Variable optical polarization during high state in gamma-ray loud narrow
  line Seyfert 1 galaxy 1H 0323+342
  We present results of optical polarimetric and multi-band photometric
observations for gamma-ray loud narrow-line Seyfert 1 galaxy 1H 0323+342. This
object has been monitored by 1.5 m Kanata telescope since 2012 September but
following a gamma-ray flux enhancement detected by Fermi-LAT on MJD 56483 (2013
July 10) dense follow-up was performed by ten 0.5-2.0 m telescopes in Japan
over one week. The 2-year R_C-band light curve showed clear brightening
corresponding to the gamma-ray flux increase and then decayed gradually. The
high state as a whole lasted for ~20 days, during which we clearly detected
optical polarization from this object. The polarization degree (PD) of the
source increased from 0-1% in quiescence to ~3% at maximum and then declined to
the quiescent level, with the duration of the enhancement of less than 10 days.
The moderate PD around the peak allowed us to precisely measure the daily
polarization angle (PA). As a result, we found that the daily PAs were almost
constant and aligned to the jet axis, suggesting that the magnetic field
direction at the emission region is transverse to the jet. This implies either
a presence of helical/toroidal magnetic field or transverse magnetic field
compressed by shock(s). We also found small-amplitude intra-night variability
during the 2-hour continuous exposure on a single night. We discuss these
findings based on the turbulent multi-zone model recently advocated by Marscher
(2014). Optical to ultraviolet (UV) spectrum showed a rising shape in the
higher frequency and the UV magnitude measured by Swift/UVOT was steady even
during the flaring state, suggesting that thermal emission from accretion disk
is dominant in that band.
