| Blazar is a special subclass of active galactic nuclei(AGN),whose jets are directed towards the observer,with significant multi-wavelength variation and high degree of polarization.Variation of blazars can be caused by a variety of mechanisms,such as geometric factors related to changes in viewing angle or triggered by intrinsic shock acceleration.However,there is still a lack of strong evidence to identify the dominant mechanism in specific targets.Because of the polarization carries a great deal of information about the physical processes happening inside the sources,the observation of the polarization can easily help us to limit the mechanisms of variation.In this paper,we mainly study the generation and the characteristics of optical polarization in blazars,and analyze the variation mechanisms of CTA 102 in long and short timescales.In the first part of this paper,based on the basic theory of single particle radiation,the polarization caused by synchrotron radiation is reviewed by assuming that charged particles move helically in a constant magnetic field with extreme relativistic velocity.By considering the case of electron groups,the polarization degree of radiation generated by electron groups with different energy spectrum distributions can be obtained.Considering the electromagnetic model of the transient gamma-ray burst(GRB)and the jet structure of the hollow cylinder geometry,the polarizations of the two structures are calculated respectively.The large polarization degree of the GRB,the bimodal distribution and the 90° abrupt change of the polarization angle of the AGN jet are reconstructed.In the second part of this paper,taking blazar CTA 102 as an example,the variation phenomena in long and short timescales are analyzed in detail,including spectral indices,polarization variation and correlation between optical flux and γ-ray flux.The multiwavelength light curves are spanned about 10 years,and in short timescale,five periods of γ-ray flares are selected.In this paper,the positions of emitting regions corresponding to different bands are studied.We found that the optical and γ-ray bands have the same emitting region and is located in the upstream of the X-ray emitting region.But the relative distance from the emitting regions to the radio core cannot be determined by the correlation between the light curves.In optical band,the spectral indices of CTA 102 changes from redder-when-brighter(RWB)to bluer-when-brighter(BWB).While in γ-ray band,the spectral indices change from softer-when-brighter(SWB)to flatter-when-brighter(FWB).In this paper we found that the polarization of CTA 102 increases with the flux at long timescale.But in short timescales,the polarization and the flux can be positively correlated,negatively correlated or irrelevant.This complex correlation can be explained by multiple emitting zones.Through numerical simulation,we found that in helical jet model,there is clockwise or counterclockwise rotation on the q-u plane;and in the shock model which using the viewing angle as the variable,the oscillation behavior appears on the q-u plane.In addition,the polarization degrees of CTA 102,rotations on the q-u plane and the relationship between optical flux and γ-ray flux can,be explained by the two-components model.By fitting the curve of the relation between optical flux and-γ-ray flux,it can be concluded that the long-term variation of CTA 102 is caused by the change of Doppler factors,and the γ-ray radiation is generated through SSC process.Finally,the correlation between the polarization degree and the optical flux of RoboPol polarization monitoring samples is analyzed.We find that the polarization degree of each target also can be positively correlated,negatively correlated or irrelevant with the optical flux in long timescale.The correlation was related to the classification of blazars.The polarization degree of FSRQ was positively correlated with the flux,while a variety of correlations were possible in BL Lac. |
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