GMRT observation towards detecting the Post-reionization 21-cm signal
  We have analyzed 610 MHz GMRT observations towards detecting the redshifted
21-cm signal from z=1.32. The multi-frequency angular power spectrum C_l(Delta
nu) is used to characterize the statistical properties of the background
radiation across angular scales ~20" to 10', and a frequency bandwidth of 7.5
MHz with resolution 125 kHz. The measured C_l(Delta nu) which ranges from 7
mK^2 to 18 mK^2 is dominated by foregrounds, the expected HI signal
C_l^HI(Delta nu) ~10^{-6}- 10^{-7} mK^2 is several orders of magnitude smaller.
The foregrounds, believed to originate from continuum sources, is expected to
vary smoothly with Delta nu whereas the HI signal decorrelates within ~0.5 MHz
and this holds the promise of separating the two. For each l, we use the
interval 0.5 < Delta nu < 7.5 MHz to fit a fourth order polynomial which is
subtracted from the measured C_l(Delta nu) to remove any smoothly varying
component across the entire bandwidth Delta nu < 7.5 MHz. The residual
C_l(Delta nu), we find, has an oscillatory pattern with amplitude and period
respectively ~0.1 mK^2 and Delta nu = 3 MHz at the smallest l value of 1476,
and the amplitude and period decreasing with increasing l. Applying a suitably
chosen high pass filter, we are able to remove the residual oscillatory pattern
for l=1476 where the residual C_l(Delta nu) is now consistent with zero at the
3-sigma noise level. We conclude that we have successfully removed the
foregrounds at l=1476 and the residuals are consistent with noise. We use this
to place an upper limit on the HI signal whose amplitude is determined by x_HI
b where x_HI and b are the HI neutral fraction and the HI bias respectively. A
value of x_HI b greater than 7.95 would have been detected in our observation,
and is therefore ruled out at the 3-sigma level. (abridged)
