Spectroscopic Monitoring of Blazar S5 0716+714: Brightness-Dependent Spectral Behavior

In this paper, we report 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 similar to 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 toward 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(p), and a power law of the curvature of synchrotron spectrum and its nu(p), simulation well reproduces the brightness-dependent BWB trend of S5 0716+714. The BWB trend is seemingly controlled by the shift of nu(p)with respect to the observational window, and effectively may be dominated by the variations of electron average energy and magnetic field in the emitting region.