* Abstract: Cosmological magnetic fields could be one of cosmic relics which can be used as probes of the early universe and high energy particle physics. In this talk, motivated by the possible existence of a cosmological magnetic field with non-trivial helicity, we propose a parity-odd statics method to measure the magnetic helicity, using a gamma ray observation. We evaluate this statistic with gamma ray data obtained from Fermi satellite observation at high galactic latitudes.

Observed values are found to be non-zero at the 2-sigma level and also deviate at the same level from values obtained from simulated data. Assuming that the excess is indeed due to a helical cosmological magnetic field, our results indicate the existence of helical magnetic fields with the field strength, 10^-14 G on about 10Mpc scales.