Comparative intra-night optical variability of high and low polarization quasars with dominant radio cores

Abstract

We present the initial results of a programme to investigate the dependence of intra-night optical variability (INOV) of powerful AGN, on the degree of optical polarization. For the first systematic study of this kind, we employed 104-cm Sampurnanand telescope (ST) and 201-cm Himalayan Chandra telescope (HCT) to carry out R-band monitoring of a sample of 8 optically bright radio core dominated quasars showing low optical polarization (P < 3 %) (LPCDQ). Selected from literature in an biased manner, having flat or inverted spectrum at giga hertz frequencies, these LPCDQs were monitored on a total of 23 nights, by monitoring only one of them per night (average duration of monitoring being 5.6 hr per night). Using the CCD as an N-star photometer, a INOV detection threshold of approx 1-2% could be achieved for these densely sampled differential light curves (DLCs). The microvariations were clearly detected on many nights, with peak to peak INOV amplitude ranging from 1.2% to 5.3% (median 2%). While the presence of dominant radio core at centimetre wavelenghts in all these quasars signifies relativistic beaming, they differ strongly from BL Lac objects in showing a very low optical polarization (median P_op0.4 %). From our observations we determine the duty cycle (DC) of 43pm10%, for INOV for these LPCDQs. The corresponding value for BL Lac objects is known to be around 60%. Thus, using these measurements we evaluate the comaparative roles of optical polarization and relativistic beaming on the phenomenon of intra-night optical variability.