Correlation between optical flux and polarization variations in Flat Spectrum Radio Quasars on diverse timescales

TL;DR

This study investigates the correlation between optical flux and polarization in eight flat-spectrum radio quasars over ten years, revealing diverse relationships on different timescales and implications for jet magnetic field structures.

Contribution

It provides a systematic analysis of flux-polarization correlations across multiple timescales in quasars, highlighting complex behaviors and challenging simple models of jet emission.

Findings

Positive flux-PD correlation on long timescales in 34 of 79 cycles

Short-term positive correlation in 47 of 55 epochs

Correlation between optical and gamma-ray fluxes observed in 14 epochs

Abstract

Study of the polarization behaviour in blazars is a powerful tool to discern the role of magnetic field in the variable emission process in their relativistic jets. We present here results of our systematic investigation on the correlation between optical flux and polarization variations for eight flat-spectrum radio quasars on various timescales using data from the Steward Observatory that covers a period of $\sim$10 years. On long time scales ($\sim$several months), from a total of 79 observing cycles, in 34 observing cycles, we found a significant positive correlation between optical flux and optical polarization degree (PD), negative correlation in 3 cycles and no correlation in 42 cycles. On short time scales ($\sim$few days), in 47 out of a total of 55 epochs, we found a positive correlation between optical flux and PD, while, on the remaining 8 epochs, an anti-correlation was detected between the two quantities. Moreover, we noticed a significant positive correlation between optical and $\gamma-$ray fluxes on 14 epochs and a negative correlation between the two on one epoch. While the observed optical flux changes well fit in the shock-in-jet model, the observed changes in PD are not explainable by changes in the power-law spectral index of the relativistic electrons in the jet. Instead, the observed varied correlations between optical flux and PD could be due to multi-zone emission regions or the enhanced flux coinciding with the emergence of a new emission knot with its magnetic field either aligned or misaligned with the large scale magnetic field.