Persistence of blazar state in flat-spectrum radio quasars

Abstract

Flat-spectrum radio quasars (FSRQs) whose brightness is dominated by a relativistically beamed core, are frequently found in 'blazar state' commonly inferred from a high optical polarization ( > 3%), and/or a large continuum variability. Here we use these two prime optical markers to investigate continuance of an FSRQ in blazar (or non-blazar) state over an exceptionally long time baseline spanning 4 decades. Our basic sample is a well-defined, unbiased set of 80 FSRQs whose blazar state stood confirmed during 1980s from optical polarimetry. Four decades later, blazar state of each FSRQ is ascertained here from variability of their optical light-curves of typical duration $\sim$ 3.5 years, a low noise (rms $\sim$ 2%) and good cadence ($\sim$ 3 days), obtained under the Zwicky Transient Facility project ongoing since 2018. For about 40% of these FSRQs, blazar state could be ascertained additionally from the opto-polarimetric survey RoboPol (2013-2017). From both these databases it is found that only $\sim$ 10% of the FSRQs have undergone a blazar $\leftrightarrow$ non-blazar state transition over the past 3 - 4 decades. This reinforces the case for a long-term stability of blazar state in individual FSRQs, despite their state fluctuating more commonly on year-like time scales.