Spectropolarimetric view of the gamma-ray emitting NLS1 1H0323 + 342

Abstract

The gamma-ray emitting narrow-line Seyfert 1 galaxies are a unique class of objects that launch powerful jets from relatively lower-mass black hole systems compared to the Blazars. However, the black hole masses estimated from the total flux spectrum suffer from the projection effect, making the mass measurement highly uncertain. The polarized spectrum provides a unique view of the central engine through scattered light. We performed spectropolarimetric observations of the gamma-ray emitting narrow-line Seyfert 1 galaxy 1H0323 + 342 using SPOL/MMT. The degree of polarization and polarization angle are 0.122 ± 0.040 per cent and 142 ± 9 degrees, while the H α line is polarized at 0.265 ± 0.280 per cent. We decomposed the total flux spectrum and estimated broad H α full width at half maximum of 1015 km s−1. The polarized flux spectrum shows a broadening similar to the total flux spectrum, with a broadening ratio of 1.22. The Monte Carlo radiative transfer code ‘STOKES’ applied to the data provides the best fit for a small viewing angle of 9–24 deg and a small optical depth ratio between the polar and the equatorial scatters. A thick broad-line region with significant scale height can explain a similar broadening of the polarized spectrum compared to the total flux spectrum with a small viewing angle.