Accelerated Evolution of Ly$\alpha$ Luminosity Function at $\textit{z}
  \gtrsim 7$ Revealed by the Subaru Ultra-Deep Survey for Ly$\alpha$ Emitters
  at $\textit{z}=7.3$
  We present the ultra-deep Subaru narrowband imaging survey for Lya emitters
(LAEs) at $z=7.3$ in SXDS and COSMOS fields with a total integration time of
106 hours. Exploiting our new sharp bandwidth filter, NB101, installed on
Suprime-Cam, we have reached $L(Lya)=2.4\times10^{42} \ erg \ s^{-1}$
($5\sigma$) for $z=7.3$ LAEs, about 4 times deeper than previous Subaru $z
\gtrsim 7$ studies, which allows us to reliably investigate evolution of Lya
luminosity function (LF), for the first time, down to the luminosity limit same
as those of Subaru $z=3.1-6.6$ LAE samples. Surprisingly, we only find three
and four LAEs in SXDS and COSMOS fields, respectively, while one expects a
total of $\sim 65$ LAEs by our survey in the case of no Lya LF evolution from
$z=6.6$ to $7.3$.We identify a decrease of Lya LF from $z=6.6$ to $7.3$ at the
$>90\%$ confidence level from our $z=7.3$ Lya LF.Moreover, the evolution of Lya
LF is clearly accelerated at $z>6.6$ beyond the measurement uncertainties
including cosmic variance. Because no such accelerated evolution of
UV-continuum LF or cosmic star-formation rate (SFR) is found at $z\sim 7$, but
suggested only at $z>8$ (Oesch et al. 2013, Bouwens et al. 2014), this
accelerated Lya LF evolution is explained by physical mechanisms different from
a pure SFR decrease but related to Lya production and escape in the process of
cosmic reionization. Because a simple accelerating increase of IGM neutral
hydrogen absorbing Lya would not reconcile with Thomson scattering optical
depth measurements from WMAP and Planck, our findings may support new physical
pictures suggested by recent theoretical studies, such as the existence of HI
clumpy clouds within cosmic ionized bubbles selectively absorbing Lya and the
large ionizing photon escape fraction of galaxies making weak Lya emission.
