Radio-loud Narrow Line Seyfert 1 under a different perspective: a revised black hole mass estimate from optical spectropolarimetry

Several studies indicate that radio-loud (RL) Active Galactic Nuclei (AGN) are produced only by the most massive black holes (BH), $M_{rm BH} sim 10^8$-$10^{10} M_odot$. This idea has been challenged by the discovery of RL Narrow Line Seyfert 1 (RL NLSy1), having estimated masses of $M_{rm BH}$$sim$$10^6$-$10^7$ M$_odot$. However, these low $M_{rm BH}$ estimates might be due to projection effects. Spectropolarimetry allows us to test this possibility by looking at RL NLSy1s under a different perspective, i.e., from the viewing angle of the scattering material. We here report the results of a pilot study of VLT spectropolarimetric observations of the RL NLSy1 PKS 2004-447. Its polarization properties are remarkably well reproduced by models in which the scattering occurs in an equatorial structure surrounding its broad line region, seen close to face-on. In particular, we detect a polarized H$alpha$ line with a width of $sim$ 9,000 km s$^{-1}$, $sim 6$ times broader than the width seen in direct light. This corresponds to a revised estimate of $M_{rm BH}$$sim$$6times10^8$ M$_odot$, well within the typical range of RL AGN. The double-peaked polarized broad H$alpha$ profile of the target suggests that the rare combination of the orientation effects and a broad line region dominated by the rotation might account for this class of objects, casting doubts on the virial estimates of BH mass for type-I AGN.