  We present a new estimator for the cross-correlation signal between line
intensity maps to probe the Epoch of Reionization. The proposed estimator is
the hitherto neglected antisymmetric component of the cross-correlation, under
the exchange of line-of-sight positions. We consider the cross-correlation
between HI and CO fluctuations, and forecast the improvement in precision on
reionization parameters when the antisymmetric contribution is accounted for.
As a way to break the degeneracy between astrophysics and cosmology in the
intensity mapping power spectrum, we study the ratio between the antisymmetric
and symmetric components. While our results depend on the highly uncertain
astrophysical modelling, we show that in most standard scenarios including the
antisymmetric contribution as a complementary probe can lead to a significant
gain in information.
