Minute-Scale Rapid Variability of Optical Polarization in Narrow-Line Seyfert 1 Galaxy: PMN J0948+0022

TL;DR

This study reports rapid optical polarization variability in the RL-NLSy1 galaxy PMN J0948+0022, revealing a compact emission region and providing insights into jet structures distinct from blazars.

Contribution

First detection of minute-scale polarized flux variability in a narrow-line Seyfert 1 galaxy, indicating a highly ordered magnetic field in a compact emission region.

Findings

Polarization degree reached 36% at peak.

No significant time lag over 10 minutes between flux and polarization.

Evidence of synchrotron radiation from a 10^14 cm region.

Abstract

We report on optical photopolarimetric results of the radio-loud narrow line Seyfert 1 (RL-NLSy1) galaxy PMN J0948+0022 on 2012 December to 2013 February triggered by flux enhancements in near infrared and gamma-ray bands. Thanks to one-shot polarimetry of the HOWPol installed to the Kanata telescope, we have detected very rapid variability in the polarized-flux light curve on MJD 56281 (2012 December 20). The rise and decay times were about 140 sec and 180 sec, respectively. The polarization degree (PD) reached 36 +/- 3% at the peak of the short-duration pulse, while polarization angle (PA) remained almost constant. In addition, temporal profiles of the total flux and PD showed highly variable but well correlated behavior and discrete correlation function analysis revealed that no significant time lag of more than 10 min was present. The high PD and minute-scale variability in polarized flux provides a clear evidence of synchrotron radiation from a very compact emission region of 10^14 cm size with highly ordered magnetic field. Such micro variability of polarization are also observed in several blazar jets, but its complex relation between total flux and PD are explained by multi-zone model in several blazars. The implied single emission region in PMN J0948+0022 might be reflecting a difference of jets between RL-NLSy1s and blazars.