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

TL;DR

This study presents spectroscopic observations of blazar S5 0716+714, revealing brightness-dependent spectral behavior and a shift in synchrotron peak frequency, with implications for understanding jet physics.

Contribution

It introduces a detailed analysis of brightness-dependent spectral trends in S5 0716+714, supported by simulations linking spectral shifts to electron energy and magnetic field variations.

Findings

Brightness-dependent BWB chromatism observed

Spectral behavior varies with brightness state

Simulation reproduces observed spectral trends

Abstract

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.