Rotation of the optical polarization angle associated with the 2008 {\gamma}-ray flare of blazar w comae

Abstract

An $R$--band photopolarimetric variability analysis of the TeV bright blazar W Comae, between 2008 February 28 and 2013 May 17, is presented. The source showed a gradual tendency to decrease its mean flux level with a total change of ~3~mJy. A maximum and minimum brightness states in the $R$-band of 14.25$\pm$0.04 and 16.52$\pm$0.1~mag respectively were observed, corresponding to a maximum variation of $\Delta$F = 5.40 mJy. We estimated a minimum variability timescale of $\Delta$t=3.3 days. A maximum polarization degree $P$=33.8\%$\pm$1.6\%, with a maximum variation of $\Delta$P = 33.2\%, was found. One of our main results is the detection of a large rotation of the polarization angle from 78\degr to 315\degr ($\Delta\theta\sim$237\degr) that coincides in time with the $\gamma$-ray flare observed in 2008 June. This result indicates that both optical and $\gamma$-ray emission regions could be co-spatial. During this flare, a correlation between the $R$-band flux and polarization degree was found with a correlation coefficient of $r_{F-p}=0.93\pm$0.11. From the Stokes parameters we infer the existence of two optically thin synchrotron components that contribute to the polarized flux. One of them is stable with a constant polarization degree of 11\%. Assuming a shock-in jet model during the 2008 flare, we estimated a maximum Doppler factor $\delta_D\sim 27$ and a minimum of $\delta_D\sim 16$; a minimum viewing angle of the jet $\sim$2\degr.0; and a magnetic field $B \sim$ 0.12~G.