Multi-colour optical monitoring of eight red blazars

Abstract

Context. We present the observational results of multi-colour optical monitoring of eight red blazars from 2003 September to 2004 February. Aims. The aim of our monitoring is to investigate the spectral variability as well as the flux variations at short and long time scales. Methods. The observations were carried out using the 1.0 m robotic telescope of Mt. Lemmon Optical Astronomy Observatory, in Arizona, USA, the 0.6 m telescope of Sobaeksan Optical Astronomy Observatory and the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory, in the Republic of Korea. Results. During the observations, all sources show strong flux variations with amplitudes of larger than 0.5 mag. Variations with amplitudes of over 1 mag are found in four sources. Intraday variations with amplitudes larger than 0.15 mag, and a rapid brightness increase with a rate of ∼0.2 mag per day in four days, are detected in S5 0716+71. We investigate the relationship between the colour index and source brightness for each source. We find that two out of three FSRQs tend to be redder when they are brighter, and, conversely, all BL Lac objects tend to be bluer. In particular, we find a significant anti-correlation between the V − I colour index and R magnitude for 3C 454.3. This implies that the spectrum became steeper when the source was brighter, which is opposite to the common trend for blazars. In contrast, significant positive correlations are found in 3C 66A, S5 0716+71, and BL Lac. However, there are only very weak correlations for PKS 0735+17 and OJ 287. Conclusions. We propose that the different relative contributions of the thermal versus non-thermal radiation to the optical emission may be responsible for the different trends of the colour index with brightness in FSRQs and BL Lac objects.

Intra-night optical multiband variability of BL Lacertae during the 2011 outburst

“…14, fast flares exhibited a strong bluerwhen-brighter (BWB) trend in inter-night variation (bottom panel), but it is messy in the intra-night variation (top panel). This is consistent with the common trend for blazars when they are in active states (Gu et al 2006;Wu et al 2007). According to Hu et al (2006), the colour results are not strongly affected by the host galaxy, since it has similar colours to the AGN.…”

Multiband optical variability studies of BL Lacertae

“…The source brightness was found to have strong BWB chromatism on intraday timescales. The BWB trend has been found to be predominant in BL Lacertae by all optical observations of this source till date during both flaring and steady states (Ghosh et al 2000;Gu et al 2006) which was revealed by our observations also. Villata et al (2004b) found that the intraday flares followed BWB trend strongly with a slope of ∼ 0.4 which is also the case for our data with the slopes of the intra-night LCs more than ∼ 0.4 in majority of cases.…”

“…Optical observations can also indicate the possible presence of other components in addition to synchrotron continuum. There is good evidence for the presence of IDV and colour variability in some BL Lacertae objects and our source is among those (e.g., Carini et al 1992, Clements & Carini 2001, Gu et al 2006, H.E.S.S. Collaboration et al 2011).…”

The nature of optical and near-infrared variability of BL Lacertae

“…A bluer-when-brighter behaviour was also detected by other authors in datasets more or less extended in time (e.g. Zhang et al 2004;Stalin et al 2006;Gu et al 2006).…”