Constraining warm dark matter power spectrum using the cross-correlation
  of HI 21 cm signal and the Lyman-$\alpha$ forest
  We have considered the prospects for measuring the cross Warm Dark Matter
(WDM) power spectrum of the redshifted HI 21-cm signal and the Lyman-$\alpha$
forest and thereby constraining WDM mass using observations with upcoming
radio-interferometers - the Ooty Wide Field Array (OWFA) and SKA1-mid, and a
spectroscopic survey of the quasars. We have considered a quasar survey with a
mean observed quasar number density of $\bar{{\rm n}}_{\rm Q} = 48$ deg$^{-2}$
over a collecting area of 14455 deg$^{2}$, and a mean spectroscopic SNR = 5.
Our analysis with OWFA shows that it is possible to measure the WDM power
spectrum in several $k$-bins at $k < 0.4$ Mpc$^{-1}$ with ${\rm SNR} > 5$ using
an observation of 200 hours each in 100 different fields-of-view for $m_{\rm
WDM} = 0.25$ keV. Considering the possibility of the joint measurement of the
parameters, the warm dark matter density parameter $\Omega_{{\rm WDM}}$, and
the dark energy density parameter $\Omega_{\Lambda0}$, we find that the
relative error on the $1-\sigma$ measurement of the parameter $\Omega_{{\rm
WDM}}$ is $\sim 0.8$ for a fiducial $m_{\rm WDM} = 0.25$ keV. We further find
that it is possible to have a measurement of the suppression of power from the
Cold Dark Matter (CDM) power spectrum at a confidence level of $\sim
7.2-\sigma$ and $\sim 2.7-\sigma$ in two different $k$-bins over the $k$-range
$0.1 \leq k \leq 3.13$ Mpc$^{-1}$ for $m_{\rm WDM} = 0.15$ keV. Considering the
analysis with SKA1-mid, we find that for a fiducial $m_{\rm WDM}= 0.25$ keV,
the suppression in the cross power spectrum can be measured at $\sim 10 -
\sigma$ around $k \sim 0.2 \rm{Mpc}^{-1}$ for a total observing time of $20000$
hrs distributed uniformly over $50$ independent pointings where the available
$k$-range is binned as $\Delta k = k/5$.
