Probing the Heart of Active Narrow-line Seyfert 1 Galaxies with VERA Wideband Polarimetry

TL;DR

This study used VLBI wideband polarimetry to investigate the nuclear regions of radio-loud narrow-line Seyfert 1 galaxies, revealing high Faraday rotation measures and properties similar to blazar-like quasars, suggesting an early AGN evolutionary stage.

Contribution

First detection of Faraday rotation in NLSy1s' parsec-scale cores, showing their environment is gas-rich and similar to FSRQs, providing insights into early AGN evolution.

Findings

Detected significant Faraday rotation in NLSy1 cores.

NLSy1s have higher core RM than BL Lac objects.

Polarimetric properties resemble those of FSRQs.

Abstract

We explored the parsec-scale nuclear regions of a sample of radio-loud narrow-line Seyfert 1 galaxies (NLSy1s) using the VLBI Exploration of Radio Astronomy (VERA) wideband (at a recording rate of $16\,\mathrm{Gbps}$) polarimetry at 22 and 43 GHz. Our targets include 1H 0323+342, SBS 0846+513, PMN J0948+0022, 1219+044, PKS 1502+036 and TXS 2116-077, which are all known to exhibit $\gamma$-ray emission indicative of possessing highly beamed jets similar to blazars. For the first time, we unambiguously detected Faraday rotation toward the parsec-scale radio core of NLSy1s, with a median observed core rotation measure (RM) of $2.7\times 10^3\,{\rm rad\,m^{-2}}$ (or $6.3\times 10^3\,{\rm rad\,m^{-2}}$ for redshift-corrected). This level of RM magnitude is significantly larger than those seen in the core of BL Lac objects (BLOs; a dominant subclass of blazars), suggesting that the nuclear environment of NLSy1s is more gas-rich than that in BLOs. Interestingly, the observed parsec-scale polarimetric properties of NLSy1s (low core fractional polarization, large core RM and jet-EVPA misalignment) are rather similar to those of flat-spectrum radio quasars (FSRQs). Our results are in accordance with the scenario that NLSy1s are in an early stage of AGN evolution with their central black hole masses being smaller than those of more evolved FSRQs.