intergalactic magnetic field strength, which is consistent with an earlier independent estimate that only used the peak location (Tashiro et al. 2014). We discuss features in the observed $Q(R)$ that further support the intergalactic magnetic field hypothesis and make predictions for future tests.