Blazar is a special type of active galactic nucleus and its jet has a smaller angle to the line of sight. Its color becomes bluer when it becomes brighter (BWB), which is often regarded as evidence of shock in jet. In addition, the BWB may also be related to physical processes such as the turbulence in the post-shock region. Observationally, the BWB is shown as a correlation between color and brightness.

PhD student FENG Haicheng, Prof. LIU Hongtao, and their collaborators, from Yunnan Observatories of the Chinese Academy of Sciences, investigated the optical variability of blazar, based on spectroscopic observations. Their finding was published online in The Astrophysical Journal.

From September 2019 to March 2020 (Epoch2), 106 spectroscopic observations were successfully performed with the 2.4 m optical telescope located at Lijiang Observatory of Yunnan Observatories for TeV gamma-ray blazar S5 0716+714. Significant correlations are found between color and brightness and also their variation rates. In addition, the color variations are ahead of the brightness variations.

Compared to their previous observations (Epoch1), Epoch2 is at a higher state. The BWB trends at Epoch1 and Epoch2 are obviously different in the diagrams of color vs. brightness and of their variation rates. On the contrary, they are basically the same in the diagram of the relative variation rate of brightness vs. the color variation rate. This indicates that the main variability mechanisms at the different brightness states are the same. A significantly brightness-dependent BWB chromatism emerges in the total data of Epoch1 and Epoch2. The BWB trend likely becomes saturated at the highest state.

However, many other observations do not show any correlation between color and brightness or show only weak correlations. The discrepancy of the color–brightness correlations is a crucial issue that can help us to understand more detailed radiation properties in jets.

The BWB trend is seemingly controlled by the shift of the peak frequencies of synchrotron radiation 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. Electron energy-dependent acceleration probability mechanisms, and/or on stochastic acceleration mechanisms can produce the log-parabolic synchrotron spectra of blazars. These synchrotron spectra successfully reproduce the distribution of observed data in the color-brightness diagram.

This study enables people to better understand the radiation mechanism and the variation mechanism of blazar.