Optical variability properties of high luminosity AGN classes

Abstract

1We present the results of a comparative study of the intra-night optical variability (INOV) characteristics of radio-loud and radio-quiet quasars, which involves a systematic intra-night optical monitoring of seven sets of high luminosity AGNs covering the redshift range z 0.2 to z 2.2. The sample, matched in the optical luminosity -redshift (M B -z) plane, consists of seven radio-quiet quasars (RQQs), eight radio lobe-dominated quasars (LDQs), six radio core-dominated quasars (CDQs) and five BL Lac objects (BLs). Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to detect INOV with amplitudes as low as 1%. Present observations cover a total of 113 nights (720 hours) with only a single quasar monitored as continuously as possible on a night. Considering cases of only unambiguous detections of INOV we have estimated duty cycles (DCs) of 17%, 12%, 20% and 72% respectively for RQQs, LDQs, CDQs, and BLs. The low amplitude and low DC of INOV shown by RQQs compared to BLs can be understood in terms of their having optical synchrotron jets which are modestly misdirected from us. From our fairly extensive dataset, no unambiguous general trend of a correlation between the INOV amplitude and the apparent optical brightness of the quasar is noticed. This suggests that the physical mechanisms of INOV and long term optical variability (LTOV) do not have a one-to-one relationship and different factors are involved. Also, the absence of a clear negative correlation between the INOV and LTOV characteristics of blazars of our sample points towards an inconspicuous contribution of accretion disk flucutations to the observed INOV. The INOV duty cycle of the AGNs observed in this program suggests that INOV is associated predominantly with highly polarised optical components. We also report new VLA imaging of two RQQs (1029+329 & 1252+020) in our sample which has yielded a 5 GHz detection of one of them (1252+020; S = 1 mJy).

Intranight optical variability of radio-quiet weak emission line quasars – III

“…With a significantly enlarged sample of 30 DLCs of RQWLQs in the present study, we now find that their INOV duty cycle is about 5 per cent, at a typical INOV detection threshold of around 5 per cent and a monitoring duration of about 3−5 hours. In our programme, two of the RQWLQs were found in two sessions to exhibit very strong INOV (amplitude ψ > 10%), a level never observed in our 2-decade long INOV programme (summarized in GGWSS13, Goyal et al 2012;Stalin et al 2004b), except for BL Lacs and HPCDQs. The two RQWLQs, namely J090843.25+285229.8 (ψ ∼ 31% on 10.02.2013, Table 4) and J140710.26+241853.6 ( ψ ∼ 36% on 03.05.2014, Figure 1, Table 4), are thus currently the best available candidates for the elusive population of radio-quiet BL Lacs and hence need to be followed up.…”

“…A point worth emphasizing here is that while applying the F η −test, it is specially important to use the correct rms errors on the photometric data points. It has been found that the magnitude errors returned by the routines in the data reduction softwares of DAOPHOT and IRAF, are normally underestimated by a factor η ranging between 1.3 and 1.75, as shown in various studies (e.g., Gopal-Krishna et al 1995;Garcia et al 1999;Sagar et al 2004;Stalin et al 2004b;Bachev et al 2005). Recently Goyal et al (2012) estimated the best-fit value of η to be 1.5.…”

“…Optical flux variation on hourlike time scale, which is commonly known as 'intranight optical variability' (INOV), has emerged as a useful probe of AGN (Wagner & Witzel 1995;Ulrich et al 1997;Wiita 2006). The past two decades have witnessed a large number of INOV studies covering different classes of AGN, in order to study the physical processes underlying this phenomenon occurring in the different AGN classes (Miller et al 1989;Heidt & Wagner 1996;Carini et al 1990Carini et al , 1991Carini et al , 2007Gopal-Krishna et al 1995Stalin et al 2004b ⋆ E-mail: parveen@aries.res.in (PK); gopaltani@gmail.com (GK); hum@aries.res.in (HC) Goyal et al 2013;Chand et al 2014;de Diego 2014). These studies have led to theoretical models for INOV (e.g, see Ulrich et al 1997;Czerny et al 2008;Wiita 2006).…”

Nature of intranight optical variability of BL Lacertae

“…The enhanced F-test values are presented in Table 1. The duty cycle (in per cent) (DC) of BL Lacertae is computed following the definition of Romero et al (1999) that later was used by many authors (e.g., Stalin et al 2004;Goyal et al 2012, and references therein),…”

On the Photometric Error Calibration for the Differential Light Curves of Point-like Active Galactic Nuclei

“…Not only are some of these simultaneously monitored stars used for measuring any variations in the seeing disk during the course of the monitoring session, but, more importantly, they are used as non-varying standards relative to which the light curve of the target AGN can be drawn. Such 'differential light curves' (DLCs) are also drawn for the candidate 'comparison stars' themselves and used to check for the presence of INOV of those stars, in which case they are disqualified as comparison stars (e.g., Miller & Wiita 1991;Stalin et al 2004b;Wiita 2006). A key advantage of using DLCs is that the effects of any fluctuations in the atmospheric attenuation and even in the seeing disk are mostly canceled out, and this way the variability detection threshold is pushed down enormously (e.g., Howell & Jacoby 1986;Miller, Carini, & Goodrich 1989;Gilliland et al 1993;Howell et al 2005).…”