Abstract
The sub-hour timescale variability commonly observed in blazars—widely known as intra-day or microvariability—has been extensively studied in optical photo-polarimetric bands over the past 25–30 years. In addition, there have been comprehensive theoretical discussions on the topic, with various models and scenarios proposed; however, the phenomenon still remains relatively poorly understood. Here we present the summary of our optical microvariability studies over the past few years based on multi-frequency photo-polarimetric Whole Earth Blazar Telescope (WEBT) observation campaigns. The primary objective of the study was to explore the characteristics of the source microvariability on timescales of a few minutes to a few days using exceptionally dense photo-polarimetric observations. The results show that the source often displays fast variability with an amplitude as large as 0.3 mag within a few hours, as well as color variability on similar time scales often characterized by “bluer-when-brighter” trend. Similarly, the correlation between variability in flux and polarization appears to depend upon the configuration of the optical polarization angle relative to the positional angle of the innermost radio core of the jet. Other fascinating observations include a sudden and temporary disappearance in the observed variability lasting for ∼6 h. In addition, the modeling of individual microflares strongly suggests that the phenomenon of microvariability can be best explained by convolved emission from compact emission sites distributed stochastically in the turbulent jet. Besides, analysis of some of the well resolved micro-flares exhibiting high degrees of polarization points towards a complex magnetic geometry pervading the jet with the possible presence of small-scale regions of highly ordered and enhanced magnetic field similar to so-called “magnetic islands”.
Highlights
Blazars define a subclass of radio galaxies exhibiting some extreme properties, such as high luminosity, rapid flux variability across the electromagnetic spectrum, highly polarized non-thermal emission, etc
The understanding of the physical conditions leading to the observed rapid variability, along with other active galactic nuclei (AGN) processes, might be considered key to the further understanding of galactic evolution and thereby cosmic evolution in general
Galaxies 2016, 4, 56 microvariability studies could be one of the most relevant tools that can offer important insight into the physical conditions prevailing in the innermost regions of blazar jets, including the nature of the dominant particle acceleration and energy dissipation mechanism, magnetic field geometry, jet content, etc
Summary
Blazars define a subclass of radio galaxies exhibiting some extreme properties, such as high luminosity, rapid flux variability across the electromagnetic spectrum, highly polarized non-thermal emission, etc. These extreme properties are believed to be the result of Doppler boosting of the emission taking place in the relativistic outflows which emanate from the region near black hole and travel nearly along the line-of-sight (e.g., [1]). The understanding of the physical conditions leading to the observed rapid variability, along with other active galactic nuclei (AGN) processes, might be considered key to the further understanding of galactic evolution and thereby cosmic evolution in general Telescope (WEBT) [18,19] observation campaigns in 2009 and 2014
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