Probing individual sources during reionization and cosmic dawn using SKA
  HI 21-cm observations
  Detection of individual luminous sources during the reionization epoch and
cosmic dawn through their signatures in the HI 21-cm signal is one of the
direct approaches to probe the epoch. Here, we summarize our previous works on
this and present preliminary results on the prospects of detecting such sources
using the SKA1-low experiment. We first discuss the expected HI 21-cm signal
around luminous sources at different stages of reionization and cosmic dawn. We
then introduce two visibility based estimators for detecting such signal: one
based on the matched filtering technique and the other relies on simply combing
the visibility signal from different baselines and frequency channels.
  We find that that the SKA1-low should be able to detect ionized bubbles of
radius $R_b \gtrsim 10$ Mpc with $\sim 100$ hr of observations at redshift $z
\sim 8$ provided that the mean outside neutral Hydrogen fraction $ x_{\rm HI}
\gtrsim 0.5$. We also investigate the possibility of detecting HII regions
around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock
et al. 2011. We find that a $5 \sigma$ detection is possible with $600$ hr of
SKA1-low observations if the QSO age and the outside $ x_{\rm HI} $ are at
least $\sim 2 \times 10^7$ Myr and $\sim 0.2$ respectively.
  Finally, we investigate the possibility of detecting the very first X-ray and
Ly-$\alpha$ sources during the cosmic dawn. We consider mini-QSOs like sources
which emits in X-ray frequency band. We find that with a total $\sim 1000$ hr
of observations, SKA1-low should be able to detect those sources individually
with a $\sim 9 \sigma$ significance at redshift $z=15$. We summarize how the
SNR changes with various parameters related to the source properties.
