Optical polarimetry of the blazar CGRaBS~ J0211+1051 from MIRO

Abstract

We report the detection of high polarization in the first detailed optical linear polarization measurements on the BL Lac object CGRaBS J0211+1051, which flared in $\gamma$-rays on 2011 January 23 as reported by Fermi. The observations were made during 2011 January 30 - February 3 using photo-polarimeter mounted at the 1.2m telescope of Mt Abu InfraRed Observatory(MIRO). The CGRaBS J0211+1051 was detected to have $\sim21.05\pm 0.41%$ degree of polarization (DP) with steady position angle (PA) at 43$^\circ$ on 2011 January 30. During Jan 31 and Feb 1, while polarization shows some variation, position angle remains steady for the night. Several olarization flashes occurred during February 2 and 3 resulting in changes in the DP by more than 4% at short time scales ($\sim$ 17 to 45 mins). A mild increase in the linear polarization with frequency is noticed during the nights of February 2 & 3. The source exhibited significant inter-night variations in the degree of polarization (changed by about 2 to 9%) and position angle (changed by 2 to 22$^\circ$) during the five nights of observations. The intra-night activity shown by the source appears to be related to turbulence in the relativistic jet. Sudden change in the PA accompanied by a rise in the DP could be indicative of the fresh injection of electrons in the jet. The detection of high and variable degree of polarization categorizes the source as low energy peaked blazar.