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 odd
statistic, $Q$, using gamma ray data obtained from Fermi satellite observations
at high galactic latitudes to search for such a signature. Observed values of
$Q$ are found to be non-zero; the probability of a similar signal in Monte
Carlo simulations is $\sim 0.2\%$. 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 $\sim 10^{-14}~{\rm G}$ on $\sim 10~{\rm Mpc}$ scales.
