Discovery of a Highly Polarized Optical Microflare in Blazar S5 0716+714 During the 2014 Webt Campaign

Abstract

The occurrence of low-amplitude flux variations in blazars on hourly timescales, commonly known as microvariability, is still a widely debated subject in high-energy astrophysics. Several competing scenarios have been proposed to explain such occurrences, including various jet plasma instabilities leading to the formation of shocks, magnetic reconnection sites, and turbulence. In this letter we present the results of our detailed investigation of a prominent, five-hour-long optical microflare detected during recent WEBT campaign in 2014, March 2-6 targeting the blazar 0716+714. After separating the flaring component from the underlying base emission continuum of the blazar, we find that the microflare is highly polarized, with the polarization degree ∼ (40−60)% ±(2−10)%, and the electric vector position angle ∼ (10−20) deg ±(1−8) deg slightly misaligned with respect to the position angle of the radio jet. The microflare evolution in the (Q, U ) Stokes parameter space exhibits a looping behavior with a counter-clockwise rotation, meaning polarization degree decreasing with the flux (but higher in the flux decaying phase), and approximately stable polarization angle. The overall very high polarization degree of the flare, its symmetric flux rise and decay profiles, and also its structured evolution in the Q − U plane, all imply that the observed flux variation corresponds to a single emission region characterized by a highly ordered magnetic field. As discussed in the paper, a small-scale but strong shock propagating within the outflow, and compressing a disordered magnetic field component, provides a natural, though not unique, interpretation of our findings.

Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions

“…Furthermore, symmetry analysis of the longterm optical light curves of the blazar BL Lac resulted a positive correlation between flare durations and peak fluxes [17]. As an attempt to characterize microvariability in the famous BL Lac source S5 0716+714 robustly, the source was intensely studied using extended whole earth blazar telescope (WEBT) campaigns (see [8,[18][19][20]). Earlier multiband intranight variability in the source S5 0716+714 has been studied by several authors (e.g., [21,22]).…”

Coordinated Micro-Variability CIRCE Polarimetry and SARA JKT Multi-Frequency Photometry Observations of the Blazar S5 0716+71

“…Table 2 summarizes the polarization data gleaned from the literature as well as the data presented here. In both [2,9], distinct events were seen in the micro-variability curve. The flare components were analyzed, separating them from a background component.…”

“…The motivation for the observation is to test the KRM turbulent jet model under development at Florida International University (cf. [1,2]). Micro-variability is defined as small-amplitude (∼0.1 mag), short-duration (<1 day) oscillations, which the model describes as being caused by a shock propagating down a turbulent jet.…”

Investigating the Innermost Jet Structures of Blazar S5 0716+714 Using Uniquely Dense Intra-day Photo-polarimetric Observations

“…Further, some of the well-resolved microflares were modeled by decomposing the flaring component from the relatively slowly changing background component in the light curve. The details of the modeling procedure, and the results for one of the distinctly resolved microflares are discussed in [15]. The results showed that such microflares are highly polarized (up to 60%) in general, with a flux-polarization anti-correlation trend, as shown in Figure 6.…”

“…S5 0716+714 has also been detected by γ-ray instruments such as the Compton EGRET, AGILE, Fermi-LAT satellites and also by the MAGIC Cherenkov telescope (see, e.g., [11][12][13][14] and references therein). In particular, there have been numerous optical microvariability studies of S5 0716+714 which reported many interesting results, including achromatic behavior, red noise-type power spectra, and in some cases, correlation between flux and polarization degree [15][16][17][18]. In this proceeding, we present the summary of our optical microvariability studies on S5 0716+714, over the past few years, based on multifrequency photo-polarimetric Whole Earth Blazar Telescope (WEBT) [18,19] observation campaigns in 2009 and 2014 (the data collected by the WEBT Collaboration are stored in the WEBT archive; for questions regarding their availability, please contact the WEBT President Massimo Villata: villata@oato.inaf.it).…”