Spectroscopic Monitoring of Blazar S5 0716+714: Brightness-Dependent
  Spectral Behavior
  In this paper, we report the new results of spectroscopic observations of
$\gamma$-ray blazar S5 0716+714 from 2019 September to 2020 March with the 2.4
m optical telescope at Lijiang Observatory of Yunnan Observatories. The median
cadence of observations is $\sim$ 1 day. During the second observation period
(Epoch2), the observational data reveal an extremely bright state and a
bluer-when-brighter (BWB) chromatism. The BWB trend of Epoch2 differs
significantly from that of the first observation period (Epoch1). A
significantly brightness-dependent BWB chromatism emerges in the total data of
Epoch1 and Epoch2. The BWB trend becomes weaker towards the brighter states,
and likely becomes saturated at the highest state. Based on a log-parabolic
function, a power-law of synchrotron peak flux and frequency $\nu_{\rm{p}}$,
and a power-law of the curvature of synchrotron spectrum and its
$\nu_{\rm{p}}$, simulation well reproduces the brightness-dependent BWB trend
of S5 0716+714. The BWB trend is seemingly controlled by the shift of
$\nu_{\rm{p}}$ with respect to the observational window, and effectively may be
dominated by the variations of electron average energy and magnetic field in
emitting region.
