New Limits on 21cm EoR From PAPER-32 Consistent with an X-Ray Heated IGM at z=7.7. (arXiv:1304.4991v2 [astro-ph.CO] UPDATED)

New Limits on 21cm EoR From PAPER-32 Consistent with an X-Ray Heated IGM at z=7.7. (arXiv:1304.4991v2 [astro-ph.CO] UPDATED):
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
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 (52 mK)^2 for k=0.11 h 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 models,
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.