Variable optical polarization during high state in γ-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 the 1.5-m Kanata telescope since 2012 September, but following a gamma-ray flux enhancement detected by Fermi Large Area Telescope (Fermi-LAT) on MJD 56483 (2013 July 10), dense follow-up was performed by 10 0.5-2.0-m telescopes in Japan over one week. The 2-yr 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 similar to 20 d, during which we clearly detected optical polarization from this object. The polarization degree (PD) of the source increased from 0%-1% in quiescence to similar to 3% at maximum and then declined to the quiescent level, with the duration of the enhancement being less than 10 d. 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-hr continuous exposure on a single night. We discuss these findings based on the turbulent multi-zone model recently advocated by Marscher (2014, ApJ, 780, 87). Optical to ultraviolet (UV) spectrum showed a rising shape in the higher frequency and the UV magnitude measured by Swift/UVOT (UV and Optical Telescope) was steady even during the flaring state, suggesting that thermal emission from accretion disk is dominant in that band.