Search for CP violating signature of intergalactic magnetic helicity in the gamma-ray sky
The existence of a cosmological magnetic field could be revealed by the effects of non-trivial helicity on large scales. We evaluate a CP (conjugation plus parity) odd statistic, Q ,u sing gamma-ray data obtained from Fermi satellite observations at high galactic latitudes to search forsuchasignature.ObservedvaluesofQarefoundtobenon-zero;theprobabilityofasimilar signal in Monte Carlo simulations is ∼0.2 per cent. Contamination from the Milky Way does not seem to be responsible for the signal since it is present even for data at very high galactic latitudes. Assuming that the signal is indeed due to a helical cosmological magnetic field, our results indicate left-handed magnetic helicity and field strength ∼10 −14 Go n∼10Mpc scales.