Optical Flux and Colour Variability of Blazars in the ZTF Survey
  We investigate the temporal and colour variability of 897 blazars, comprising
455 BL Lacertae objects (BL Lacs) and 442 Flat Spectrum Radio Quasars (FSRQs),
selected from the Roma-BZCAT catalogue, using the multi-band light curves from
the Zwicky Transient Facility (ZTF DR6) survey. Assessing the colour
variability characteristics over ~2 year timescales, we found that 18.5 per
cent (84 out of 455) BL Lacs showed a stronger bluer when brighter (BWB) trend,
whereas 9.0 per cent (41 out of 455) showed a redder when brighter (RWB) trend.
The majority (70 per cent) of the BL Lacs showing RWB are host galaxy
dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442) objects showed
a strong BWB trend and 17.6 per cent (78 out of 442) showed a strong RWB trend.
Hence we find that BL Lacs more commonly follow a BWB trend than do FSRQs. This
can be attributed to the more dominant jet emission in the case of BL Lacs and
the contribution of thermal emission from the accretion disc for FSRQs. In
analysing the colour behaviour on shorter time windows, we find many blazars
evince shorter partial trends of BWB or RWB nature (or occasionally both). Some
of such complex colour behaviours observed in the colour-magnitude diagrams of
the blazars may result from transitions between the jet-dominated state to the
disc-dominated state and vice versa.
