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.