Antisymmetric Cross-correlation between H I and CO Line Intensity Maps
  as a New Probe of Cosmic Reionization
  Intensity mapping of the H I 21 cm line and the CO 2.61 mm line from the
epoch of reionization has emerged as powerful, complementary, probes of the
high-redshift Universe. However, both maps and their cross-correlation are
dominated by foregrounds. We propose a new analysis by which the signal is
unbiased by foregrounds, i.e. it can be measured without foreground mitigation.
We construct the antisymmetric part of two-point cross-correlation between
intensity maps of the H I 21 cm line and the CO 2.61 mm line, arising because
the statistical fluctuations of two fields have different evolution in time. We
show that the sign of this new signal can distinguish model-independently
whether inside-out reionization happens during some interval of time. More
importantly, within the framework of the excursion set model of reionization,
we demonstrate that the slope of the dipole of H I-CO cross-power spectrum at
large scales is linear to the rate of change of global neutral fraction of
hydrogen in a manner independent of reionization parameters, until the slope
levels out near the end of reionization, but this trend might possibly depend
on the framework of reionization modelling. The H I-CO dipole may be a
smoking-gun probe for the speed of reionization, or "standard speedometer" for
cosmic reionization. Observations of this new signal will unveil the global
reionization history from the midpoint to near the completion of reionization.
