2022MNRAS.516L..18C

Flat-spectrum radio quasars (FSRQs), whose brightness is dominated by a relativistically beamed core, are frequently found in the 'blazar state' commonly inferred from a high optical polarization (> 3 per cent) and/or a large continuum variability. Here we use these two prime optical markers to investigate continuance of an FSRQ in the blazar (or non-blazar) state over an exceptionally long time baseline spanning four decades. Our basic sample is a well-defined, unbiased set of 80 FSRQs whose blazar state stood confirmed during the 1980s from optical polarimetry. Four decades later, the blazar state of each FSRQ is ascertained here from variability of their optical light curves with a typical duration of ∼3.5 yr, a low noise (rms ∼2 per cent) and good cadence (∼3 d), obtained under the Zwicky Transient Facility project, which has been ongoing since 2018. For about 40 per cent of these FSRQs, the blazar state could be ascertained additionally from the opto-polarimetric survey RoboPol (2013-2017). From both these data bases, it is found that only ∼10 per cent of the FSRQs have undergone a blazar ↔ non-blazar state transition over the past three to four decades. This reinforces the case for the long-term stability of the blazar state in individual FSRQs, despite their state fluctuating more commonly on year-like time-scales.