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

TL;DR

This study presents optical, X-ray, and gamma-ray variability analysis of 18 southern blazars over five years, revealing activity phases and spectral behaviors, with specific insights into PKS 1510-089 and PKS 2005-489.

Contribution

First comprehensive multiwavelength optical monitoring of southern blazars correlating flux, spectral, and variability behaviors over several years.

Findings

PKS 1510-089 is in an active phase.

PKS 2005-489 remains in a decade-long quiescent state.

Bluer-when-brighter trend observed in some blazars.

Abstract

We present the results of the optical monitoring of 18 southern blazars in the V and R Johnson-Cousins bands. Our main objective is to study the variations in the optical flux and theis 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 gamma-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 considired 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.