Discovery of a Highly Polarized Optical Microflare in the Blazar S5 0716+714 During 2014 WEBT Campaign
2015
The occurrence of low-amplitude flux variations in
blazarson 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 reconnectionsites, 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
blazar0716+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 $\sim (40-60)\%$$\pm (2-10)\%$, and the electric vector
position angle$\sim (10 - 20)$deg$\pm (1-8)$deg slightly misaligned with respect to the
position angleof the radio jet. The microflare evolution in the $(Q,\,U)$
Stokes parameterspace 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.
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