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

Abstract

Long-term optical, X-ray and γ-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 timescales. From the 1980s until now, the optical R band light curve spans more than 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 manifests a clear bluer-when-brighter behavior in the sense that the optical spectrum turns harder (flatter) when the brightness increases. While in the high state, the optical spectrum is stable, which means the source spectral index does not vary with brightness. The correlation analysis has been performed among optical, X-ray and γ-ray energy bands. The result indicates that the variations of γ-ray and X-ray bands are well correlated without time delay on the timescale of days, and their variations exhibit weak correlations with those of the optical band. The variations, especially outbursts, are simultaneous, but the magnitude of variations is disproportionate. The detailed analysis reveals that the main outbursts exhibit strong correlations in different γ-ray, X-ray and optical bands.