Intranight variability of UV emission from powerful blazars

Abstract

We report the first study to characterise intranight variability of the blazar class from the perspective of (rest-frame) UV emission. For this, we carried out intranight optical monitoring of 14 flat-spectrum radio quasars (FSRQs) located at high redshifts (1.5 < $z$ < 3.7), in 42 sessions of median duration $\sim$ 5.4 hr. These sources were grouped into two samples distinguished by published fractional optical polarisation: (i) nine low-polarisation sources with $p_{opt} < 3\%$ and (ii) five high-polarisation sources. Unexpectedly, a high duty cycle (DC $\sim$ 30$\%$) is found for intranight variability (with amplitude $\psi > 3\%$) of the low-polarisation sources. This DC is a few times higher than that reported for low-polarisation FSRQs located at moderate redshifts ($z$ $\sim$ 0.7) and hence typically monitored in the rest-frame blue-optical. Further, we found no evidence for an increased intranight variability of UV emission with polarisation, in contrast to the strong correlation found for intranight variability of optical emission. We briefly discuss this in the context of an existing scenario which posits that the nonthermal UV emission of blazars arises from a relativistic particle population different from that radiating up to near-infrared/optical frequencies.