Intranight variability of UV emission from powerful blazars
Date & Time :
Auditorium & Zoom
Blazars are Active Galactic Nuclei (AGN) whose observed radiation is predominantly nonthermal and arises from a relativistic jet roughly pointed and hence beamed towards the observer. These are characterized by rapid variability of continuum and polarised emission and their jets frequently exhibit superluminal motion and flux variability from radio to gamma-rays, on time scales as short as minutes. The variability studies have richly contributed to understanding the nature of their central engines and jets. However, information on the intranight variability is lacking for the ultra-violet (UV) emission from blazars. To probe the rapid UV variability of blazars, a practical approach we have adopted here is to perform intranight optical monitoring of blazars located at sufficiently high redshifts, so that the monitored optical radiation is their rest-frame UV emission. In this study, we have carried out intranight optical monitoring of 14 flat-spectrum radio quasars (FSRQs) located at high redshifts (1.5 < z < 3.7) which were grouped into two samples distinguished by low and high fractional optical polarisation, with the division taken at p_opt = 3%. Unexpectedly, a high duty cycle (DC ∼ 30%) is found for intranight variability (with amplitude ψ > 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 ∼ 0.7) and hence typically monitored in the rest-frame blue-optical. Further, we found no evidence for increased intranight variability of UV emission with polarisation, in contrast to the strong correlation found for intranight variability of optical emission. We will briefly discuss this in the context of an existing scenario that posits that the nonthermal UV emission of blazars arises from a relativistic particle population different from that radiating up to near-infrared/optical frequencies.
About Speaker :
Krishan Chand is a Senior Research Fellow (SRF) at ARIES and this is his annual review talk.