Optical monitoring in southern blazars. Analysis of variability and spectral colour behaviours

Abstract

We present the results of the optical monitoring of 18 southern blazarsin the V and R Johnson–Cousins bands. Our main objective isto study the variationsin the optical flux and their relationship with the colour and spectral variabilities. The optical observations were acquired with the 2.15 m ‘Jorge Sahade’ telescope, CASLEO, Argentina. The whole campaign comprised from 2014 April to 2019 September. In addition, X-ray data were taken from the Chandra X-ray Observatory and the Swift/XRT databases, and γ -ray data were taken from the Fermi-Large Area Telescope 3FGL catalogue. From the total of 18 blazars, we found variability in each one of the time-scales considered for 6 blazars (PKS 0208–512, PKS 1116–46, PKS 1440–389, PKS 1510– 089, PKS 2005–489, and PKS 2155–304). In particular, from the colour–magnitude and the multiwavelength analysis, we found that PKS 1510-089 (flat-spectrum radio quasar) is undergoing an activity phase. For the case of PKS 2005–489 (BL Lac), this blazar is in a quiescent state, in which it has been for more than a decade, and it is compatible with its bluer-when-brighter moderate tendency, possibly due to the presence of shocks within the jet.