Blazar is a special type of active galactic nucleus and its jet has a smaller angle to the line of sight. There is a special variation behavior that blazar’s color becomes bluer when it becomes brighter (BWB), which is often regarded as evidence of shock in jet and shows a correlation between color and brightness.

Recently, FENG Haicheng, Prof. LIU Hongtao and the collaborators from Yunnan observatories of the Chinese Academy of Sciences investigated the optical variability of blazar based on quasi-simultaneous spectroscopic and multiband photometric observations. Their finding was published online in The Astrophysical Journal.

From November 2018 to March 2019, 45 spectroscopic observations and 44 photometric multiband 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.

The researchers found that the spectral and photometric data show BWB trends for S5 0716+714, and significant correlations between color and brightness and also their variation rates. Besides, they revealed that the color variations are ahead of the brightness variations.

However, many other observations did 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 understand more detailed radiation properties in jets.

The appearance of the observed BWB may depend on the relative position of the observational frequency ranges with respect to the peak frequencies of synchrotron radiation emitted by a relativistic jet, e.g., at the left of the peak frequencies.

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

"The reason why color variation is ahead of brightness variation is not clear. The variability might be potentially related to the activity of the central black hole in blazar,” Prof. LIU said.