New Limits on 21cm EoR From PAPER-32 Consistent with an X-Ray Heated IGM
  at z=7.7
  We present new constraints on the 21cm Epoch of Reionization (EoR) power
spectrum derived from 3 months of observing with a 32-antenna,
dual-polarization deployment of the Donald C. Backer Precision Array for
Probing the Epoch of Reionization (PAPER) in South Africa. In this paper, we
demonstrate the efficacy of the delay-spectrum approach to avoiding
foregrounds, achieving over 8 orders of magnitude of foreground suppression (in
$\textrm{mK}^2$). Combining this approach with a procedure for removing
off-diagonal covariances arising from instrumental systematics, we achieve a
best 2-sigma upper limit of $(41\,\textrm{mK})^2$ for $k=0.27
h\textrm{Mpc}^{-1}$ at $z=7.7$. This limit falls within an order of magnitude
of the brighter predictions of the expected 21cm EoR signal level. Using the
upper limits set by these measurements, we generate new constraints on the
brightness temperature of 21cm emission in neutral regions for various
reionization models. We show that for several ionization scenarios, our
measurements are inconsistent with cold reionization. That is, heating of the
neutral intergalactic medium (IGM) is necessary to remain consistent with the
constraints we report. Hence, we have suggestive evidence that by $z=7.7$, the
HI has been warmed from its cold primordial state, probably by X-rays from
high-mass X-ray binaries or mini-quasars. The strength of this evidence depends
on the ionization state of the IGM, which we are not yet able to constrain.
This result is consistent with standard predictions for how reionization might
have proceeded.
