Journal Article

Optical variability and colour behaviour of 3C 345

Jianghua Wu, Xu Zhou, Jun Ma and Zhaoji Jiang

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 418, issue 3, pages 1640-1648
Published in print December 2011 | ISSN: 0035-8711
Published online December 2011 | e-ISSN: 1365-2966 | DOI:
Optical variability and colour behaviour of 3C 345

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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 a redder-when-brighter trend. Based 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 mag in the i band. Intranight 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 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.

Keywords: galaxies: active; quasars: individual: 3C 345

Journal Article.  5132 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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