The optical variability of flat-spectrum radio quasars in the SDSS stripe 82 region

TL;DR

This study analyzes optical variability in 29 flat-spectrum radio quasars over nine years, finding that the common bluer-when-brighter trend dominates, while the redder-when-brighter trend is rare and possibly linked to thermal emission.

Contribution

It provides the first detailed investigation of the frequency of redder-when-brighter trends in FSRQs using SDSS Stripe 82 data.

Findings

14 of 29 FSRQs show bluer-when-brighter trend

Only 1 FSRQ exhibits redder-when-brighter trend

Thermal emission from accretion disk may cause redder-when-brighter behavior

Abstract

Context. Although a bluer-when-brighter trend is commonly observed in blazars, the opposite trend of redder-when-brighter has also been found in some blazars. Aims. We investigate the frequency of the redder-when-brighter trend in flat-spectrum radio quasars (FSRQs). Methods. We investigate the optical variability of 29 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released multi-epoch data covering about nine years. We determined the spectral index by fitting a powerlaw to SDSS ugriz photometric data, and explored the relationship between the spectral index and source brightness. Results. For all FSRQs studied, we detect variations in r band flux of overall amplitude between 0.24 mag and 3.46 mag in different sources. Fourteen of 29 FSRQs display a bluer-when-brighter trend. However, only one source exhibits a redder-when-brighter trend, which implies that this behavior is rare in our FSRQ sample. In this source, the thermal emission from the accretion disk may be responsible for the redder-when-brighter trend.