Short-term flux and colour variations in low-energy peaked blazars

Abstract

We have measured multiband optical flux and colour variations for a sample of 12 low-energy peaked blazars (LBLs) on short, day-to-month, time-scales. Our sample contains six BL Lacertae objects (BL Lacs) and six flat spectrum radio quasars (FSRQs). These photometric observations, made during 2008 September to 2009 June, used five optical telescopes, one in India and four in Bulgaria. We detected short-term flux variations in 11 of these blazars and colour variability in eight of them. Our data indicate that six blazars (3C 66A, AO 0235+164, S5 0716+714, PKS 0735+178, OJ 287 and 3C 454.3) were observed in pre- or post-outburst states, five (PKS 0420−014, 4C 29.45, 3C 279, PKS 1510−089 and BL Lac) were in a low state, while one (3C 273) was in an essentially steady state. The duty cycles for flux and colour variations on short time-scales in these LBLs are ∼92 and ∼33 per cent, respectively. The colour versus magnitude correlations seen here support the hypothesis that BL Lac objects tend to become bluer with increase in brightness; however, FSRQs may show the opposite trend, and there are exceptions to these trends in both categories of blazar. We briefly discuss emission models for active galactic nuclei that might explain our results.