(Proposal ID) S17B-063

(PI) Itoh, Ryosuke

(Proposal Title)
Optical circular polarization variability study of BL Lac object S5 0716+714

(Abstract)
We request to obtain full Stokes (I,Q,U,V) measurements in 4 broad band optical
filters (B,V,R,I), for the classical BL Lac object S5 0716+714, to study the 
total flux, fractional linear, fractional circular polarization variability 
for a continuous monitoring duration of 4 hours each (4.5 hours in total 
including calibration overheads). The proposed observations using FOCAS in 
imaging polarimetry mode, will give signal-to-noise ratio > 1000 in few seconds
of exposure time in each filter, enabling us to detect total flux, fractional 
linear and circular polarization with <0.2% accuracies. The measurement in 4 
broad band filters will enable us to discern the intrinsic origin of circular 
polarization and obtain strict constraints on the underlying plasma condition 
near the event horizon of the central supermassive black hole, specifically, 
the magnetic field and the proton content. The pioneering character of the 
proposed study is related to the fact that, till now, no systematic 
investigation of circular polarization of blazar sources in various optical 
filters and on hourly timescales has been carried out.