Optical polarization properties of February 2010 outburst of the blazar Mrk 421

Abstract

In this paper, we explore the behavior of optical polarization during the multi-wavelength outburst of the blazar Mrk 421 detected in February 2010. We use optical polarization measurements in the wavelength range 500–700 nm from SPOL observations available between January 1, 2010 and March 31, 2010 (MJD 55197-55286) including the period of multi-wavelength flaring activity detected from the source around February 16–17, 2010 (MJD 55243-55244). We also use near simultaneous optical and radio flux measurements from SPOL in V and R bands and OVRO at 15 GHz respectively. We find that the emissions in the optical and radio bands do not show any significant change in the source activity unlike at X-ray and $\gamma$-ray energies during the outburst. The optical and radio flux measurements are found to be consistent with the long term quiescent state emission of the source. Moreover, the linear polarization in the wavelength range 500–700 nm decreases to a minimum value of 1.6$\%$ during the X-ray and $\gamma$-ray outburst which is significantly lower than the long term average value of $\sim 4.2\%$. The angle of polarization varies between $114^{\circ }\mbox{--}163^{\circ }$ with a preferred average value of $\sim 137^{\circ }$ during this period. We estimate the degree of polarization intrinsic to the jet taking into account the host galaxy contamination in R band and compare this with the theoretical synchrotron polarization estimated for a power law distribution of relativistic electrons gyrating in an emission region filled with ordered and chaotic magnetic fields. The intrinsic linear polarization estimated for different epochs during the above period is found to be consistent with the theoretical synchrotron polarization produced by the relativistic electrons with power law spectral index $\sim 2.2$. We find that the behavior of optical polarization possibly supports the two emission zone hypothesis for blazars in which X-ray and $\gamma$-rays are produced in one region whereas the optical emission takes place from another region permeated with ordered and chaotic magnetic fields. The decrease in linear polarization during the X-ray and $\gamma$-ray outburst can be attributed to the sudden dominance of chaotic magnetic field over the ordered field in the optical emission region in the relativistic jet of the blazar Mrk 421.