First observational constraints on tensor non-Gaussianity sourced by
  primordial magnetic fields from cosmic microwave background
  Primordial magnetic fields (PMFs) create a large squeezed-type
non-Gaussianity in tensor perturbation, which generates non-Gaussian
temperature fluctuations in the cosmic microwave background (CMB). We for the
first time derive an observational constraint on such a tensor non-Gaussianity
from observed CMB maps. Analyzing temperature maps of the WMAP 7-year data, we
find that such a tensor non-Gaussianity is consistent with zero. This gives an
upper bound on PMF strength smoothed on $1 ~ {\rm Mpc}$ as $B_{1 ~ \rm Mpc} <
3.1 ~{\rm nG}$ at $95%$ C.L.
