Long-term multi-wavelength variations of Fermi blazar 3C 279

TL;DR

This study compiles long-term multi-wavelength data of blazar 3C 279, revealing strong variability, a possible quasi-periodic cycle, and complex spectral behaviors across optical, X-ray, and gamma-ray bands.

Contribution

It provides the first comprehensive long-term multi-wavelength analysis of 3C 279, identifying a potential 5.6-year quasi-periodic variation and detailed spectral behavior patterns.

Findings

Detected a 5.6-year quasi-periodic variation in optical light curve.

Found strong correlations between gamma-ray and X-ray variations without time delay.

Observed spectral index changes with brightness in low state, but stability in high state.

Abstract

The long-term optical, X-ray and $\gamma$-ray data of blazar 3C 279 have been compiled from $Swift$-XRT, $RXTE$ PCA, $Fermi$-LAT, SMARTS and literature. The source exhibits strong variability on long time scales. Since 1980s to now, the optical $R$ band light curve spans above 32 yr, and a possible 5.6-yr-long quasi-periodic variation component has been found in it. The optical spectral behavior has been investigated. In the optical band, the mean spectral index is -1.71. The source exhibits an obvious special spectral behavior. In the low state, the source shows a clear bluer-when-brighter behavior in a sense that the optical spectrum turns harder (flatter) when the brightness increases. While in the high state, the optical spectrum is stable, that means the source spectral index does not vary with the brightness. The correlation analysis has been performed among optical, X-ray and $\gamma$-ray energy bands. The result indicates that the variations of $\gamma$-ray and X-ray bands are well correlated without time delay on the time scale of days, and their variations exhibit weak correlations with those of optical band. The variations, especial outbursts, are simultaneous, but the magnitude of variations is disproportionate. The detailed analysis reveals that the main outbursts exhibit strong correlations in different $\gamma$-ray, X-ray and optical bands.