Intranight variability of ultraviolet emission from powerful blazars

Abstract

We report the first study to characterize intranight variability of the blazar class from the perspective of (rest-frame) ultraviolet (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 ∼5.4 h. These sources were grouped into two samples distinguished by published fractional optical polarization: (i) nine low-polarization sources with popt < 3 per cent and (ii) five high-polarization sources. Unexpectedly, a high duty cycle (DC ∼ 30 per cent) is found for intranight variability (with amplitude ψ > 3 per cent) of the low-polarization sources. This DC is a few times higher than that reported for low-polarization FSRQs located at moderate redshifts (z ∼ 0.7) and hence typically monitored in the rest-frame blue-optical. Furthermore, we found no evidence for an increased intranight variability of UV emission with polarization, 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 non-thermal UV emission of blazars arises from a relativistic particle population different from that radiating up to near-infrared/optical frequencies.