Optical Variability and Colour Behaviour of 3C 345

TL;DR

This study analyzes the optical variability and colour behaviour of the FSRQ 3C 345 over 3.5 years, revealing complex chromatic trends influenced by emission features, redshift, and observational passbands.

Contribution

It provides the first detailed multi-year optical variability and colour analysis of 3C 345, highlighting the impact of emission lines and observational factors on colour behaviour.

Findings

Both bluer-when-brighter and redder-when-brighter trends observed.

Emission features like Mg II line influence colour behaviour.

Colour trends depend on redshift, passbands, and emission line strengths.

Abstract

The colour behaviour of blazars is a subject of much debate. One argument is that the BL Lac objects show bluer-when-brighter chromatism while the flat-spectrum radio quasars (FSRQs) display redder-when-brighter trend. Base on a 3.5-year three-colour monitoring programme, we studied the optical variability and colour behaviour of one FSRQ, 3C 345. There is at least one outburst in this period. The overall variation amplitude is 2.640 mags in the $i$ band. Intra-night variability was observed on two nights. The bluer-when-brighter and redder-when-brighter chromatisms were simultaneously observed in this object when using different pairs of passbands to compute the colours. The bluer-when-brighter chromatism is a shared property with the BL Lacs, while the redder-when-brighter trend is likely due to two less variable emission features, the Mg\,{\sc ii} line and the blue bump, at short wavelengths. With numerical simulations, we show that some other strong but less variable emission lines in the spectrum of FSRQs may also significantly alter their colour behaviour. Then the colour behaviour of an FSRQ is linked not only to the emission process in the relativistic jet, but also to the redshift, the passbands used for computing the colour and the strengths of the less variable emission features relative to the strength of the non-thermal continuum.