Quasi-simultaneous Spectroscopic and Multi-band Photometric Observations
  of Blazar S5 0716+714 during 2018-2019
  In order to study short timescale optical variability of $\gamma$-ray blazar
S5 0716+714, quasi-simultaneous spectroscopic and multi-band photometric
observations were performed from 2018 November to 2019 March with the 2.4 m
optical telescope located at Lijiang Observatory of Yunnan Observatories. The
observed spectra are well fitted with a power-law $F_{\lambda}=A\lambda
^{-\alpha}$ (spectral index $\alpha >0$). Correlations found between
$\dot{\alpha}$, $\dot{A}$, $\dot{A}/A$, $\dot{F_{\rm{\lambda}}}$, and
$\dot{F_{\rm{\lambda}}}/F_{\rm{\lambda}}$ are consistent with the trend of
bluer-when-brighter (BWB). \textbf{The same case is for colors, magnitudes,
color variation rates, and magnitude variation rates of photometric
observations.} The variations of $\alpha$ lead those of $F_{\rm{\lambda}}$.
Also, the color variations lead the magnitude variations. The observational
data are mostly distributed in the I(+,+) and III(-,-) quadrants of coordinate
system. Both of spectroscopic and photometric observations show BWB behaviors
in S5 0716+714. The observed BWB may be explained by the shock-jet model, and
its appearance may depend on the relative position of the observational
frequency ranges with respect to the synchrotron peak frequencies, e.g., at the
left of the peak frequencies. \textbf{Fractional variability amplitudes are
$F_{\rm{var}}\sim 40\%$ for both of spectroscopic and photometric observations.
Variations of $\alpha$ indicate variations of relativistic electron
distribution producing the optical spectra. }
