Multi-band optical-NIR variability of blazars on diverse timescales

TL;DR

This study presents extensive optical and near-infrared monitoring of three highly active blazars, analyzing their variability across multiple timescales and wavelengths to understand their spectral behavior and underlying physical processes.

Contribution

It provides a comprehensive multi-band variability analysis of blazars over several years, including new observations, correlation studies, and spectral energy distribution insights.

Findings

Weak correlations with no significant time lags found.

Sources predominantly follow a bluer-when-brighter trend.

Characteristic variability timescales estimated using structure functions.

Abstract

To search for optical variability on a wide range of timescales, we have carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in B, V, R and I pass-bands were determined for $\sim$ 7000 new optical observations from 114 nights made during 2011 - 2014, with an average length of $\sim$ 4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse timescales. Discrete correlation functions were computed among B, V, R, and I observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using B, V, R, I, J and K pass-band data. We found that the sources almost always follows a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.