The optimal redshift for detecting ionized bubbles in HI 21-cm maps
  The detection of individual ionized bubbles in HI 21-cm maps is one of the
most promising, direct probes of the epoch of reionization(EoR). Atleast 1000
hrs of observation would be required for such a detection with either the
currently functioning GMRT or the upcoming MWA. Considering the large
investment of telescope time it is essential to identify the``optimal
redshift'' where the prospects of a detection are most favourable. We find that
the optimal redshift is determined by a combination of instrument dependent
factors and the evolution of the neutral fraction x_HI. We find that the
redshift range 8.1 +/- 1.1 and 9.8 +/- 1 are optimum for detecting ionized
bubbles with the GMRT and MWA respectively. The prospects of a detection, we
find, are more favourable in a scenario with late reionization with x_HI
\approx 0.5 at z \approx 7.5 as compared to an early reionization model where
x_HI \approx 0.5 at z \approx 10. In the late reionization scenario, for both
instruments a 3 \sigma detection is possible for bubbles of comoving radius
R_b>= 30 Mpc with 1000 hrs of observation. Future observations will either lead
to the detection of ionized bubbles, or in the event of non-detection, lead to
constraints on the product x_HI R_b^\gamma for the observational volume, where
\gamma=1.5 and 2 for GMRT and MWA respectively.
