Abstract
We analyze the broadband activity of the flat spectrum radio quasar PKS 1222+216 from 2008 to 2015 using multi-frequency monitoring which involves γ-ray data from the Fermi Large Area Telescope, total intensity and linear polarization observations from different optical telescopes in R band, and imaging of the inner jet structure with the Very Long Baseline Array (VLBA) at 43 GHz. During the observations, the source showed several dramatic flares at γ rays and optical bands, with the rising branch of a γ-ray flare accompanied by a rapid rotation of the polarization position angle (EVPA), a fast increase of the degree of polarization in the optical band, brightening of the VLBI core, and appearance of a new superluminal component in the parsec-scale jet. The rapid variability of the optical linear polarization may be explained by a strong turbulence in the jet plasma. We find a correlation between the γ rays, optical R band, and 43 GHz variability on a long-term scale (months and years), and a good general alignment between EVPAs in R band and at 43 GHz, while the correlation between short-term variations (days and weeks) is weaker. Synchronous activity across the bands supports the idea that the emission regions responsible for the γ-ray and optical flares are co-spatial and located in the vicinity of the mm-wave core of the parsec-scale jet. However, these connections do not completely explain the challenging behaviour of PKS 1222+216, since there are some γ-ray flares which are not accompanied by jet events, and vice versa. We need a continuation of multi-frequency monitoring along with high resolution imaging of the parsec-scale jet to understand in detail the origin of high energy emission in blazars.
Highlights
The flat spectrum radio quasar PKS 1222+216 (4C+21.34; z = 0.43) ended a prolonged state of quiescence in a dramatic manner in late 2009
A number of γ-ray and optical flares occurred during the two subsequent years
The optical R-band EVPA has a preferred direction near 0◦, which coincides with the EVPA values of the Very Long Baseline Array (VLBA) map peak at 43 GHz and the jet flow direction (Figures 1e and 3d)
Summary
A series of pronounced γ-ray flares over a nearly two-year period were accompanied by millimeter-wave outbursts and optical flares, as well as a detection of >0.2 TeV γ-rays in June 2010 [1]. This established the object as a bona fide blazar, whose emission across most of the electromagnetic spectrum is dominated by nonthermal radiation from a compact. Telescope and VLBA images at 43 GHz. Telescope and VLBA images at 43 GHz We have used these observations to improve constraints on the location of the emission regions responsible for the optical and γ-ray flares by comparison of individual events (flares, polarization variability, jet kinematics) and cross-correlation analysis of light curves
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