  According to the analysis of the $^{14}$C content of two Japanese trees over
a period of approximately 3000 years at high time resolution, Miyake (2012)
found a rapid increase at AD 774-775 and later on at AD 992-993 (Miyake 2013).
This corresponds to a high-energy event happened within one year that input
$\gamma$-ray energy about 7$\times{}$10$^{24}$erg to the Earth, leaving the
origin a mystery. Such strong event should have an unusual optical counterpart,
and have been recorded in historical literature. We searched Chinese historical
materials around AD 744-775 and AD 992-993, but no remarkable event was found
except a violent thunderstorm in AD 775. However, the possibility of a
thunderstorm containing so much energy is still unlikely. We conclude the event
caused the $^{14}$C increase is still unclear. This event most probably has no
optical counterpart, and short gamma-ray burst, giant flare of a soft gamma-ray
repeater and terrestrial $\gamma$-ray flash may all be the candidates.
