Spectral variability of the blazar 3C 279 in the optical to X-ray band during 2009-2018

TL;DR

This study analyzes the long-term spectral variability of blazar 3C 279 across optical to X-ray bands over nine years, revealing changes in correlation patterns and a delay between optical and X-ray emissions, interpreted through a synchro-Compton model.

Contribution

It provides a detailed multi-state analysis of 3C 279's spectral variability and introduces a framework for understanding state-dependent emission correlations in blazars.

Findings

Correlation of optical properties changes after MJD 55500

More frequent activity after MJD 57700

65-day optical delay relative to X-ray in state 2

Abstract

We report on spectral variability of the blazar 3C 279 in the optical to X-ray band between MJD 55100 and 58400 during which long-term radio variability was observed. We construct light curves and band spectra in each of the optical ($2\times10^{14}$-$1.5\times10^{15}$ Hz) and X-ray (0.3-10 keV) bands, measure the spectral parameters (flux $F$ and spectral index $\alpha$), and investigate correlation between $F$ and $\alpha$ within and across the bands. We find that the correlation of the optical properties dramatically change after $\sim$MJD 55500 and the light curves show more frequent activity after $\sim$MJD 57700. We therefore divide the time interval into three "states" based on the correlation properties and source activity in the light curves, and analyze each of the three states separately. We find various correlations between the spectral parameters in the states and an intriguing 65-day delay of the optical emission with respect to the X-ray one in state 2 (MJD 55500-57700). We attempt to explain these findings using a one-zone synchro-Compton emission scenario.