Abstract
We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical flare in early 2015. The optical flare was accompanied by a powerful γ -ray flare and the detection of very-high-energy γ -ray emission. We analyze total and polarized intensity images obtained with the VLBA at 43 GHz and discover a new bright polarized superluminal knot, which was ejected from the VLBI-core during the peak of the flare.
Highlights
The BL Lac object S4 0954+658 (z = 0.367) displays significant flux and polarization variability on both intra-day and longer timescales
The flare in February–March of 2015 was the brightest optical state ever observed for the blazar
High-amplitude intra-night variations were detected in both optical light and fractional polarization
Summary
We combine observations from cm-wave to γ-ray energies, and compare the light curves with the evolution of the parsec-scale jet structure of the source at 43 GHz. 2. Optical (R-band) flux densities were obtained from photometric observations at the 0.4 m telescope of St. Petersburg State U. Identification of components in the jet across epochs is based on analysis of their flux, position angle, distance from the core, size, degree of polarization, and EVPA (Electric Vector Position Angle).
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