Optical variability of radio-intermediate quasars

Abstract

We report the results of our intensive intranight optical monitoring of eight optically bright 'radio-intermediate quasars' (RIQs) having flat or inverted radio spectra. The monitoring was carried out in R band on 25 nights during 2005-2009. On each night only one RIQ was monitored for a minimum duration of ~4 h (the average being 5.2 h per night). Using the CCD as an N-star photometer, an intranight optical variability (INOV) detection threshold of ~1-2 per cent was achieved for the densely sampled differential light curves derived from our data. These observations amount to a large increase over those reported hitherto for this rare and sparsely studied class of quasars which can, however, play an important role in understanding the link between the dominant varieties of powerful active galactic nucleus, namely the radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and blazars. Despite the probable presence of relativistically boosted nuclear jets, inferred from their flat/inverted radio spectra, clear evidence for INOV in our extensive observations was detected only on one night. Furthermore, flux variation between two consecutive nights was clearly seen for one of the RIQs. These results demonstrate that as a class, RIQs are much less extreme in nuclear activity compared to blazars. The availability in the literature of INOV data for another two RIQs conforming to our selection criteria allowed us to enlarge the sample to 10 RIQs (monitored on a total of 42 nights for a minimum duration of ~4 h per night). The absence of large amplitude INOV (ψ≥ 3 per cent) persists in this enlarged sample. This extensive data base have enabled us to arrive at the first estimate for the INOV duty cycle (DC) of RIQs. The DC is found to be small (~9 per cent), increasing to ~14 per cent if the two cases of 'probable' INOV are included. The corresponding value is known to be ~60 per cent for BL Lacs and ≈15 per cent for both RLQs and RQQs, if they too are monitored for 4-6 h in each session. Our observations also provide information about the long-term optical variability of RIQs, which is found to be fairly common and reaches typical amplitudes of ≈0.1 mag. The light curves of these RIQs are briefly discussed in the context of a theoretical framework proposed earlier for linking this rare kind of quasars to the much better studied dominant classes of quasars.