Optical variability and colour behaviour of 3C 345

Abstract

The colour behaviour of blazars is a subject of much debate. One argument is\nthat the BL Lac objects show bluer-when-brighter chromatism while the\nflat-spectrum radio quasars (FSRQs) display redder-when-brighter trend. Base on\na 3.5-year three-colour monitoring programme, we studied the optical\nvariability and colour behaviour of one FSRQ, 3C 345. There is at least one\noutburst in this period. The overall variation amplitude is 2.640 mags in the\n$i$ band. Intra-night variability was observed on two nights. The\nbluer-when-brighter and redder-when-brighter chromatisms were simultaneously\nobserved in this object when using different pairs of passbands to compute the\ncolours. The bluer-when-brighter chromatism is a shared property with the BL\nLacs, while the redder-when-brighter trend is likely due to two less variable\nemission features, the Mg\\,{\\sc ii} line and the blue bump, at short\nwavelengths. With numerical simulations, we show that some other strong but\nless variable emission lines in the spectrum of FSRQs may also significantly\nalter their colour behaviour. Then the colour behaviour of an FSRQ is linked\nnot only to the emission process in the relativistic jet, but also to the\nredshift, the passbands used for computing the colour and the strengths of the\nless variable emission features relative to the strength of the non-thermal\ncontinuum.\n