Toward empirical constraints on the global redshifted 21 cm brightness temperature during the epoch of reionization

Preliminary results are presented from a simple, single-antenna experiment designed to measure the all- sky radio spectrum between 100 and 200MHz. The system used an internal comparison-switching scheme to reduce nonsmooth instrumental contaminants in the measured spectrum to 75 mK. From the observations, we place an initial upper limit of 450 mK on the relative brightness temperature of the redshifted 21 cm contribution to the spectrum due to neutral hydrogen in the intergalactic medium (IGM) during the epoch of reionization, assuming a rapid transition to a fully ionized IGM at z = 8. With refinement, this technique should be able to distinguish between slow and fast reionization scenarios. To constrain the duration of reionization to Delta z > 2, the systemic residuals in the measured spectrum must be reduced to 3 mK.